Vertical Ascent vs. To LXO First

[Kesa]

Arkie, your video demonstration is not valid, because getting 2 apoapses at the Mun without getting the Periapses at the same place means your orbits have different energies.

Raising periapse isn't necessary to go to/around the mun. Actually, the differences of energy required is the kinetic energy of ~300m/s (speed of elliptical vs no speed in straight up), which, due to Oberth effect, represents a tiny difference of ~20m/s at 70km. I did not do the math but I expect a not greater difference in theoretical dv requirement to land on the mun.

You're damn right about how to test different methods : ships to orbit and ships to straight up are really different. Dv requirement is irrelevant, because high TWR crafts lose less dv, but are often more expansive, and often have less dv. I don't think anyone is going to prove vertical ascent is the cheapest way, but if so, I bet it is not possible without lots of SRBs.

Edited December 15, 2014 by Kesa
added indent

[Superfluous J]

Mk16 parachute, mk1 command pod, TR-18A stack decoupler, FL-T400 fuel tank, and LV-T30.

I will try tonight. I look forward to see your results.

I don't have time to record anything but I just tried this and - with a paper-thin margin - I did it with a craft that totaled $9740 including the $3122 for the payload. That's $6618. I'd like a better margin (and perhaps a solar panel) but am confident I can keep the entire cost under $10k including payload.

The lifter is in 3 stages: One with 5 60% limited BACC SRBs (for the initial thrust. By the time these are done I'm basically sideways) one with a single unlimited BACC srb (that essentially gets me into orbit) and a 48-7s pushing a T400 fuel tank to get to Mun and circularlize at 30km. Total VAB dV is 6847m/s. counting payload, 4707 for the lifter.

EDIT

Okay last post/update for a while. I redesigned the ship to not thrust limit and to instead split the 5 SRBs into 2 sets of non-thrust-limited SRBs, 3 in the first set and 2 in the second. I also modified it to have 2 smaller SRBs instead of one large one in the top stage of the lifter. now I've got about a 400-500dV margin and a very easy ride up to orbit in spite of not having fins or nosecones. I'm going to tweak later tonight but I'm hoping to trim that margin down and also get the ship below $10k (It's 10,415 right now)

Assuming I get that working, I'll try to post a video or at least an Imgur album tomorrow morning.

Raising periapse isn't necessary to go to/around the mun.

I am not saying raising periapsis is necessary. I'm saying having apoapsis at the same height is not enough information to know which is better. Unless the two ships end in the same circumstances (which is why I proposed getting a payload into a specific Mun orbit) then the test is invalid.

Edited December 15, 2014 by 5thHorseman

[Kesa]

I am not saying raising periapsis is necessary. I'm saying having apoapsis at the same height is not enough information to know which is better. Unless the two ships end in the same circumstances (which is why I proposed getting a payload into a specific Mun orbit) then the test is invalid.

Wanted to include it upon saying you were damn right for the comparison method, I just forgot

[arkie87]

Wanted to include it upon saying you were damn right for the comparison method, I just forgot

Yes, i think me and 5thHorsemen discussed before that having the same apoapsis isnt fair since lko method will have less velocity relative to the mun when it arrives, and will therefore, require less deltaV to get into an orbit.

I don't have time to record anything but I just tried this and - with a paper-thin margin - I did it with a craft that totaled $9740 including the $3122 for the payload. That's $6618. I'd like a better margin (and perhaps a solar panel) but am confident I can keep the entire cost under $10k including payload.

The lifter is in 3 stages: One with 5 60% limited BACC SRBs (for the initial thrust. By the time these are done I'm basically sideways) one with a single unlimited BACC srb (that essentially gets me into orbit) and a 48-7s pushing a T400 fuel tank to get to Mun and circularlize at 30km. Total VAB dV is 6847m/s. counting payload, 4707 for the lifter.

EDIT

Okay last post/update for a while. I redesigned the ship to not thrust limit and to instead split the 5 SRBs into 2 sets of non-thrust-limited SRBs, 3 in the first set and 2 in the second. I also modified it to have 2 smaller SRBs instead of one large one in the top stage of the lifter. now I've got about a 400-500dV margin and a very easy ride up to orbit in spite of not having fins or nosecones. I'm going to tweak later tonight but I'm hoping to trim that margin down and also get the ship below $10k (It's 10,415 right now)

Assuming I get that working, I'll try to post a video or at least an Imgur album tomorrow morning.

I am not saying raising periapsis is necessary. I'm saying having apoapsis at the same height is not enough information to know which is better. Unless the two ships end in the same circumstances (which is why I proposed getting a payload into a specific Mun orbit) then the test is invalid.

Which aerodynamics model are you using, 5thHorsemen? You have to be using FAR or the tests will not be comparable (if i use stock, then my method will not work since terminal velocity is a killer in stock).

[arkie87]

Yes, i think me and 5thHorsemen discussed before that having the same apoapsis isnt fair since lko method will have less velocity relative to the mun when it arrives, and will therefore, require less deltaV to get into an orbit.

Which aerodynamics model are you using, 5thHorsemen? You have to be using FAR or the tests will not be comparable (if i use stock, then my method will not work since terminal velocity is a killer in stock).

Also, were your tests done with 0.25 or

0.90

[Superfluous J]

Which aerodynamics model are you using, 5thHorsemen? You have to be using FAR or the tests will not be comparable (if i use stock, then my method will not work since terminal velocity is a killer in stock).

