Planning Suicide Burns with Manoeuvre Nodes - Why does it not work?

[tavert]

of course a "suicide burn" is more efficient!

Just think logically!

The longer you fight gravity, the more fuel you will waste. So if your landing burn is short, the percentage of wasted fuel is less.

And a short landing burn is, of course, a single one with 100% of thrust ---> and i think that is called "suicide burn" (?)

A big disadvantage is, that it is pretty risky.

What i do:

I try to burn one time with ~90%, so i will never crash into the ground(just more thrust) and i also can go back with the thrust so i get down the fastest way

The counterintuitive part about landing is the tradeoff between gravity losses and steering losses. If you point retrograde the whole time you don't have any steering losses, but since your altitude drops during the burn, gravity speeds you up and that costs extra fuel to counteract. If instead you come in very low and horizontally and adjust your direction of thrust to maintain constant altitude as you slow down, you do suffer steering losses from not pointing retrograde, but since your altitude doesn't change you don't get accelerated by gravity. Integrated over an entire landing burn, the constant-altitude method is more efficient than a retrograde suicide burn.

See http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse for some math and numbers, at least for the constant altitude method. Simulations in game or with mathematical predictions will verify the comparison between constant-altitude and retrograde.

[Andersenman]

I'm trying to understand, but you're confusing me:

[...] but since your altitude drops during the burn, gravity speeds you up and that costs extra fuel to counteract. If instead you come in very low and horizontally and adjust your direction of thrust to maintain constant altitude as you slow down, you do suffer steering losses from not pointing retrograde, but since your altitude doesn't change you don't get accelerated by gravity.

As I understand, I always have to counter gravity, regardless of my radial movement: If I hover, I have to spend full dv against gravity. If I'm flying sub-orbitally, I have to spend some dv to prevent "not missing the ground while [not quite] falling". Only when I'm truly orbiting, I don't have to counter gravity with thrust. ... with these three cases applying at any altitude, and any change in altitude. Right? So why are you saying that I wouldn't need extra fuel to counteract if I was maintaining altitude?

Also, I am trying to understand landing (and launching for that matter) as an orbital change that moves the apoapsis from one altitude to another by burns roughly around the point where the orbit happens to intersect surface. Is this wrong somewhere?

And just to make sure, I'm not arguing for a particular landing method, I'm trying to understand where I might be wrong; and ultimately, what fudge factors I need when interpreting the manoeuvre node system's burn times and times to node. (I believe that it should work somehow as I have seen that it becomes more and more accurate the shorter the burn time, the flatter the trajectory and the smaller the time to node.)

See http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse for some math and numbers, at least for the constant altitude method. Simulations in game or with mathematical predictions will verify the comparison between constant-altitude and retrograde.

Seen and marvelled at. Higher algebra is sadly not one of my strengths which is frustrating knowing that Maths never lies ...

Edited January 6, 2014 by Andersenman

[Kasuha]

The actual point here is simple. What is landing in terms of orbital mechanics? You decrease your apoapsis to the level of terrain and periapsis deep under the surface.

The most efficient maneuver to decrease your apoapsis is retrograde burn at periapsis.

And the most efficient maneuver to decrease your periapsis is retrograde burn at apoapsis.

Suicide burn means you're decreasing both at once at an arbitrary point of your trajectory. Therefore - not optimal.

The constant altitude approach means you're first decreasing your apoapsis while you're at periapsis - till you circularize. That's optimal maneuver. Then you decrease your periapsis, while you're sitting at the apoapsis. Again, optimal maneuver. Some loses come from the fact that you need to keep yourself at or near that apoapsis (actually slightly behind it) but these are minimal.

Optimal suicide burn looks pretty much like the constant altitude landing - you start burning at periapsis set to exactly the right height above the terrain, pass through circularization and manage to pull the apoapsis right behind you as you burn retrograde. After careful calculations, you could pull that maneuver and I believe it is the most efficient way of landing. But that's not your usual suicide burn, it's a hybrid of the two approaches which needs to be carefully prepared and executed.

Edited January 6, 2014 by Kasuha

[tavert]

I'm trying to understand, but you're confusing me:

Sorry! Let's try a few more interpretations of this (Kasuha's is also a new way of thinking about it, I like - though I wouldn't describe the losses as "minimal," they're quite significant at lower TWR).

As I understand, I always have to counter gravity, regardless of my radial movement: If I hover, I have to spend full dv against gravity. If I'm flying sub-orbitally, I have to spend some dv to prevent "not missing the ground while [not quite] falling". Only when I'm truly orbiting, I don't have to counter gravity with thrust. ... with these three cases applying at any altitude, and any change in altitude. Right?

