Vertical Ascent vs. To LXO First

[arkie87]

arkie,

*DOH* I did it again! Glossed over a statement without providing the data. Sorry...

Optimized packages for a .8t payload from Munar surface to Kerbin aerocapture:

Take care,

-Slashy

I trust your results, since you said you optimized it in the game...

If i doubted your results, or asked you to prove it, then i expect you to provide data

But thanks for the data; data makes Kerbals happy

I assume by optimize, you mean, dumped all excess fuel? Out of curiosity, were you optimizing for cost or deltaV?

I see a lot of testing in my future...

Is this KSP or Portal?

But it's certainly not unlikely that such a situation could arise; a heavier and more expensive engine ends up making a smaller and cheaper overall vehicle. In fact, this happens fairly often, but I had never considered factoring gravity losses into that before.

Now you lost me again -- can you explain this logic?

Edited December 18, 2014 by arkie87

[numerobis]

The main thing I've learned from this thread is that I want to run some simulations of

(1) liftoff until my speed at SoI exit is 375 m/s or whatever.

( perfect horizontal liftoff from 0m assuming a spherical planet and a point spacecraft, making sure that we never get below 0m.

Then vary TWR and Isp, and list the results in terms of deltaV and fuel mass expended. But I have software to release, so this won't be for today.

[arkie87]

The main thing I've learned from this thread is that I want to run some simulations of

(1) liftoff until my speed at SoI exit is 375 m/s or whatever.

( perfect horizontal liftoff from 0m assuming a spherical planet and a point spacecraft, making sure that we never get below 0m.

Then vary TWR and Isp, and list the results in terms of deltaV and fuel mass expended. But I have software to release, so this won't be for today.

( is a lot easier to simulate. I already have a matlab code that does that I can vary TWR and ISP and show the optimum TWR for a given ISP...

[GoSlash27]

Now you lost me again -- can you explain this logic?

Well... The Isp thing, LV-N vs 48-7S comes to mind. At some point it becomes lighter and cheaper to go nuke.

Or other factors, like structural complexity, where the cost and mass of putting together a franken-booster eats up the weight and cost savings by going to a more suitable, though inferior and costly engine.

That sort of thing happens fairly often, but the idea of treating low t/w as a cost and efficiency penalty hadn't occurred to me.

Even adding multiples of the same engine could make for a lighter and cheaper overall vehicle if the penalty in mass and cost is offset by the reduction in fuel and tankage.

I'm going to have to test the cost/ benefit of t/w in terms of overall mass and cost and see if I can figure out a decent function for it. Perhaps there's a lower bound above 1G where it's most efficient. This would have to tie into engine mass and Isp somehow...

Best,

-Slashy

Edited December 18, 2014 by GoSlash27

[The_Rocketeer]

But that is clearly not the answer I am looking for, since that answer, based on orbital mechanics, doesnt account for real-world problems. There could easily be an example in the Kerbol System where one is inside a deep crater somewhere, where, given how much distance they have to go vertically, it will pay to just continue. Or there can be another place in the Kerbol system, where, due to reasons I am unfamiliar with, it would pay to go vertical. This is what i was asking for in the OP-- if others were aware of any or could calculate any.

I wouldn't assert it without hard evidence (I've learned this gets me nowhere with you ), but I think the answer is probably still a flat no.

I say this because we already have a few examples that show it. In stock LKO ascents, it's conventional and optimal to first vertically ascend to ~10km to avoid the thick atmosphere and then kick over to orbital prograde - this is basically the sort of profile you're describing as avoiding real world problems. For a Kerbin-sized body, a 10km ascent to avoid terrain would be ultra massive! Remember we're not talking about the altitude above datum (sea level for Kerbin) of the tallest mountain on a body, we're talking about the difference in height between the launch site and the nearest peak. Even with this huge ascent it still doesn't make vertical ascent more optimal.

So, what you need to know is the greatest difference in terrain altitude of any body in the Kerbolar system over a distance, which scales proportionate the the body's equatorial circumference and is small enough such that you would hit it if you flew on a prograde trajectory directly from the local terrain's low-point (launch site) to the high-point.