You may not know this but if one of us uses stock and the other uses FAR, the one who uses stock will almost assurdly lose. By using FAR (which I did) I saved well over a grand of dV which was about a 12-15% savings on fuel. That's even more than I expect to save over the "straight up" method.

There's no way I could get that ship into orbit of Mun with a $10k rocket in stock.

And as a non-preview-build user who did the tests before 0.90 came out, I of course didn't use it However, I don't think there were any changes between the versions so I'll happily try it in .90 if you feel it's necessary.

[arkie87]

You may not know this but if one of us uses stock and the other uses FAR, the one who uses stock will almost assurdly lose. By using FAR (which I did) I saved well over a grand of dV which was about a 12-15% savings on fuel. That's even more than I expect to save over the "straight up" method.

There's no way I could get that ship into orbit of Mun with a $10k rocket in stock.

And as a non-preview-build user who did the tests before 0.90 came out, I of course didn't use it However, I don't think there were any changes between the versions so I'll happily try it in .90 if you feel it's necessary.

I doubt anything has changed between 0.9 and 0.25. But right now, i cannot get access to FAR for 0.90 so module manager wont enable it... so i have to wait...

And of course i was aware that deltaV requirements are different. That's why i was asking. If you were using stock, i could build a better rocket with FAR without trying :-P

[arkie87]

I'm sorry if I'm repeating someone else, but...

I think there's a massive elephant in the room here that everyone's overlooked.

In your video, you use 1 Jumbo tank of fuel for each launch. You use a tiny amount more for the vertical ascent launch, plus a tiny boost from the stage separation (in other words a little bit more dV). Then you go to the map view.

Compare those two map screens:

In the prograde launch, you have an apoapsis much in excess of Mun's orbital altitude and a more circular orbit. In the vertical ascent launch, you have an apoasis ~ Mun's orbital altitude, and no circularisation to speak of. You've also spent a drop more fuel.

The former indicates a higher energy state for your craft - you've literally gone further for marginally less fuel. In other words, your dV calculations are wrong.

Am I really the first to spot this?

How are my dV calculations wrong? I report that vertical is less efficient, not vice versa (but only marginally)?

And i try to adjust for the overshoot with the maneuver node as best I can...

[arkie87]

I doubt anything has changed between 0.9 and 0.25. But right now, i cannot get access to FAR for 0.90 so module manager wont enable it... so i have to wait...

And of course i was aware that deltaV requirements are different. That's why i was asking. If you were using stock, i could build a better rocket with FAR without trying :-P

On second thought, cost of parts might be different, if balance of the career mode has changed. Can you verify you craft has not changed price?

[The_Rocketeer]

How are my dV calculations wrong? I report that vertical is less efficient, not vice versa (but only marginally)?

And i try to adjust for the overshoot with the maneuver node as best I can...

Looks like you got that in just before I deleted my post. I retracted it because I realised I hadn't allowed for dV changes due to Isp. Even so, tank for tank clearly indicates you get considerably more orbital energy for your fuel using a prograde burn.

It's also pretty well established that your displayed quantities are as dodgy as a £6 note (as we say in the UK). That was in a post I hadn't read yet at the time of writing.

Edited December 16, 2014 by The_Rocketeer

[The_Rocketeer]

A few points, however.

1. All of these (many) threads have left me a little confused regarding what exactly you're talking about. Technically LKO (or LXO) implies reaching a stable orbit (before making a Hoffman transfer manoeuvre), which requires raising periapsis to a safe altitude. Obviously this involves fuel losses compared with a vertical intercept. Since that's apparently not what you're talking about, I'll try to address the question of the text rather than the question of the title.

2. You haven't given any in game example of a Mun launch to provide data for your point. This entire discussion is hypothetical, yet you criticise some for not providing hard data to support their view. Where's yours?

3. The argument (for a vacuum launch) boils down to this:

Vertical launch gains:

1. burn only in desired direction of velocity

2. shorter distance to intercept/escape

3. no terrain danger

Vertical launch losses:

1.acceleration due to gravity.

"LKO" > intercept gains:

1. Null net gravity losses while passing planet, i.e. 'gravity free' acceleration

2. Therefore higher velocity at equivalent distance to escape/intercept

2a. Consequently less time to escape/intercept, and lower total gravity losses from this point

"LKO" > intercept losses:

1. dV losses due to burning in directions other than desired velocity

2. overall time due to further total distance

3. losses due to terrain avoidance

The extent to which each is advantageous is a question of TWR primarily because of TIME.

In a vertical ascent, the slower the acceleration the more time that passes, the worse the gravity losses become. Therefore the losses are minimised by very rapid acceleration, i.e. high TWR.

High TWR, however, doesn't greatly help a 'LKO' launch because the planet is in the way and has to be navigated around. Extra thrust in directions other than desired velocity (to speed up getting past the planet) just result in a more circular orbit. Therefore, forced higher TWR actually impedes the efficiency of this launch approach.

That's how I see it anyway. Feel free to correct my mistakes, but do so acknowledging that I made them in good faith.

[GoSlash27]

Arkie,

Given the results you've achieved in the t/w ratio thread, you might want to revisit this.

Assuming a 10G thrust is sufficient to achieve Vt in the vertical mode, you could test such a craft for total DV to munar apoapsis, then compare it to the results you'd get from the same craft when flying prograde and throttling back to maintain it's Vt.

I think if you do it that way, you'll see more disparity in total DV between the two methods than you had before.

This is what I had done in my test; the vertical launch was conducted at 2G acceleration, while the prograde test was done with the acceleration roughly proportional to the sine of the pitch angle.