Right.

So why are you saying that I wouldn't need extra fuel to counteract if I was maintaining altitude?

Not quite, sorry if it came across that way. You are counteracting gravity to avoid falling, with the off-retrograde component of your thrust. But since your velocity vector is perpendicular to the force of gravity (moving horizontally, 0 vertical speed), gravity does not change the magnitude of your velocity, only its direction. It's as if you're orbiting a lighter planet at a slower speed, simulating the difference in planet mass with your vertical thrust component counteracting some of the gravity.

In a retrograde burn on the other hand, as you drop in altitude, your velocity vector is no longer perpendicular to the force of gravity. The non-zero component of gravity that is parallel to your velocity causes the magnitude of your velocity to increase, and the perpendicular component changes the direction. That change in speed due to nonzero vertical speed is known as "gravity losses." It slows you down on ascent, and speeds you up during landing, if you allow your altitude to change.

(I believe that it should work somehow as I have seen that it becomes more and more accurate the shorter the burn time, the flatter the trajectory and the smaller the time to node.)

This is true. For higher and higher TWR, the differences between retrograde and constant-altitude landing method approach 0.

Edited January 6, 2014 by tavert

[Kasuha]

I wouldn't describe the losses as "minimal," they're quite significant at lower TWR

That depends on what you consider significant. Trigonometric functions play in your favor, even when burning 45 degrees from retrograde, 70% of your thrust still goes to decreasing your horizontal velocity. And you usually don't need to spend too much time burning at such angles unless your TWR is really, really close to 1.

This is true. For higher and higher TWR, the differences between retrograde and constant-altitude landing method approach 0.

I disagree. Regardless of TWR, landing from low periapsis is always more efficient than braking from suborbital trajectory with high apoapsis.

Edited January 6, 2014 by Kasuha

[Andersenman]

Just to confirm:

That change in speed due to nonzero vertical speed is known as "gravity losses." It slows you down on ascent, and speeds you up during landing, if you allow your altitude to change.

"Gravity loss" is essentially the decrease in speed I see along a non-circular orbit on the way from Pe to Ap? And likewise, the increase in speed from Ap to Pe would be a "gravity gain"?

Edited January 6, 2014 by Andersenman

[blizzy78]

The following video by Kosmo-not describes about the most efficient and safe way of landing.

While the video is pretty good, it does not teach when to start burning and how fast. That is, I always struggle because I don't know if I'm too early or too late. Burning at full thrust too early will lead to skimming so that you don't come in short. Skimming in turn leads to fighting gravity for too long, wasting fuel along the way.

What I would like to know is this: If I'm this distance out from my desired landing site, how big a deceleration would I need if I'm going this fast to come in at (ideally) 0 m/s?

[Andersenman]

What I would like to know is this: If I'm this distance out from my desired landing site, how big a deceleration would I need if I'm going this fast to come in at (ideally) 0 m/s?

This is what the node system has been helpful with. It only starts to make me stop short the higher and faster I'm coming in. Put a node above the target, guesstimating surface rotation as needed, pull retrograde until your new orbit drops you steeply onto your destination, then burn like in the video (ie. ignoring the blue node marker), starting at time-to-node minus burn time.

So far the node method had been making me stop short while Kosmo-not's method looks like it takes ages to stop laterally – I wouldn't be surprised if both together would cancel out somehow and make for a half-accurate landing after all. Try it! (While I'm sitting at work.)

Oh, and please do tell how it went.

Edited January 6, 2014 by Andersenman

[Kasuha]

While the video is pretty good, it does not teach when to start burning and how fast. That is, I always struggle because I don't know if I'm too early or too late. Burning at full thrust too early will lead to skimming so that you don't come in short. Skimming in turn leads to fighting gravity for too long, wasting fuel along the way.

What I would like to know is this: If I'm this distance out from my desired landing site, how big a deceleration would I need if I'm going this fast to come in at (ideally) 0 m/s?

That video is not about pinpoint landings. It's about how to land wherever you manage to stop. You need to start burning so you circularize at your periapsis (you can use a maneuver to help you with timing), then you brake, descend, and land.

Horizontal landing is not very good for pinpoint landings, although I did use it to land on Tylo's poles (it took good 1/8 of the circumference to brake). It means you may need to spend some more energy for hovering than you would ultimately need to just land anywhere.

You may try this approach:

Put your periapsis sufficient distance ahead of your target and circularize at that point using a maneuver. Use that maneuver also to put your trajectory right above your target. Then put another maneuver above your target and kill all horizontal velocity at that point. And start braking about 7/10 of the maneuver time ahead.