If this was what you wanted to investigate from the outset, I think you could probably have been a little clearer about it in the OP. I certainly had no such impression at all, and I've read that post about 10 times now. On the other hand, it does explain why you were so dismissive of general arguments, since the sort of scenario you're talking about is incredibly locally specific. I had thought of the deep pits near the Munar north pole as a possible location to test whether vertical ascent could ever be better, but apart from the fact that they probably aren't deep enough for that to work anyway, there's still no reason I can think of why you would ever want to launch into an escape trajectory perpendicular to the ecliptic plane! Then again, this would also hurt the horizontal launch more than vertical due to loss of sidereel velocity.

In other words, I don't say that it's impossible for vertical to ever be better than horizontal, but the conditions needed to create this will almost certainly be self-imposed and not a consequence of 'natural' features.

As ever I don't want to get drawn into another debate, but I hope this is food for thought. I find all of these discussions educational to a greater or lesser extent, but mostly I approach them as mental exercises and a though experiment rather than anything more practical, and my contributions often reflect that. Nonetheless, I'm glad this discussion has reached a (relatively) amicable end.

Edit:

In fact, on re-reading that I realised an even more obvious point: Kerbin's atmosphere is basically the same as a terrain obstacle, since it forces an increase in final orbit altitude that wouldn't be necessary without it. For a Kerbin SOI escape, it's been shown to be optimal to use an LKO escape rather than a vertical burn. Doesn't this really sum up (and conclude) the whole 'situational' debate? Allowing that Kerbin is a relatively large body, it's still a humungously large obstacle relative to the planetary surface and g force! And that's not even taking into consideration losses to drag, which are higher overall for LKO.

Edited December 18, 2014 by The_Rocketeer

[GoSlash27]

Rocketeer,

I'll take the liberty of assisting in this one

I have learned a few things as well, particularly, just how bad it is to raise apoapsis at all by going vertical at all if you plan to go horizontal to get into orbit. From now on, i will tap throttle to get off surfaces and then aim as horizontal as possible

-arkie

Best,

-Slashy

[The_Rocketeer]

Wait, what? I'm not sure what that quote is supposed to mean in reference to my post... who are you assisting (and how)?

[arkie87]

Well... The Isp thing, LV-N vs 48-7S comes to mind. At some point it becomes lighter and cheaper to go nuke.

Or other factors, like structural complexity, where the cost and mass of putting together a franken-booster eats up the weight and cost savings by going to a more suitable, though inferior and costly engine.

That sort of thing happens fairly often, but the idea of treating low t/w as a cost and efficiency penalty hadn't occurred to me.

Even adding multiples of the same engine could make for a lighter and cheaper overall vehicle if the penalty in mass and cost is offset by the reduction in fuel and tankage.

I'm going to have to test the cost/ benefit of t/w in terms of overall mass and cost and see if I can figure out a decent function for it. Perhaps there's a lower bound above 1G where it's most efficient. This would have to tie into engine mass and Isp somehow...

Best,

-Slashy

Ok, had you specified "heavier more expensive but higher ISP engine" i would have understood the logic

But it's certainly not unlikely that such a situation could arise; a heavier and more expensive engine ends up making a smaller and cheaper overall vehicle. In fact, this happens fairly often, but I had never considered factoring gravity losses into that before.

[arkie87]

I wouldn't assert it without hard evidence (I've learned this gets me nowhere with you ), but I think the answer is probably still a flat no.

I say this because we already have a few examples that show it. In stock LKO ascents, it's conventional and optimal to first vertically ascend to ~10km to avoid the thick atmosphere and then kick over to orbital prograde - this is basically the sort of profile you're describing as avoiding real world problems. For a Kerbin-sized body, a 10km ascent to avoid terrain would be ultra massive! Remember we're not talking about the altitude above datum (sea level for Kerbin) of the tallest mountain on a body, we're talking about the difference in height between the launch site and the nearest peak. Even with this huge ascent it still doesn't make vertical ascent more optimal.

So, what you need to know is the greatest difference in terrain altitude of any body in the Kerbolar system over a distance, which scales proportionate the the body's equatorial circumference and is small enough such that you would hit it if you flew on a prograde trajectory directly from the local terrain's low-point (launch site) to the high-point.