Your previous test had you going full throttle in the prograde maneuver and you lost a ton of DV due to drag.

Best,

-Slashy

Edited December 16, 2014 by GoSlash27

[GoSlash27]

^ Addendum to the above...

After establishing that the prograde profile is much more efficient than the vertical, it should then become apparent that using engines with enough thrust to generate 10 Gs of acceleration in the prograde mode is wasteful, since you don't need anywhere near that mass of engines.

Taking this into account would make the disparity even greater.

Best,

-Slashy

[arkie87]

Looks like you got that in just before I deleted my post. I retracted it because I realised I hadn't allowed for dV changes due to Isp. Even so, tank for tank clearly indicates you get considerably more orbital energy for your fuel using a prograde burn.

It's also pretty well established that your displayed quantities are as dodgy as a £6 note (as we say in the UK). That was in a post I hadn't read yet at the time of writing.

How are my displays dodgy? can you elaborate? Even if you dont trust KER, its clear that for LKO burn, i used just about the entire first stage. And for vertical burn, I used entire first stage plus a bit of second stage... so even if you dont trust deltaV values, you have to trust fuel usage, which says they are really close.

A few points, however.

1. All of these (many) threads have left me a little confused regarding what exactly you're talking about. Technically LKO (or LXO) implies reaching a stable orbit (before making a Hoffman transfer manoeuvre), which requires raising periapsis to a safe altitude. Obviously this involves fuel losses compared with a vertical intercept. Since that's apparently not what you're talking about, I'll try to address the question of the text rather than the question of the title.

I could be talking about either raising periapsis first, or not. It doesnt matter too much, since, it is my understanding, they are close either way.

2. You haven't given any in game example of a Mun launch to provide data for your point. This entire discussion is hypothetical, yet you criticise some for not providing hard data to support their view. Where's yours?

My discussion isnt hypothetical at all.... I have provided calculations (and a video, if you are referring to LKO; many videos if you consider related arguments). Calculations or actual trials are hard data. Even logical arguments can be accepted under certain conditions (since they can be refuted). But what i oppose, is people merely asserting that they are right and I am wrong, without giving any reason whatsoever.

3. The argument (for a vacuum launch) boils down to this:

Vertical launch gains:

1. burn only in desired direction of velocity

2. shorter distance to intercept/escape

3. no terrain danger

Vertical launch losses:

1.acceleration due to gravity.

Not sure why #2 is an advantage? Unless you mean shorter burn time, which is the advantage of high TWR in general, not vertical ascent.

If we are talking about in a vacuum i.e. Mun, then yes, there are no other advantages... Also, while on earth, burning vertically will cause you to travel approx 100 km during ascent before engines need to cut out, burning vertical from Mun might take you just as far (a bit less since acceleration will be faster due to higher TWR), and 100km is one Mun radius, so gravity drops by 4.

"LKO" > intercept gains:

1. Null net gravity losses while passing planet, i.e. 'gravity free' acceleration

2. Therefore higher velocity at equivalent distance to escape/intercept

2a. Consequently less time to escape/intercept, and lower total gravity losses from this point

#1: It is not a gravity free acceleration, unless you assume impulse burn. You have to accelerate up to orbital velocity, at the very least, while aiming at the angle necessary to prevent you from falling, which is wasteful.

#2: I dont follow. the advantage of LXO, is that once you are in a stable orbit, you can burn perpendicular to gravity to avoid gravity losses. So the advantage of of going to LXO first only comes if periapsis burn is large enough

#3: I dont see what time has to do with it, persay. And i dont think you can say definitively the paripasis will have less time spent in SoI, since vertical goes straight up to SoI exit, while LXO approach has to swing around the entire planet first (even if it's going faster, though i dont think it is-- escape velocity is a function of altitude and, as long as you dont hit the planet/atmosphere while doing it, the direction you choose )...these are "hand waivy" logical arguments. And I can think of hand-waivy logical arguments which could provide the other conclusion. Thus, i much prefer either hard trials or math.

"LKO" > intercept losses:

1. dV losses due to burning in directions other than desired velocity

2. overall time due to further total distance

3. losses due to terrain avoidance

The extent to which each is advantageous is a question of TWR primarily because of TIME.

In a vertical ascent, the slower the acceleration the more time that passes, the worse the gravity losses become. Therefore the losses are minimised by very rapid acceleration, i.e. high TWR.

Yes, I agree and having been saying this the entire TIME (pun intended). It is actually my origional argument. And it's not about overall time, only about time spent burning (since fuel flow is constant).

High TWR, however, doesn't greatly help a 'LKO' launch because the planet is in the way and has to be navigated around. Extra thrust in directions other than desired velocity (to speed up getting past the planet) just result in a more circular orbit. Therefore, forced higher TWR actually impedes the efficiency of this launch approach.

Yes, this is the main advantage of LKO; you can burn in stable orbit where due to centripetal forces, gravity is essentially zero. Thus, low TWR and more efficient engines can be used, saving mass thereby increasing deltaV. I am not arguing with that point.

That's how I see it anyway. Feel free to correct my mistakes, but do so acknowledging that I made them in good faith.

That is a great attitude to have

[arkie87]

Arkie,

Given the results you've achieved in the t/w ratio thread, you might want to revisit this.

Assuming a 10G thrust is sufficient to achieve Vt in the vertical mode, you could test such a craft for total DV to munar apoapsis, then compare it to the results you'd get from the same craft when flying prograde and throttling back to maintain it's Vt.

