Transfering to Mun: First to LKO or Directly to Mun?

[arkie87]

I've found that one LV-T45 can turn just fine in stock. And the basic non-controlling fins hold a (real) gravity turn in FAR just fine if you do it just right. It may take a few tries but once you know how the rocket climbs it's pretty easy.

That's what i thought. Can you elaborate on "doing it right" and "know how the rocket climbs. I dont see how you can use that information to inform your flying?

I don't understand why you're allowing the largest SRB but not the simplest controlling fins, but if I can use the non-controlling ones sure, I can do that.

Because controlling fins arent unlocked yet in my career mode save but big SRB's are, as mentioned in original post....

[Superfluous J]

That's what i thought. Can you elaborate on "doing it right" and "know how the rocket climbs. I dont see how you can use that information to inform your flying?

Not really. Every rocket controls differently. Unless you go straight up of course But generally speaking you find the point where you tap it to the right and then barely control it anymore. Usually you're going 50-100m/s when that happens, assuming you don't have a crazy high TWR.

Because controlling fins arent unlocked yet in my career mode save but big SRB's are, as mentioned in original post....

That's fair, though I'd suggest you could have made better decisions in your unlocking

[arkie87]

Not really. Every rocket controls differently. Unless you go straight up of course But generally speaking you find the point where you tap it to the right and then barely control it anymore. Usually you're going 50-100m/s when that happens, assuming you don't have a crazy high TWR.

That's fair, though I'd suggest you could have made better decisions in your unlocking

Well, i dont think i had the biggest SRB's unlocked...

I usually gun for solar panels and science first, so aerodynamics is second

[GoSlash27]

Sorry I took this long. I was doing other things...

My DV figures from my test using the Pathfinder:

1) Prograde gravity turn with direct injection: 5230 m/sec

2) Prograde orbit followed by injection: 5,260 m/sec (+30 m/sec)

3) Vertical direct injection: 5,570 m/sec (+340 m/sec)

Now to see the video...

Best,

-Slashy

[GoSlash27]

Yeah, I was afraid I was gonna see that.

The test using the gravity turn is flawed. You're accelerating far too rapidly, which grossly inflates the drag losses. This makes the vertical ascent appear more competitive than it otherwise would be because it spends less time accruing ridiculously wasteful drag losses..

You should try turning down the heat to where you don't have a vertical meteor and compare them that way.

IAC, the original question has been officially and definitively answered: The most efficient profile to the Mun is a prograde gravity turn direct injection.

Best,

-Slashy

Edited December 12, 2014 by GoSlash27

[The_Rocketeer]

Now put that in your pipe and puff it....

[arkie87]

Sorry I took this long. I was doing other things...

My DV figures from my test using the Pathfinder:

What is pathfinder? I'd like to see how it works And what assumptions it makes for a "gravity turn" i.e. TWR and angle profile vs. altitude/velocity (so i can put that profile into my matlab ascent code and play around)

1) Prograde gravity turn with direct injection: 5230 m/sec

2) Prograde orbit followed by injection: 5,260 m/sec (+30 m/sec)

3) Vertical direct injection: 5,570 m/sec (+340 m/sec)

Now to see the video...

Best,

-Slashy

That's not bad. Only 10% more fuel for a much easier maneuver that you cant (read: is much harder to) screw up.

Yeah, I was afraid I was gonna see that.

The test using the gravity turn is flawed. You're accelerating far too rapidly, which grossly inflates the drag losses. This makes the vertical ascent appear more competitive than it otherwise would be because it spends less time accruing ridiculously wasteful drag losses..

You should try turning down the heat to where you don't have a vertical meteor and compare them that way.

Optimal vertical velocity during ascent is terminal velocity, no? With FAR, terminal velocity is nearly unreachable unless you have a huge TWR (much higher than i achieve at liftoff), no?

Accordingly, I assume what you meant it that i started turning sideways too early, and therefore encounter more horizontal drag than needed at too low an altitude. But you cannot possibly mean that it is more efficient if you already have a high TWR vehicle to not use full throttle during ascent (assuming you are below terminal velocity)? Regardless, will try launching with reduced thrust and see what happens.