If this was what you wanted to investigate from the outset, I think you could probably have been a little clearer about it in the OP. I certainly had no such impression at all, and I've read that post about 10 times now. On the other hand, it does explain why you were so dismissive of general arguments, since the sort of scenario you're talking about is incredibly locally specific. I had thought of the deep pits near the Munar north pole as a possible location to test whether vertical ascent could ever be better, but apart from the fact that they probably aren't deep enough for that to work anyway, there's still no reason I can think of why you would ever want to launch into an escape trajectory perpendicular to the ecliptic plane! Then again, this would also hurt the horizontal launch more than vertical due to loss of sidereel velocity.

In other words, I don't say that it's impossible for vertical to ever be better than horizontal, but the conditions needed to create this will almost certainly be self-imposed and not a consequence of 'natural' features.

As ever I don't want to get drawn into another debate, but I hope this is food for thought. I find all of these discussions educational to a greater or lesser extent, but mostly I approach them as mental exercises and a though experiment rather than anything more practical, and my contributions often reflect that. Nonetheless, I'm glad this discussion has reached a (relatively) amicable end.

Edit:

In fact, on re-reading that I realised an even more obvious point: Kerbin's atmosphere is basically the same as a terrain obstacle, since it forces an increase in final orbit altitude that wouldn't be necessary without it. For a Kerbin SOI escape, it's been shown to be optimal to use an LKO escape rather than a vertical burn. Doesn't this really sum up (and conclude) the whole 'situational' debate? Allowing that Kerbin is a relatively large body, it's still a humungously large obstacle relative to the planetary surface and g force! And that's not even taking into consideration losses to drag, which are higher overall for LKO.

I can understand in specific cases, like the Mun example, it is not the most efficient, but the Mun case was just an example. Admittedly, in the OP, i thought i specifically asked if there are other known cases that anyone knows of where it might be more efficient, but i think i either deleted it or decided against it, since i thought it would be better to focus on this specific example. So i will take credit for that confusion My bad.

That said, you haven't addressed this case, where horizontal method doesnt work:

The only example that comes to mind is Gilly-- since it is so misshapen (surface roughness-to-diameter ratio is the largest of any body in the Kerbol System), that you can rarely launch east without also climbing. However, you could also throw a ball into orbit around it, so any deltaV savings would be minimal.

This is a much better answer than a categorical "no" because it illustrates how ugly the planet/moon would have to be for this effect to dominate, and since it is only likely to occur on smaller bodies, any deltaV savings would be minimal anyway. This is what i was looking to investigate-- if there are any other cases in the Kerbol system where this would be practical-- and in examining the Mun problem with you guys, I think it made the Gilly answer clear.

Once again, think of my threads are more scientific exploration, rather than me suggesting one approach is right. If someone tells me X wont work, when i have provided math or video to show it will, i expect at least a math or video in return

[arkie87]

Wait, what? I'm not sure what that quote is supposed to mean in reference to my post... who are you assisting (and how)?

I dont think he is assisting anyone. He is just clarifying some confusion.

The case i presented in the OP mathematically showed that an ideal vertical burn is more efficient than a 10 km hop followed by a horizontal burn. However, this thread has made it abundantly clear that a vertical 10 km hop is extremely inefficient if you plan to do a horizontal burn, thus, the math in the OP was a bit biased...

[The_Rocketeer]

I can understand in specific cases, like the Mun example, it is not the most efficient, but the Mun case was just an example. Admittedly, in the OP, i thought i specifically asked if there are other known cases that anyone knows of where it might be more efficient, but i think i either deleted it or decided against it, since i thought it would be better to focus on this specific example. So i will take credit for that confusion My bad.

That said, you haven't addressed this case, where horizontal method doesnt work:

Once again, think of my threads are more scientific exploration, rather than me suggesting one approach is right. If someone tells me X wont work, when i have provided math or video to show it will, i expect at least a math or video in return

With respect, I think you've misread or misunderstood my post. I'm giving an example of a trajectory that is analogous to avoiding terrain obstacles, such as in the case of the misshapen-ness of Gilly, where a vertical terrain-avoidance manoeuvre, whether initial or simultaneous to horizontal acceleration, would be necessary. This is equivalent to the sort of vertical manoeuvre used in stock KSP to avoid the extreme drag of the 'souposphere', in ascent profile if not in catastophic consequence.