So to confirm i understand, you are talking about LKO not LMO (this thread is about LMO)?

You essentially want me to confirm that terminal velocity is most efficient in FAR i.e. if my TWR is scaled up for FAR i.e. like 10+, then i will indeed need to back off the throttle, just like in stock. I dont see the pressing need to do this.

I think if you do it that way, you'll see more disparity in total DV between the two methods than you had before.

This is what I had done in my test; the vertical launch was conducted at 2G acceleration, while the prograde test was done with the acceleration roughly proportional to the sine of the pitch angle.

Your previous test had you going full throttle in the prograde maneuver and you lost a ton of DV due to drag.

Best,

-Slashy

How did i lose a ton of deltaV due to drag, if i was below terminal velocity the entire time?

^ Addendum to the above...

After establishing that the prograde profile is much more efficient than the vertical, it should then become apparent that using engines with enough thrust to generate 10 Gs of acceleration in the prograde mode is wasteful, since you don't need anywhere near that mass of engines.

Taking this into account would make the disparity even greater.

Best,

-Slashy

This is once again referring to the Kerbin case and not the Munar case, which this thread is about.

Regardless, I agree. My point, as others have said (in one of my threads ), is that increasing TWR increases deltaV due to burn becoming more like an impulse, but require more mass, and so reduce deltaV as well. Which one is larger depends on the circumstance.

Furthermore, I am not arguing this case. I am arguing, for the Kerbin case, that with a given vehicle with high TWR already. In career mode (before 0.90), all we care about is kerbucks, not mass or complexity or efficiency. In practice, SRB's are dirt cheap and easily increase TWR. So whereas one skipper costs 2850 without the fuel, the tallest SRB costs only 1800, with the fuel!!

Thus, it's probably cheaper to go with SRB's, which already will have high TWR, with FAR aeryodynamics. As a side point, in stock, high TWR is a waste of fuel since it cannot be utilized while climbing (until you reach gravity turn) since terminal velocity is soooooo low.

However, 5thHorseman will argue that even if you are using SRB's with FAR installed, it pays to throttle them back (before launch) and do a gravity turn and forgo the extra thrust, which is available at 0 mass increase; if i had a mainsail, no one would tell me i should reduce the thrust to be equivalent to a skipper, since i have already paid for the weight of a mainsail!

[The_Rocketeer]

Arkie,

You seem to not be understanding how the advantage of a gravity slingshot arises.

Imagine we could remove the physical presence of a planet/moon, but keep the gravity well and launch/start locations the same.

When we 'launch' from the side closer to our escape/intercept point, we have relatively less distance to go, so each quanta of acceleration is essentially worth more by starting from here - we basically don't need as much in total. However, net acceleration will be reduced by losses to gravity - our rate of acceleration is thrust minus gravity.

When we 'launch' from the other side, assuming we now don't need to fly to orbit and avoid an unhealthy terrain interface, we can also burn directly towards our intended escape/intercept target. Because gravity pulls us down, we accelerate at thrust plus gravity - so, we accelerate at 2x gravity faster than from the other start point. When we reach the centre of the gravity well, this situation is reversed - we're now accelerating at thrust minus gravity again. But, we're now travelling at a given velocity, and still net accelerating, which means that the time it took to get to the centre is more than the time it will take to reach surface level on the other side. In other words, the amount of acceleration we gained due to gravity falling from the 'surface' to the 'centre' will be more than the amount we lose climbing from the 'centre' to the 'surface' on the other side. This is what I mean by "gravity free acceleration", and it's in no way an insubstantial contribution to delta-V. When we get to the surface on the other side, we'll have an even higher velocity than at the centre, whereas the other craft launching from here will have 0 vertical velocity. So, from here to the intercept will take us less time than the other craft - and our gravity losses from here to target will be lower - even though the other craft didn't have as far to travel in the beginning. Although to the other craft 1 unit of acceleration is worth more than it is to our craft, we simply have much, much more of it.

Now putting the physical planet back in, the amount of dV it takes to move around the planet/moon does eat into the net gains from the assistance of gravity, but most of this energy is stored as angular velocity because it is mostly applied perpendicular to gravity (except for the little bit used to climb to the high atmosphere).

My point 3 was intended to restate the argument in a clear way and simplify the points over which you disagree in a way that can be measured. It wasn't making any point, but a statement of the variables in this debate.

I also specified it as applying to a vacuum, and I haven't referred anywhere to atmosphere - this is because this thread is (supposed to be) talking about the Mun or other bodies besides Kerbin (perhaps you are confused, having started so many threads, about which one you're currently in). This ties in with my point 2, which was that you haven't given a demonstration of a launch from MUN! This whole discussion is foundationless if you don't actually practically test the case you are studying. Make your comparative launches from the surface of the Mun and show us some relevant numbers, uncompromised by terminal velocity and drag losses etc.

Also, your 'dodgy displays' were discussed at length a few pages ago. Perhaps you should scan back thru.

Edit:

Saying that a vertical rocket has a higher TWR is stupid. You need to test the SAME rocket, with the SAME TWR, more or less like you did in your video. It is an advantage to have less distance to travel if the force of gravity is the same. This is not because one rocket has a higher TWR, it's because your destination is nearer!

Let's get down to simple scientific process here.

Hypothesis: if TWR is high enough, launching vertically is more efficient than launching on a prograde gravity turn for a given intercept at the same altitude.

Test: Fly both flight profiles. For accurate testing of the effect of TWR alone, a body without an atmosphere is ideal. Also, throttle should be set to maximum since reduced throttle settings effectively decrease TWR which could compromise results.