That said, what is an approximate "optimal" gravity turn i.e. should i begin turning nearly instantly, or wait until i get to X altitude? What is X altitude? What if i just burn up to 30 or 40 km, then turn 90 degrees, and launch nearly sideways?

IAC, the original question has been officially and definitively answered: The most efficient profile to the Mun is a prograde gravity turn direct injection.

Best,

-Slashy

That wasn't the original question (though it was the one i posed to you in PM-- see the OP), though i initially mistakenly thought that a vertical ascent could be more efficient under optimal conditions.

Speaking of which, how do i change it to say "answered"?

[arkie87]

Optimal vertical velocity during ascent is terminal velocity, no? With FAR, terminal velocity is nearly unreachable unless you have a huge TWR (much higher than i achieve at liftoff), no?

Accordingly, I assume what you meant it that i started turning sideways too early, and therefore encounter more horizontal drag than needed at too low an altitude. But you cannot possibly mean that it is more efficient if you already have a high TWR vehicle to not use full throttle during ascent (assuming you are below terminal velocity)? Regardless, will try launching with reduced thrust and see what happens.

That said, what is an approximate "optimal" gravity turn i.e. should i begin turning nearly instantly, or wait until i get to X altitude? What is X altitude? What if i just burn up to 30 or 40 km, then turn 90 degrees, and launch nearly sideways?

I tried ascending with reduced thrust such that I didn't get re-entry effects during liftoff and everything i tried was much worse than what i had already done...so please describe to me in detail what to do. Or, make a video of you doing it better with the same craft

[panzer1b]

I prefer LKO 1st, then whatever i wanna do after. Its USUALLY most efficient provided i do the burn right, and well most of the time i fly SSTOs, most of which while CAPABLE of gunning it straight up, wasting alot of fuel (most SSTOs have 1 nuke and 1-3 rapiers, bad TWR unless i kick in rapiers in rocket mode), amke more sense to do it normally, get LKO, and then do the space stuff. 2nd, i prefer to get a nice orbit and then use maneuver nodes to optimize the burn to whatever or wherever i wanna go. Makes more sense as well, as then i can timewarp nicely and it doesnt exactly take much longer then a straight up blast to mun.

[Superfluous J]

But you cannot possibly mean that it is more efficient if you already have a high TWR vehicle to not use full throttle during ascent (assuming you are below terminal velocity)?

Of course not.

It's also more efficient to fire your RCS jets if you brought 100x the RCS fuel than you'll ever need in a regular mission, but it's even better to redesign your rocket to be more efficient.

[Empiro]

Sorry I took this long. I was doing other things...

My DV figures from my test using the Pathfinder:

1) Prograde gravity turn with direct injection: 5230 m/sec

2) Prograde orbit followed by injection: 5,260 m/sec (+30 m/sec)

3) Vertical direct injection: 5,570 m/sec (+340 m/sec)

Now to see the video...

Best,

-Slashy

I'm not too surprised to see this. One way to look at gravity turn -> injection is that at some point during your burn, your PE goes from under ground to above the surface, and at that point you're "in orbit", even though you're in the atmosphere at ~30-40km. You're maximizing the Oberth effect as you continue to burn into a transfer to the Mun. The atmosphere is so thin at that point that the drag losses are pretty minimal.

[GoSlash27]

What is pathfinder? I'd like to see how it works And what assumptions it makes for a "gravity turn" i.e. TWR and angle profile vs. altitude/velocity (so i can put that profile into my matlab ascent code and play around)

My profile is outlined in detail here. It's not for FAR, tho'.

That's not bad. Only 10% more fuel for a much easier maneuver that you cant (read: is much harder to) screw up. Depends on your definition of "bad". That probe normally escapes Kerbin's SOI entirely and completes both normalization *and* inclination burns on less fuel than the vertical munar intercept took. I personally would never waste 340 m/sec if I didn't have to.

Optimal vertical velocity during ascent is terminal velocity, no? With FAR, terminal velocity is nearly unreachable unless you have a huge TWR (much higher than i achieve at liftoff), no?

I don't know enough about FAR aerodynamics to answer this question, but "optimal" acceleration doesn't light you up like a Christmas tree on the way up, regardless of pressure gradients.