You also haven't actually shown that the horizontal manoeuvre doesn't work on Gilly, you've simply suggested that it might not.

I accept that your thread is exploring the issue, but frankly it's best to begin by exploring what you already know rather than assuming that you need some new information or data. If you find you have an answer that seems unsubstantiated by your current knowledge, then it's time to break out the experiments.

It's also not for others to disprove your explorative theory, but for you to prove it. Scientific process doesn't begin with a hypothesis and then attempt to show why it can't be true - that is proving a negative and is a logical fallacy.

Edit:

Where I refer to ~10km vertical ascent, please note this is on Kerbin and not on Mun, and is because of the stock atmosphere's drag. Nonetheless, in stock and with Kerbin's gravity it is still more efficient to do this than just keep going straight up.

Edited December 18, 2014 by The_Rocketeer

[arkie87]

With respect, I think you've misread or misunderstood my post. I'm giving an example of a trajectory that is analogous to avoiding terrain obstacles, such as in the case of the misshapen-ness of Gilly, where a vertical terrain-avoidance manoeuvre, whether initial or simultaneous to horizontal acceleration, would be necessary. This is equivalent to the sort of vertical manoeuvre used in stock KSP to avoid the extreme drag of the 'souposphere', in ascent profile if not in catastophic consequence.

You also haven't actually shown that the horizontal manoeuvre doesn't work on Gilly, you've simply suggested that it might not.

I accept that your thread is exploring the issue, but frankly it's best to begin by exploring what you already know rather than assuming that you need some new information or data. If you find you have an answer that seems unsubstantiated by your current knowledge, then it's time to break out the experiments.

It's also not for others to disprove your explorative theory, but for you to prove it. Scientific process doesn't begin with a hypothesis and then attempt to show why it can't be true - that is proving a negative and is a logical fallacy.

For the case presented in the OP, i did prove it with math (and not experiment). For the case of Gilly it's just a hypothesis, but i really do not care to prove or disprove it. Do you doubt that it is the case anyway?

[The_Rocketeer]

For the case presented in the OP, i did prove it with math (and not experiment). For the case of Gilly it's just a hypothesis, but i really do not care to prove or disprove it. Do you doubt that it is the case anyway?

Well... this is what you said:

In the OP itself, i even ask if there are even any better scenarios that people are aware of where the two approaches might be similar.

I assume Rocketeer will categorically assert: "no, never". (don't mean to put words in your mouth if you wouldnt say this... so feel free to correct me)

But that is clearly not the answer I am looking for, since that answer, based on orbital mechanics, doesnt account for real-world problems. There could easily be an example in the Kerbol System where one is inside a deep crater somewhere, where, given how much distance they have to go vertically, it will pay to just continue. Or there can be another place in the Kerbol system, where, due to reasons I am unfamiliar with, it would pay to go vertical. This is what i was asking for in the OP-- if others were aware of any or could calculate any.

The only example that comes to mind is Gilly-- since it is so misshapen (surface roughness-to-diameter ratio is the largest of any body in the Kerbol System), that you can rarely launch east without also climbing. However, you could also throw a ball into orbit around it, so any deltaV savings would be minimal.

This is a much better answer than a categorical "no" because it illustrates how ugly the planet/moon would have to be for this effect to dominate, and since it is only likely to occur on smaller bodies, any deltaV savings would be minimal anyway. This is what i was looking to investigate-- if there are any other cases in the Kerbol system where this would be practical-- and in examining the Mun problem with you guys, I think it made the Gilly answer clear.

To which I replied:

I wouldn't assert it without hard evidence (I've learned this gets me nowhere with you ), but I think the answer is probably still a flat no.