Apparatus: One rocket with high TWR. For control/comparison, another rocket with a normalised TWR could also be used for both profiles.

Gather data:

Analysis:

Conclusion:

All yours...

Edited December 16, 2014 by The_Rocketeer

[arkie87]

Arkie,

You seem to not be understanding how the advantage of a gravity slingshot arises.

Imagine we could remove the physical presence of a planet/moon, but keep the gravity well and launch/start locations the same.

When we 'launch' from the side closer to our escape/intercept point, we have relatively less distance to go, so each quanta of acceleration is essentially worth more by starting from here - we basically don't need as much in total. However, net acceleration will be reduced by losses to gravity - our rate of acceleration is thrust minus gravity.

When we 'launch' from the other side, assuming we now don't need to fly to orbit and avoid an unhealthy terrain interface, we can also burn directly towards our intended escape/intercept target. Because gravity pulls us down, we accelerate at thrust plus gravity - so, we accelerate at 2x gravity faster than from the other start point. When we reach the centre of the gravity well, this situation is reversed - we're now accelerating at thrust minus gravity again. But, we're now travelling at a given velocity, and still net accelerating, which means that the time it took to get to the centre is more than the time it will take to reach surface level on the other side. In other words, the amount of acceleration we gained due to gravity falling from the 'surface' to the 'centre' will be more than the amount we lose climbing from the 'centre' to the 'surface' on the other side. This is what I mean by "gravity free acceleration", and it's in no way an insubstantial contribution to delta-V. When we get to the surface on the other side, we'll have an even higher velocity than at the centre, whereas the other craft launching from here will have 0 vertical velocity. So, from here to the intercept will take us less time than the other craft - and our gravity losses from here to target will be lower - even though the other craft didn't have as far to travel in the beginning. Although to the other craft 1 unit of acceleration is worth more than it is to our craft, we simply have much, much more of it.

Now putting the physical planet back in, the amount of dV it takes to move around the planet/moon does eat into the net gains from the assistance of gravity, but most of this energy is stored as angular velocity because it is mostly applied perpendicular to gravity (except for the little bit used to climb to the high atmosphere).

My point 3 was intended to restate the argument in a clear way and simplify the points over which you disagree in a way that can be measured. It wasn't making any point, but a statement of the variables in this debate.

You have made these logical arguments before, but I'd prefer mathematical ones or experimental trials. Since you propose an experiment later on, let's focus on those so as not to derail this conversation, ok?

I also specified it as applying to a vacuum, and I haven't referred anywhere to atmosphere - this is because this thread is (supposed to be) talking about the Mun or other bodies besides Kerbin (perhaps you are confused, having started so many threads, about which one you're currently in). This ties in with my point 2, which was that you haven't given a demonstration of a launch from MUN! This whole discussion is foundationless if you don't actually practically test the case you are studying. Make your comparative launches from the surface of the Mun and show us some relevant numbers, uncompromised by terminal velocity and drag losses etc.

While you might not have mentioned anything regarding an atmosphere, 5thHorsemen and I were discussing the Kerbin problem, so i wasnt sure which problem you were discussing. So maybe i should have insisted he discuss it with me in the proper thread.

I think mathematical theory and experiment are both valid. I would agree experiment is preferred though, since theory might make certain simplifications or assumptions, but in no way is a mathematical theoretical discussion "foundationless".

Also, your 'dodgy displays' were discussed at length a few pages ago. Perhaps you should scan back thru.

I really dont have the patience to debate with someone who will tell me that i should look back up a few pages to see what you were talking about. I spent hours debating with GoSlash, who did that, and it took us a few days to finally come to an agreement. If you want to have a productive and constructive debate, please cite what you are referring to (like i am doing by citing you now). Otherwise, misunderstandings will compound.

Saying that a vertical rocket has a higher TWR is stupid. You need to test the SAME rocket, with the SAME TWR, more or less like you did in your video. It is an advantage to have less distance to travel if the force of gravity is the same. This is not because one rocket has a higher TWR, it's because your destination is nearer!

Please cite where i said this... because i agree with you, saying that would be stupid. I think you are misunderstanding me... I also wouldnt recommend saying something like that, as it could get the negative attention of the forum moderators. Try to keep it civil.

Let's get down to simple scientific process here.

Hypothesis: if TWR is high enough, launching vertically is more efficient than launching on a prograde gravity turn for a given intercept at the same altitude.

Test: Fly both flight profiles. For accurate testing of the effect of TWR alone, a body without an atmosphere is ideal. Also, throttle should be set to maximum since reduced throttle settings effectively decrease TWR which could compromise results.

Apparatus: One rocket with high TWR. For control/comparison, another rocket with a normalised TWR could also be used for both profiles.

Gather data:

Analysis:

Conclusion:

All yours...

Hypothesis is more like: if TWR is high, launching vertical might be comparable or better (i never said always) than an LXO-to-X burn, depending on the intended target. In this case, its SoI change and Kerbin-capture.

I would be happy to do this test. I can use hyper edit to get craft to proper location, i hope...

[GoSlash27]

Arkie,

Sorry, yeah I as referring to Kerbin. Your vertical launch would not have exceeded Vt, but your prograde launch would have.

On an airless body, there is no question that Prograde is preferable.

The math is greatly simplified, and the reason prograde is preferable is so simple, that you'll laugh when you hear it:

DV on an airless body is merely a*t. When accelerating vertically, your acceleration is reduced by 1.63m/sec each second you burn, whereas if you burn horizontally, it's not.