Accordingly, I assume what you meant it that i started turning sideways too early, and therefore encounter more horizontal drag than needed at too low an altitude. But you cannot possibly mean that it is more efficient if you already have a high TWR vehicle to not use full throttle during ascent (assuming you are below terminal velocity)? Regardless, will try launching with reduced thrust and see what happens.

That said, what is an approximate "optimal" gravity turn i.e. should i begin turning nearly instantly, or wait until i get to X altitude? What is X altitude? What if i just burn up to 30 or 40 km, then turn 90 degrees, and launch nearly sideways?

Again, I don't know enough about FAR profiles to answer to this. Your issue wasn't with when you made the turn, but rather that you're generating so much drag due to the acceleration.

If you're bleeding DV like a stuck pig due to drag losses, the vertical ascent is going to seem less wasteful than it actually is in comparison.

That wasn't the original question (though it was the one i posed to you in PM-- see the OP), though i initially mistakenly thought that a vertical ascent could be more efficient under optimal conditions.

Sure it was. You used the word "efficient" several times in the OP. Your other criteria are your personal preferences, so there are no wrong answers there. Those are subjective opinion rather than fact. If you wanna go vertical because you find it easier, that's your business.

For the rest of us, most of us will probably prefer one of the other 2 methods because maintaining control during a gravity turn isn't a problem for us.

Speaking of which, how do i change it to say "answered"?

If you go into advanced edit on the OP, it will give you the option to fix that.

Best,

-Slashy

[Laie]

Coming late to the party... has a munar transfer orbit already been considered? That is, launching to a very high apoapsis that about touches on the Mun's SOI?

You'd still do a gravity burn to get there, but it would hardly be a turn. No circularization, but ofc lifting the periapsis out of the atmosphere. Fine-tuning the rendezvous encounter with miniscule delta-v to adjust the orbital period.

[arkie87]

Of course not.

It's also more efficient to fire your RCS jets if you brought 100x the RCS fuel than you'll ever need in a regular mission, but it's even better to redesign your rocket to be more efficient.

Granted. But that's why i worded what i said carefully

[arkie87]

My profile is outlined in detail here. It's not for FAR, tho'.

That's not bad. Only 10% more fuel for a much easier maneuver that you cant (read: is much harder to) screw up. Depends on your definition of "bad". That probe normally escapes Kerbin's SOI entirely and completes both normalization *and* inclination burns on less fuel than the vertical munar intercept took. I personally would never waste 340 m/sec if I didn't have to.

Sorry. I guess i missed that. Looks fine, but for FAR it has to be gradual (assuming that's what you intended).

I don't know enough about FAR aerodynamics to answer this question, but "optimal" acceleration doesn't light you up like a Christmas tree on the way up, regardless of pressure gradients.

As far as i know, this just isnt true. Optimal velocity is always terminal velocity regardless of aerodynamics model used (assuming a 1/2 rho V^2 C_d relationship for drag), even if terminal velocity causes you to light up like a Christmas tree. This can be derived as follows:

Fuel Usage is proportional to Thrust*time = Thrust*Distance/Velocity

Thrust is related to the other forces by conservation of momentum:

sum(F) = m*a = Thrust - Drag - Weight

If we assume object is already at optimal velocity, then a = 0, so:

Eq. (3) Thrust = Drag + Weight

So fuel usage can be expressed as:

Fuel Usage ~= (Drag + Weight)*Distance/Velocity = (1/2 rho V^2 C_d + mg)*Distance / V = (1/2 rho V C_d + m*g/V)*Distance

If we want to find the optimal velocity, we differentiate Fuel Usage w.r.t. V, and set it equal to zero:

1/Distance * d(Fuel Usage)/dV = 1/2 rho C_d - m*g/V^2 = 0

Re-arranging, we find the relationship:

1/2 rho V^2 C_d = m*g

The LHS = Drag Force

The RHS = Weight

So at optimal velocity, drag equals weight, which occurs at terminal velocity.

(If you already knew this, then i apologize. But perhaps it helped someone else).

Again, I don't know enough about FAR profiles to answer to this. Your issue wasn't with when you made the turn, but rather that you're generating so much drag due to the acceleration.