[tavert]

Just to confirm:

"Gravity loss" is essentially the decrease in speed I see along a non-circular orbit on the way from Pe to Ap? And likewise, the increase in speed from Ap to Pe would be a "gravity gain"?

Yes.

That depends on what you consider significant. Trigonometric functions play in your favor, even when burning 45 degrees from retrograde, 70% of your thrust still goes to decreasing your horizontal velocity. And you usually don't need to spend too much time burning at such angles unless your TWR is really, really close to 1.

I disagree. Regardless of TWR, landing from low periapsis is always more efficient than braking from suborbital trajectory with high apoapsis.

Check the thread I linked to earlier. At low TWR it costs a lot more dV.

Did I say anything about how high your periapsis is? No. Just whether you perform your braking burn by pointing retrograde the whole time, vs changing direction to maintain constant altitude. Otherwise a like-for-like comparison would have the same incoming orbit.

[Kasuha]

Did I say anything about how high your periapsis is? No.

I think we're working with rather vague definition of suicide burn.

In my opinion, though, common idea of suicide burn is that you first decrease your periapsis below terrain to get on a suborbital trajectory. Because only after that you can have things like time to impact which give you clues about when to start the burn. And because you want to be efficient, you do so at apoapsis. Which leaves you on high apoapsis suborbital trajectory with no chance to brake at or near periapsis.

Edited January 6, 2014 by Kasuha

[HarvesteR]

The estimated burn time does factor in the changing TWR, as it gets calculated based on the vessel's current acceleration (and expecting full throttle).

What it doesn't do is antecipate how much that burn time is going to change as your TWR increases, so while it may start out saying 20 seconds for a burn, if you were to time the burn with a chronometer you'd notice the estimated seconds go by slightly faster than real-time, since it's recalculating as the craft mass decreases and your acceleration increases.

It does work out to zero burn time at zero dV remaining though, which is the essential bit.

Doing a single braking burn to stop at a very precise spot is a VERY tricky stunt to pull off though, hence the name of the maneuver. In fact, it's probably a good thing if you're coming up short... the alternative would be far less desirable.

You can try not burning at 100% though, maybe leave it at 90-ish to compensate, and you'll end up closer to the ground.

Do keep in mind you're essentially playing a game of chicken with the ground, and the ground doesn't care nearly as much about losing.

Cheers

[I_Killed_Jeb]

I think we're working with rather vague definition of suicide burn.

In my opinion, though, common idea of suicide burn is that you first decrease your periapsis below terrain to get on a suborbital trajectory. Because only after that you can have things like time to impact which give you clues about when to start the burn. And because you want to be efficient, you do so at apoapsis. Which leaves you on high apoapsis suborbital trajectory with no chance to brake at or near periapsis.

I don't know if this is true. At the point of landing, all of the orbital kinetic and potential (if you land at 0m) energies have to go down to zero, so while you can get into a suborbital trajectory with less burn at apoapsis, you still have to burn off the energy at some point in the trajectory.

[Kasuha]

I don't know if this is true. At the point of landing, all of the orbital kinetic and potential (if you land at 0m) energies have to go down to zero, so while you can get into a suborbital trajectory with less burn at apoapsis, you still have to burn off the energy at some point in the trajectory.

It is common knowledge (in KSP at least) that the most efficient way to decrease your periapsis is to burn retrograde at apoapsis. It is possible to do it at arbitrary point (including at periapsis) but that costs you more dv. And even that has optimal and suboptimal approaches. I was a bit ironic about it, in the sense that common knowledge of optimal approaches still leads you to doing things inefficiently.

But my point was that suicide burn usually means suborbital trajectory with high apoapsis. OP was definitely referring to such approach, if you check the first post of this thread. In fact, this approach is relatively good for pinpoint landings because if your speed is high enough, the planet won't rotate your target place too far off your trajectory. If you have enough fuel, that is.

At the point of landing you definitely don't have zero potential or kinetic energy. You have kinetic energy corresponding to the planet's rotation. And you still have energy which would allow you to stay in orbit with apoapsis of your landing altitude if the body under you suddenly shrunk into a black hole. But that's only partially relevant here.

Of course you need to drop that energy difference somewhere. But depending on where on your trajectory you do so, it may cost you less or more dv.

Edited January 6, 2014 by Kasuha

[Andersenman]

But my point was that suicide burn usually means suborbital trajectory with high apoapsis. OP was definitely referring to such approach, if you check the first post of this thread.

To clarify this: I did not mean to refer to high-Ap sub-orbitals alone, but ANY sub-orbit that ends in a full burn and a touchdown without unnecessarily extensive lingering, and also why the node system isn't always accurate enough to be considered helpful in planning such landings.