So if I were to accelerate at (say) 2G horizontally for

200 seconds, it would give me 652m/sec + 9 m/sec for the rotation. If you burn vertically at the same rate, you only get 356.

Now... you can reduce this disparity by increasing acceleration, but the difference between the two methods can only approach zero as acceleration approaches infinity. it can never exceed it.

But there's no good reason to add the mass of bigger engines, since the prograde burn will get optimal results with engines producing exactly local G acceleration at liftoff. There's no benefit to adding thrust in a prograde launch.

Adding thrust in the vertical mode will reduce (but not eliminate) the disparity between the two modes in theory, but in practice any gains you get from increasing thrust will be erased by the penalty of the big engines required to produce it.

Best,

-Slashy

Edited December 16, 2014 by GoSlash27

[The_Rocketeer]

I'm not making logical arguments or entering into the debate on one side or another at all. I'm simply stating how certain physical concepts apply to this scenario. If I'm coming across as argumentative that's not my intention.

On the other hand, you seem to be going to great lengths to take issue with various aspects of my commentary. When I explain something at length it's not because I want to convince you of anything, it's because you appear to be unaware of the factual basis behind a broader concept, e.g. gravity slingshot.

If you are literally satisfied that LKO is more efficient, as you suggest you are, why hasn't this conversation been closed? I'm now really struggling to understand why anybody is even still talking about this.

Since you belabour the point (which by the way actually stemmed from a post I already redacted) and since you apparently can't be bothered to scroll back a few pages in your own thread... HERE:

Your numbers from KER are misleading you. FAR requires a minimum of 3,500 m/sec to establish orbit according to the wiki.

Yeah, something weird is happening in KER, since during launch, deltaV values for upper stage were changing when they shouldnt be since the fuel wasnt being shared...

From KER, i think the numbers are as follows (KER reports two numbers. I assume the first is current stage and second is total, so i report total)

KER said 47.428 ton, but I summed parts manually in spreadsheet and came out to only 46.670 ton. Either way, 42.6 ton is incorrect.

Similarly, KER said 15.458 ton at burnout, but i calculated 14.67 ton. Don't know where you got 10.7 ton...

...

I think your number here is incorrect because you used incorrect masses. By my calculations, using ISP 320 (at sea level for mainsail), i get 3633 m/s deltaV in lifter stage and using ISP = 360 (vaccum for mainsail), i get 4087 m/s deltaV. Since i only have approx 7 m/s deltaV left for LKO-to-mun burn, lets be conservative and assume, correct deltaV spent is the total of 4087 m/s...

I think the values you used were incorrect...

...

Yes, it is a LV-T30 engine to maximize TWR

...

Your conclusions here are misinformed since you used incorrect numbers. See above.

...

I didnt use KER for terminal velocity--only FAR.

I admit KER's numbers seem fishy, when it comes to mass and deltaV though....

3 different answers. I got the figures from the video in the lower right hand corner where it said "total mass".

And you certainly wouldn't use sea level Isp figures for that. They don't apply on Kerbin above 5Km or so. vacuum is more accurate.

You seemed more than happy to use the numbers that add- on generated before, now all of a sudden they're "fishy"??

I no longer have any confidence in any numbers you're posting here. How do you know if your spreadsheet is right, or if FAR is right, or if KER is right?

*My* numbers from my test are 100% solid. No questions, no variations.

If you really want to *know* you should open this up for peer review.

Post your craft file and have somebody else who uses FAR confirm whether your rocket is *truly* more efficient than the norm or less.

I'm thinkin' it's a lot less and your numbers are gacked.

etc etc etc.

Remember now? Your numbers are wonky.

Edit: I'm not going to argue about this if you turn around and tell me I'm wrong. In fact, I don't think my efforts are being appreciated really at all, so perhaps I should just bow out and leave you to your circular reasoning.

Oh, and when I say that 'this statement is stupid' that's not infringing forum rules. It's only if I say 'you're stupid' that I could possibly get in trouble. And since that's couched in 's, that also doesn't count. I certainly don't need cautioning on how say things by you.

Edited December 16, 2014 by The_Rocketeer

[numerobis]

Finally, when you build up horizontale speed, you are granted a centrifuge force (blue arrow, pic 3). I won't express it with gravitational parm, but as it grows like a square, if we call vh the horizontale velocity and v0 the orbital velocity at current altitude (eg 2300m/s for kerbin), we can write it :

g * (vh^2/v0^2)

This isn't quite right. First off, it should be g - vh^2/v0^2: minus, not times (otherwise it says gravity is zero when you aren't moving and it's g when you're at orbital speed).

Secondly, when I take that as my apparent gravity and thrust horizontal plus just up enough to beat that apparent gravity, I end up rising -- which I shouldn't. So something is weird. Either a bug in the math here, or a bug in my code (likely).

[arkie87]

Secondly, when I take that as my apparent gravity and thrust horizontal plus just up enough to beat that apparent gravity, I end up rising -- which I shouldn't. So something is weird. Either a bug in the math here, or a bug in my code (likely).

Why? If you thrust horizontally and up enough to cancel gravity at the current velocity, your velocity increases, and now, your apparent gravity is even less, so you should rise...

[arkie87]

I'm not making logical arguments or entering into the debate on one side or another at all. I'm simply stating how certain physical concepts apply to this scenario. If I'm coming across as argumentative that's not my intention.