If you're bleeding DV like a stuck pig due to drag losses, the vertical ascent is going to seem less wasteful than it actually is in comparison.

Again, this just isnt true. In stock aerodynamics, terminal velocity is ~110 m/s at sea level, and at about 10 km it quickly increases too infinity faster than you can accelerate (if i am not mistaken). In FAR, terminal velocity starts off much higher (>600 m/s)--due to the more realistic aerodynamics-- such that you can never reach it unless you have a huge TWR.

I suspect a lot of these discrepencies in arguments occur because of stock vs. FAR aerodynamics. In stock, you have to limit your thrust on takeoff often (indeed, i used to do this when i was still using stock aerodynamics) very often, since it was easy to hit terminal velocity. In FAR, it's rarely possible. This clearly effects optimal vehicle TWR. In stock it might be lower-- like 1.4-- since you will be limited by terminal velocity. In far, i imagine it would be higher, since otherwise, it would be like taking off in stock below terminal velocity...

Sure it was. You used the word "efficient" several times in the OP. Your other criteria are your personal preferences, so there are no wrong answers there. Those are subjective opinion rather than fact. If you wanna go vertical because you find it easier, that's your business.

I admitted that word was mistaken. I thought i remember hearing someone mention it's more efficient, but i was clearly wrong, and already admitted that. The remaining purpose of the thread is not to debate which is more efficient, but rather, on the other advantages and disadvantages of both methods.

For the rest of us, most of us will probably prefer one of the other 2 methods because maintaining control during a gravity turn isn't a problem for us.

Once again, i will just say if you are using stock aerodynamics, then this thread doesn't really apply to you, as maintaining control during turn is easier and doesnt result in spin outs.

If you go into advanced edit on the OP, it will give you the option to fix that.

Thanks!

[DuoDex]

Personally I go with the circularization burn, then a Mun Transfer. But it's really your choice.

[arkie87]

So i found out where I can obtain terminal velocity. FAR flight display includes it. I have posted a video of how terminal velocity varies with altitude. There ain't no way you are reaching terminal velocity with FAR

[Kesa]

I don't know what the op meant by "directly to the Mun", but at least for those who assume it's a Vertical ascent vs LKO debate, here is my answer (btw it took me a long time to figure it out) :

LKO :

Basically, all the energy you have from your horizontal velocity at periapsis is converted into potential energy when you are at apoapsis (and very little amount of kinetic energy for your velocity at apoapsis, which should be less than 300m/s, so less than 1/100 of your initial kinetic energy. Speaking about Dv, it means you have to be 15m/s faster at periapsis than for a vertical ascent).

That why you do not lose any dv/energy by circularizing first.

Vertical ascent :

At first glance, there are several advantages :

- your trajectory is easier.

- you go out of the atmo faster, therefore having less drag.

- you don't watse time/fuel to achieve an orbit

- It's quicker.

But they aren't that advantageous :

- since you have to eyeball the good timing to launch your rocket, trajectory might actually be harder than setting mmaneuver node. Since Mods/calculator of launch window assume you come from LKO (guees why), you can't use them. (you can still use them for interplanetary travel).

- to achieve LKO, you begin to turn when the atmo get thinner, so the difference is not that big.

- as stated before you don't lose anything achieving lko.

- Further more, gravity sucks by far more energy than drag. Imagine you got TWR = 2. by accelerating upward, you will have an acceleration of 10 m/s. But if you turn your rocket in order to stay at the same vertical speed, you are accelerating effectively at 17m/s (10*sqrt(3), even more when approaching orbital velocity). Even with TWR = 10 (~half the weight of your rocket is engines, and you hit the red bar of g force), I bet you'd waste 1/10th of your fuel.

- It's not actually quicker. First, you need to wait up to ~one day (7h12 hours precisely) for the good time to launch, while LKO period is up to 30 min. Then, you also can accelerate more than needed from LKO to be quicker to the mun.

Finally, the only advantage of a vertical ascent is that it does not need you to know how to orbit.