I say this because we already have a few examples that show it. In stock LKO ascents, it's conventional and optimal to first vertically ascend to ~10km to avoid the thick atmosphere and then kick over to orbital prograde - this is basically the sort of profile you're describing as avoiding real world problems. For a Kerbin-sized body, a 10km ascent to avoid terrain would be ultra massive! Remember we're not talking about the altitude above datum (sea level for Kerbin) of the tallest mountain on a body, we're talking about the difference in height between the launch site and the nearest peak. Even with this huge ascent it still doesn't make vertical ascent more optimal.

etc

In other words, your "situation in which it's better to launch vertically" is vanishingly specific, and in my opinion is extremely improbable. I'm not attempting to prove that it so, but I am demonstrating the extent to which we can already understand it's necessary limits, by showing that even with a very large vertical detour for Kerbin's atmosphere, we still have a more efficient approach in a LKO > intercept/escape profile than a vertical ascent > intercept/escape profile for that body.

I hope you understand what I'm driving at - I'm surprised that reciprocal comprehension seems so difficult, but perhaps the fault is my own. In any case, I don't plan to say any more on the subject since I think I've put forward everything I have to contribute, and as I've said I don't wish to debate the detail. My wish is simply to provide some material for general consideration.

[arkie87]

Brief paper describing nondimensionalization and differential equation.

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Edited December 18, 2014 by arkie87

[arkie87]

Well... this is what you said:

To which I replied:

etc

In other words, your "situation in which it's better to launch vertically" is vanishingly specific, and in my opinion is extremely improbable. I'm not attempting to prove that it so, but I am demonstrating the extent to which we can already understand it's necessary limits, by showing that even with a very large vertical detour for Kerbin's atmosphere, we still have a more efficient approach in a LKO > intercept/escape profile than a vertical ascent > intercept/escape profile for that body.

I hope you understand what I'm driving at - I'm surprised that reciprocal comprehension seems so difficult, but perhaps the fault is my own. In any case, I don't plan to say any more on the subject since I think I've put forward everything I have to contribute, and as I've said I don't wish to debate the detail. My wish is simply to provide some material for general consideration.

I understand that my case is astonishingly specific. My point is simple:

Until we explore the math and/or do tests, it's not obvious how common or uncommon it is. If we do that math, and find out it's not important, then that is a result as well.

[GoSlash27]

Wait, what? I'm not sure what that quote is supposed to mean in reference to my post... who are you assisting (and how)?

Sorry, I was attempting to assist both of you, and failed

You were explaining that even with terrain in the way, it's a straightforward process to tip over immediately and accelerate into an orbital escape, and I cited his quote to me (which I would not have assumed you saw) that he had already decided this himself as a practical matter over the course of the thread.

I just thought that sharing that quote with you would clear up any confusion.

Sorry!

-Slashy

[arkie87]

Sorry, I was attempting to assist both of you, and failed

You were explaining that even with terrain in the way, it's a straightforward process to tip over immediately and accelerate into an orbital escape, and I cited his quote to me (which I would not have assumed you saw) that he had already decided this himself as a practical matter over the course of the thread.

I just thought that sharing that quote with you would clear up any confusion.

Sorry!

-Slashy

Well, one thing we can agree on, is that we are all dysfunctional at communicating with each other...

[GoSlash27]

I understand that my case is astonishingly specific. My point is simple:

Until we explore the math and/or do tests, it's not obvious how common or uncommon it is. If we do that math, and find out it's not important, then that is a result as well.

Well... we would have to find places where

1)the local terrain is sufficiently steep to preclude the possibility of going east (or whatever direction you intend to go)

2) also high enough that you're better off going vertical after having cleared the obstacle, and

3) is situated exactly in the spot where it gives you an escape to where you're going.

The combination of all 3 at the same time... I don't believe any such place exists other than down an equatorial mohole very close to Moho's orbital prograde/ retrograde or a canyon on Eeloo's equator.

There are sheer cliff terrain anomalies on the poles of bodies all over the system, but any DV you save from launching vertical there will be lost by subsequent correction burns.

Only places I can think of...

In short, I'm not thinking that this is something that can be answered by testing and math. This is more of an astrogeographical question.

Best,

-Slashy

Edited December 18, 2014 by GoSlash27

[arkie87]

Well... we would have to find places where

1)the local terrain is sufficiently steep to preclude the possibility of going east (or whatever direction you intend to go)

2) also high enough that you're better off going vertical after having cleared the obstacle, and

3) is situated exactly in the spot where it gives you an escape to where you're going.