This is perfect. That is exactly what i want and expect

On the other hand, you seem to be going to great lengths to take issue with various aspects of my commentary. When I explain something at length it's not because I want to convince you of anything, it's because you appear to be unaware of the factual basis behind a broader concept, e.g. gravity slingshot.

By all means, explain all concepts to me. Sometimes I am already aware of them, sometimes i'm not...

If you are literally satisfied that LKO is more efficient, as you suggest you are, why hasn't this conversation been closed? I'm now really struggling to understand why anybody is even still talking about this.

The reason the thread isnt closed is because we are still debating it... For the Mun case, my math suggests that with high TWR, direct vertical ascent is more efficient since you have to climb first. If you want to refute it, either try both cases yourself, or perform your own mathematical calculations. Logical arguments do not trump math or experiment.

Since you belabour the point (which by the way actually stemmed from a post I already redacted) and since you apparently can't be bothered to scroll back a few pages in your own thread... HERE:

etc etc etc.

Remember now? Your numbers are wonky.

Those were GoSlashly's opinions. I am skeptical of deltaV calculations in KER (though it might just be the result of ISP varying with altitude or something), but you cannot argue with the similar fuel consumptions, which is independent of anything KER outputted. My fuel consumption results indicate that the results are similar for Kerbin case (and by the way, what we are discussing now is the Kerbin case, which is another reason i was confused which case--mun or kerbin-- you wanted to discuss).

Edit: I'm not going to argue about this if you turn around and tell me I'm wrong. In fact, I don't think my efforts are being appreciated really at all, so perhaps I should just bow out and leave you to your circular reasoning.

I dont see where I am telling you I you are wrong? I am debating; I am presenting counter arguments. If you want me to accept your opinion as fact without being allowed to present counter arguments until I either understand the flaw in my reasoning, or convince you of mine, then please stop debating with me. That is not scientific.

Oh, and when I say that 'this statement is stupid' that's not infringing forum rules. It's only if I say 'you're stupid' that I could possibly get in trouble. And since that's couched in 's, that also doesn't count. I certainly don't need cautioning on how say things by you.

Calling my reasoning "circular" and saying "remember now? Your numbers are wonky" is antagonizing, belligerent, condescending, and rude. If you dont mean to be rude, the i recommend adding a kerbal-smiley face...

[Superfluous J]

However, 5thHorseman will argue that even if you are using SRB's with FAR installed, it pays to throttle them back (before launch) and do a gravity turn and forgo the extra thrust, which is available at 0 mass increase; if i had a mainsail, no one would tell me i should reduce the thrust to be equivalent to a skipper, since i have already paid for the weight of a mainsail!

Actually I won't argue that. That was an early test and right after posting I instantly realized that 5 SRBs at 60% thrust wastes dV. I redesigned the rocket to use 3 SRBs at 100%, drop them, and then use 2 others at 100%. Almost the same TWR with extra dV and a longer burn time.

(And I'm still curious to see your sub-$10k craft)

Edited December 16, 2014 by 5thHorseman

[LethalDose]

Okay, it sounds like we're getting back onto discussions about launching from the Mun. I've generated the data below to contribute to this discussion. Three assent methods from the Mun were used: Vertical assent (using an iterative simulation), 10 km parking orbit before escape (using orbital velocity calculations) and direct assent from the surface (also using orbital velocity calculations). The spreadsheet is available here.

VERTICAL ASCENT

I've created a spreadsheet that simulates a direct vertical launch from a planetary body at a given altitude, fuel mass, dry mass, thrust, and ISP. The simulation took into account changes in acceleration during the launch due to spent fuel and increasing altitude. The initial orbital velocity of the vessel due to the body's rotation is not accounted for in these simulations. The following constants were used for all simulations:

• dt (time increment): 0.1 s
  
• Starting altitude: Sea Level + 0 m
  
• Planetary radius: 200 km (Mun)
  
• mu: 6.51 x 10¹⁰ m³/s² (Mun)
  
• dry mass: 1 t
  
• fuel mass: 1 t
  
• ISP: 390 s
  
• g₀: 9.82 m/s²
  
• SoI boundary: 2 429 559.1m (Mun; from planetary center)
  

[values taken for the KSP wiki, where relevant]

I changed burn time (in steps of dt) until I found the shortest burn that would cross the SOI border with the lowest velocity (in other words, just barely escaping). dV spent was calaculated when the burn ended based on change in mass using the rocket equation. I repeated the process for 5 separate TWRs, with the following results:

[table=width: 200]

[tr]

[td]

Thrust (kN)

[/td]

[td]

Initial TWR

[/td]

[td]

dV spent (m/s)

[/td]

[/tr]

[tr]

[td]

5

[/td]

[td]

1.53

[/td]

[td]

1125.2

[/td]

[/tr]

[tr]

[td]

10

[/td]

[td]

3.07

[/td]

[td]

910.7

[/td]

[/tr]

[tr]

[td]

20

[/td]

[td]

6.14

[/td]

[td]

835.9

[/td]

[/tr]

[tr]

[td]

50

[/td]

[td]

15.35

[/td]

[td]

800.7

[/td]

[/tr]

[tr]

[td]

100

[/td]

[td]

30.70

[/td]

[td]

785.3

[/td]

[/tr]

[/table]

[The "initial TWR" uses local weight at sea level, with acceleration due to gravity of ~ 1.63 m/s²]

As suspected, as the TWR goes up, the efficiency drops, but it starts to hit an asymptote after a TWR of ~ 6. The values for the higher TWRs are similar to what arkie presented in the OP. It's also interesting to note that the peak velocity appeared to be at or near escape velocity for each simulated burn listed above (I didn't check escape velocity at the altitude where peak velocity was reached). This really shouldn't be too surprising.