Edited December 14, 2014 by Kesa

[arkie87]

I don't know what the op meant by "directly to the Mun", but at least for those who assume it's a Vertical ascent vs LKO debate, here is my answer (btw it took me a long time to figure it out) :

LKO :

Basically, all the energy you have from your horizontal velocity at periapsis is converted into potential energy when you are at apoapsis (and very little amount of kinetic energy for your velocity at apoapsis, which should be less than 300m/s, so less than 1/100 of your initial kinetic energy).

That why you do not lose any dv/energy by circularizing first.

Vertical ascent :

At first glance, there are several advantages :

- your trajectory is easier.

- you go out of the atmo faster, therefore having less drag.

- you don't watse time/fuel to achieve an orbit

- It's quicker.

But they aren't that advantageous :

- since you have to eyeball the good timing to launch your rocket, trajectory might actually be harder than setting mmaneuver node. Since Mods/calculator of launch window assume you come from LKO (guees why), you can't use them. (you can still use them for interplanetary travel).

- to achieve LKO, you begin to turn when the atmo get thinner, so the difference is not that big.

- as stated before you don't lose anything achieving lko.

- Further more, gravity sucks by far more energy than drag. Imagine you got TWR = 2. by accelerating upward, you will have an acceleration of 10 m/s. But if you already have circularized, you're not fighting gravity anymore, accelerating effectively at 20 m/s. Even with TWR = 10 (~half the weight of your rocket is engines, and you hit the red bar of g force), I bet you'd waste 1/10th of your fuel.

- It's not actually quicker. First, you need to wait up to ~one day (7h12 hours days actually) for the good time to launch, while LKO period is up to 30 min. Then, you also can accelerate more than needed from LKO to be quicker to the mun.

Finally, the only advantage of a vertical ascent is that it does not need you to know how to orbit.

I think you over-simplified a bit. Other advantages of vertical include:

(1) Utilization of Oberth effect the entire time (vs. LKO, you waste the initial climbing part when you turn off pro-grade); nevertheless, even theoretically, vertical requires more deltaV than LKO to Mun for this particular case. For other planets/moons, this might not be the case.

(2) With FAR, does not require control surfaces or reaction wheels or expensive thrust-vectoring engines to steer the craft into a gravity turn. Thus, strapping loads of SRB's and going vertical might be cheaper (in terms of Kerbucks) than using a mainsail/skipper and using thrust vectoring to get into LKO first.

(3) Eyeballing is easy-- just aim 90 degrees in front of the Mun or Minmus; or burn at sunrise/sunset if you are going interplanetary at optimum launch window

(4) With FAR, it is possible if you are too aggressive with gravity turn, you craft will spin out of control, thus, ruining the mission.

(5) Optimum ascent path to minimize deltaV is very sensitive and its not necessarily so easy to hit every time going to LKO first vs. vertical ascent is easy to hit optimally every time. GoSlash27 predicted approximately 300 m/s wasted deltaV using vertical ascent method, but its possible to waste that going into LKO first, if you accidentally raise your apoapsis too high or do something else sub-optimally...

[Kesa]

I think you over-simplified a bit. Other advantages of vertical include:

(1) Utilization of Oberth effect the entire time (vs. LKO, you waste the initial climbing part when you turn off pro-grade); nevertheless, even theoretically, vertical requires more deltaV than LKO to Mun for this particular case. For other planets/moons, this might not be the case.

I disagre, when accelerating vertically, you go higher very much faster than horizontally on LKO, losing speed (and gaining altitude) actually reducing the oberth effect, especially for burns that typically takes 1 minute or more. So Lko makes a better use of the oberth effect.

What's more, I guess if TWR = 3, vertical ascent will waste 1/3 of Dv. Nevertheless, you need at least TWR>1, and the higher is the better. Vertical ascent needs heavy/inefficient engines in order to achieve high twr, counterbalancing cost and size required to have steering system on a vessel that can go from lko. You can go from lko to transfer orbit with Twr =0.5 or lower, with lighter or more efficient engines.

Eyeballing is easy when you know how (and when) to do it.

As I stated, the only advantage of vertical ascent is that you don't have to know how to go in LKO. What I forgot to mention is that going to LKO might be difficult the first times (and I trust you this is even worse with FAR. Consider though that winglets are light and that the rocket get less wobbly when on upper altitude).