The combination of all 3 at the same time... I don't believe any such place exists other than down an equatorial mohole very close to Moho's orbital prograde/ retrograde or a canyon on Eeloo's equator.

There are sheer cliff terrain anomalies on the poles of bodies all over the system, but any DV you save from launching vertical there will be lost by subsequent correction burns.

Only places I can think of...

In short, I'm not thinking that this is something that can be answered by testing and math. This is more of an astrogeographical question.

Best,

-Slashy

Exactly. For it to be practical in the Kerbol system, it is an astrogeographical question (is that a word? astro=star, geo=earth... lol); however, it is interesting to find out under what conditions it could exist in theory...

And what about Gilly? Is it tidally-locked?

[Superfluous J]

And what about Gilly? Is it tidally-locked?

Gilly is not tidally locked but its gravity is so small that the fuel you spend correcting your orbit after decoupling something is significant enough to render every (reasonable) method of landing on it similar.

Really. It takes thousands of m/s to get there, and 20 or so to escape its SOI. If you instead spent 40 who'd care?

[GoSlash27]

arkie,

It is, but it's not a candidate. It doesn't have vertical (or nearly vertical) cliffs for you to sit next to, which is the only case where you couldn't establish an orbital burn without climbing vertically first for any meaningful distance. Likewise Pol. Lumpy as it is, it doesn't have cliffs (other than polar anomalies).

The overriding concern is the terrain in your immediate area. You can get a fairly low pitch escape burn early at very low altitudes above the launch site even in mountainous terrain.

Best,

-Slashy

[arkie87]

Gilly is not tidally locked but its gravity is so small that the fuel you spend correcting your orbit after decoupling something is significant enough to render every (reasonable) method of landing on it similar.

Really. It takes thousands of m/s to get there, and 20 or so to escape its SOI. If you instead spent 40 who'd care?

I'm getting sick of repeating myself. This is a theoretical question.

[GoSlash27]

Gilly is not tidally locked but its gravity is so small that the fuel you spend correcting your orbit after decoupling something is significant enough to render every (reasonable) method of landing on it similar.

Really. It takes thousands of m/s to get there, and 20 or so to escape its SOI. If you instead spent 40 who'd care?

It's not? I thought it was...

No matter either way, it's not a candidate. Without a steep cliff blocking a direction you'd want to go, there's no reason to go vertical. And yeah... the gravity's so low that you could sneeze and reach escape velocity anyway...

Best,

-Slashy

Edited December 19, 2014 by GoSlash27

[arkie87]

It's not? I thought it was...

No matter either way, it's not a candidate. Without a steep cliff blocking a direction you'd want to go, there's no reason to go vertical. And yeah... the gravity's so low that you could sneeze and reach escape velocity anyway...

Best,

-Slashy

That is true, but it might illustrate what the planet would have to look like before it becomes relevant, which is, once again, what I'm after. In the OP, i've shown that a 10km cliff is more than enough to make vertical ascent more fuel efficient. 10 km out of a 200 km radius, is about 5% roughness. The roughness on gilly is much more than 5%. And I obviously cannot test this experimentally because no such place exists (that i know of) in KSP.

Anyway, I'm gonna get back to playing KSP instead of talking about it

[GoSlash27]

That is true, but it might illustrate what the planet would have to look like before it becomes relevant, which is, once again, what I'm after. In the OP, i've shown that a 10km cliff is more than enough to make vertical ascent more fuel efficient. 10 km out of a 200 km radius, is about 5% roughness. The roughness on gilly is much more than 5%. And I obviously cannot test this experimentally because no such place exists (that i know of) in KSP.

Anyway, I'm gonna get back to playing KSP instead of talking about it

Aye, but the cliff has to actually *exist*. The Mun has no 10km cliffs (or 10km terrain anywhere, for that matter). Gilly, while lumpier, also has no cliffs anywhere that's a going concern.

You could pick a spot near the pole of nearly any body and find significant cliffs. You won't launch into any important direction, but you can at least test. Otherwise, moholes or an Eeloo canyon. I don't recommend landing in moholes

Cool. I do want to pick your brain about something real quick if you have a moment...

-Slashy

Edited December 19, 2014 by GoSlash27