10 KM PARKING ORBIT

My estimated dV to go from sea level to a circular 10 km parking orbit was 575.4 m/s* using a eastward burn from the equator. An additional 198.7 m/s to transfer from the parking orbit to the SOI boundary. The total dV for these maneuvers was 774.1 m/s. The Mun's orbital surface velocity (~ 9 m/s) was included in these calculates.

*This value seems quite low to me, especially since the circularization burn was ~ 7 m/s. A quick glance at dV maps put this value between 580 and 640, based on the altitude of the parking orbit. Assuming 640 m/s is correct for a 14 km parking orbit, the entire escape would take approximately 840 m/s. Even with this upward bump, this is still ~ 12.5% lower than arkie presented in the OP.

It should also be noted the overall dV cost of the parking orbit escape increases as the parking orbit altitude increases.

DIRECT ELLIPTIC ASCENT

The ideal direct assent was calculated assuming an instant, prograde acceleration from sea level that would create an eliptic orbit that would reach the SOI. The dV calculated for such an assent was calculated to be 766.7 m/s, and, IMO, should be considered as an absolute lower bound for a Munar escape from sea level.

CONCLUSIONS

While the simulation isn't perfect, I think the time resolution should be more than sufficient for these purposes. The alternatives to these simulations are either demonstrations on YouTube or busting out differential equations. Anyone that wants to present such evidence is welcome to it. I also think it's perfectly reasonable to compare the results of the different methods side-by-side. Anyone who wants to demonstrate anything above is incorrect, is welcome to it.

But any dissent that doesn't provide contrary evidence is a complete waste of time.

These results show that the vertical ascent reaches towards an asymptote near surface escape velocity as TWR increases. If the dV maps are to be believed, then a vertical assent from sea level will probably beat a 10 km parking orbit if you can manage a TWR of 6 or greater off of sea level, given the assumptions above. And frankly, this seems perferctly reasonable. Should some greater velocity at SOI be desired, the results may change. I leave it to others to bicker about what significance that might have, or, preferably, present reasonable evidence to the contrary.

The simulation results should, if anything, be over-estimates of required dV, for two reasons. First, this method required the vessel to still have some vertical velocity when leaving the SoI. This was not the case for the calculated escape transfer dV costs. Second, the simulation applies force to the vessel prior to the mass of the fuel being removed as exhaust, not simultaneously. Hence the accelerations and resultant velocities are slightly lower than would be observed in practice.

Frankly, after all this math, it really looks like the vertical assent is pretty reasonable option in this case, since it provides an escape with dV costs comparable to reasonable parking orbits. Basically, use whichever you prefer.

Edited December 16, 2014 by LethalDose

[arkie87]

Arkie,

Sorry, yeah I as referring to Kerbin. Your vertical launch would not have exceeded Vt, but your prograde launch would have.

Let's talk about airless body, since that is what this thread is supposed to be about.

As a side point, "prograde" is a bit vague since launching vertically is "prograde" as well. I prefer to say "horizontal" vs. "vertical" rather than "vertical" vs. "prograde".

On an airless body, there is no question that Prograde is preferable.

The math is greatly simplified, and the reason prograde is preferable is so simple, that you'll laugh when you hear it:

DV on an airless body is merely a*t. When accelerating vertically, your acceleration is reduced by 1.63m/sec each second you burn, whereas if you burn horizontally, it's not.

So if I were to accelerate at (say) 2G horizontally for

200 seconds, it would give me 652m/sec + 9 m/sec for the rotation. If you burn vertically at the same rate, you only get 356.

Now... you can reduce this disparity by increasing acceleration, but the difference between the two methods can only approach zero as acceleration approaches infinity. it can never exceed it.

But there's no good reason to add the mass of bigger engines, since the prograde burn will get optimal results with engines producing exactly local G acceleration at liftoff. There's no benefit to adding thrust in a prograde launch.

Adding thrust in the vertical mode will reduce (but not eliminate) the disparity between the two modes in theory, but in practice any gains you get from increasing thrust will be erased by the penalty of the big engines required to produce it.

Best,

-Slashy

For such a low TWR, of course, burning vertically will not pay. The discussion must involve high TWR (even if you would have had more deltaV had you chosen a lighter engine and lower TWR).

Even for an infinite TWR launch (i.e. impulse burns), as you pointed out, horizontal is probably better or, at worst, equal to vertical launch. I have admitted this. However, on the Mun, you cannot perform a horizontal burn unless you are a daredevil... That is the basis for this thread!

For the Mun case, unless you land on a hill, you probably want to burn vertical first to avoid crashing. I've seen that 10 km is a safe altitude, so if you first burn vertically to 10 km, and then turn over and burn horizontally, you will end up using more fuel, as i have calculated in OP.

Now, granted, shooting straight up to 10km is inefficient; it is more efficient with the horizontal approach to burn horizontally the whole time and raise apoapsis via centripetal acceleration. In addition, 10km is a bit high, since you can burn horizontally with a slight inclination and visually inspect to avoid hills. And the closer you are to the ground the entire time, the less fuel you will use, but also the more dangerous.

A better result might be to know the balance point i.e. if you first climb vertical, at what altitude will it use less fuel to continue going vertically than to turn horizontal and burn to LMO. I am working on this now....

Edited December 16, 2014 by arkie87