[arkie87]

I disagre, when accelerating vertically, you go higher very much faster than horizontally on LKO, losing speed (and gaining altitude) actually reducing the oberth effect, especially for burns that typically takes 1 minute or more. So Lko makes a better use of the oberth effect.

I readily admit that which utilizes Oberth effect "better" is dependent on a lot of factors such as your ship, TWR, and planet you are launching from etc.... Nevertheless, in practice, vertical ascent seem to take <10% more deltaV compared to LKO-to-mun transfer, in my experience.

What's more, I guess if TWR = 3, vertical ascent will waste 1/3 of Dv. Nevertheless, you need at least TWR>1, and the higher is the better. Vertical ascent needs heavy/inefficient engines in order to achieve high twr, counterbalancing cost and size required to have steering system on a vessel that can go from lko. You can go from lko to transfer orbit with Twr =0.5 or lower, with lighter or more efficient engines.

Yeah, but in career mode, we dont care about efficiency-- we care about cost, and in current iteration, SRB's are dirt cheap, and will get you more deltaV/Kerbuck than a more efficient engine will.

So #2 stands.

Now if they increase price of SRB's or cost of fuel, then the optimum solution will obviously change

[Kesa]

I'm pretty sure the lost due to gravity is something like 1/TWR of total DV for vertical ascent. I was thinking it was the difference between the two methods, but I forgot that going to LKO also generate gravity loss.

With an autopilot and a typical rocket (starting with TWR=2), I think you lose between 1300 and 1400 m/s to go to LKO, wich is roughly 1/4 of the bare minimum of 5600 to go to an intercept with the mun.

So I think that with a TWR > 4 (it is lower than I expected), it might worth go vertical.

Also note that by going to orbit, you only endure grav lost during the ascent to LKO, while going vertical, you have grav loss all the way, so Interplanetary vertical ascend require even greater TWR to be competitive.

Also, I don't know how many dv you lose due to drag during ascend, and if there is significant differences between vertical ascent and going to LKO.

As I see in your math, supposing instant burns, there is no significant differences between DV required to have an intercept or a mun-like orbit. The main difference will be in gravity loss. I like to think that going to orbit (or even better directly to intercept like wanderfound described) is always more efficient : with high TWR rocket, you would have gravity loss lower than 1/TWR, even lower than 1/4, but the differences tends to disapear, up to be negligible regarding the unavoidable loss in the souposphere.

As for cost, the only way to have high twr at cheap cost is SRB. But you can also use srb for an orbital insertion. I have a probe under 5.000 kerbucks that uses 1 RT10 and I think I have a 1 kerbal lander that might cost under 30.000 (using srb also).

Further more, getting the max out of srb is harder that using LF engines with customizable amount of fuel.

Edited December 14, 2014 by Kesa

[Renegrade]

So I think that with a TWR > 4 (it is lower than I expected), it might worth go vertical.

My empirical testing sometime ago definitely supported this statement. My test craft is what I like to term a "Jeb joy ride" or "missile", built on top of a high-power KW SRB with a launchpad TWR of around 4 (I don't recall the peak TWR, but it would have been pretty durned high when the SRB was just about to burn out). It out-performed the orbital/conventional approach significantly.

I later re-did the testing with a lower TWR craft (I think 2.5-ish?) built out of stock parts, and it performed better using an orbital/conventional approach.

(Using FAR in all tests.. with stock air, the atmosphere would soak up most of the energy. Also the goal was interplanetary for the tests, not the Mun)

[Wanderfound]

As for cost, the only way to have high twr at cheap cost is SRB. But you can also use srb for an orbital insertion. I have a probe under 5.000 kerbucks that uses 1 RT10 and I think I have a 1 kerbal lander that might cost under 30.000 (using srb also).

It's going to depend a bit on your definition of "cheap", but it's easy enough to get TWR > 4 into a sub-√60,000 SSTO spaceplane, with a fuel bill (AKA cost after recovery) of under √2,000:

The Aerospikes aren't intended to be kept lit for the whole ascent, of course. Although they are on this slightly more expensive one:

Edited December 14, 2014 by Wanderfound

[Sereneti]

If you go to LKO first, you can use the fuel saving on the periapsis-burns...