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Escape Towers


petrichorAM
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I was fiddling around on KSP last night and after a few tests I got an escape tower design to work the way I wanted. As I will now be using this on my next few mission programs I was just wondering, when you launch normally and nothing breaks what would be the minimum 'safe' altitude to decouple the tower when it's no longer required?

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I was fiddling around on KSP last night and after a few tests I got an escape tower design to work the way I wanted. As I will now be using this on my next few mission programs I was just wondering, when you launch normally and nothing breaks what would be the minimum 'safe' altitude to decouple the tower when it's no longer required?

Either of: When your SRBs are decoupled (at which point an escape tower is barely useful anymore, I find), or immediately after starting the gravity turn (so it can't come back down on top of you), whichever comes second.

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The LES on my craft gets dumped at about 10km, more than enough time if a disaster happens after that to stage through to the 'chutes and land safely. As others have said setting up staging so that only one or two of the Seperatrons fire when disposing of the LES is a wise move too.

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I fire my LES as a seperate stage just after the final staging of ascent (so payload+final stage). although you are right, any point where only parachutes are needed is fine.

Though often the parachutes are part of the LES, for payloads that wont be returning.

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Think Apollo dropped it at 15 km. In KSP you could drop it just after start of gravity turn, as this an common failure point.

To avoid getting hit by the tower don't fire up all the seperatons, this will send it spinning away.

Apollo dropped it at about 90km(the edge of space is considered to be 100km). In the later stages they were mostly using it to re-orient the capsule bottom-first so that the parachutes could deploy.

But KSP is massively space compressed. Kerbin has 1/10th the radius of Earth, and Low Earth Orbital Velocity is about four times higher than for what we call LKO. Earth's atmosphere doesn't ever really quite 'end' either: the Exosphere is considered to extend out to the point where the remaining gas particles aren't gravitationally bound to the Earth. Although it reaches a density so low that the remaining particles don't act like a gas anymore at about 500-1000km depending on solar activity.

As for not using all the sepatrons to eject it, I can't really attest to that. I can say that if you only manage to spin it about the Y-Axis by doing so that'll just make it worse, because that's exactly how rifles work. Getting it to unbalance in a way that sends it off to one side or another could be tricky with symmetrically placed sepatrons, since you can't normally fire only part of a symmetry group (and if it's too unbalanced it'll curve down into your rocket!)

I find it easier to just pitch it after I start the gravity turn. No fuss, no muss, no alarming close encounter.

Edited by Tiron
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Don't jettison it based on your altitude... jettison it based on when you no longer need it as an abort option.

Think Apollo dropped it at 15 km. In KSP you could drop it just after start of gravity turn, as this an common failure point.

To avoid getting hit by the tower don't fire up all the seperatons, this will send it spinning away.

Apollo dropped it much higher, during the second stage, once they were high and fast enough that the S-IVB and CSM's SPS motor could manage an Abort to Orbit if the S-II failed during the burn.

In the event that the S-IVB also failed and the craft was still to low and slow for the CSM to reach orbit alone, the SPS would be fired to maneuver away from the S-IVB and the SM would be jettisoned from the CM for landing.

If you're interested in the specifics on Apollo abort modes... http://en.wikipedia.org/wiki/Apollo_abort_modes

I jettison my LES tower when the spacecraft has enough delta V to reach orbit on it's own. At that point, I can either bring the crew home in a more controlled and deliberate manner or send up a second launch to provide whatever they need to continue the mission.

Edited by RoboRay
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I jettison my LES tower when the spacecraft has enough delta V to reach orbit on it's own. At that point, I can either bring the crew home in a more controlled and deliberate manner or send up a second launch to provide whatever they need to continue the mission.

Which has the unfortunate side effect of wasting a TREMENDOUS amount of Delta-V hauling the LET that high: Keep in mind that the rocket equation has a very nasty recursive multiplicative quality when it comes to additional payload. According to XKCD's 'What-if' column(http://what-if.xkcd.com/58/), "the fuel required to increase your speed by 1 km/s multiplies your weight by about 1.4", the way he's phrased that sounds suspiciously like he's talking about 1km/s of delta-v, but then he starts talking in terms of increasing to 8km/s (LEO Approximate orbital velocity) so I'm wondering if he left out drag and gravity losses for the sake of simplicity(Actual delta-V to LEO is around 9.3-10km/s according to a Delta-V map on wikipedia.)

The trick is, that's recursive: Each additional km/s multiplies the weight of the rocket including the fuel for the last round by 1.4 again. LKO takes around 4200-4400 m/s of delta-v total, so far as I've seen, but I'll just fudge it to 4 km/s for the sake of simplicity in stealing his numbers: 1.4*1.4*1.4*1.4=3.8416. So every ton of weight you haul to LKO takes ~4 tons of fuel(extremely fudged) to get it there. This is the reason for staging: Every bit of weight you dump on the way up gets recursed fewer times, as does all the fuel you no longer need to haul that to orbit... a small weight reduction will result in a disproportionate reduction in the amount of fuel required.

So the sooner you dump the tower, the more you reduce the extra fuel you need to haul it and the more Delta-V you have left when you get to orbit.

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Which has the unfortunate side effect of wasting a TREMENDOUS amount of Delta-V hauling the LET that high

The really unfortunate thing would be discarding your abort option before you have another abort option available to replace it, and then needing an abort option.

My LES weighs 0.3 tons. The delta V it costs me isn't of major significance.

So the sooner you dump the tower, the more you reduce the extra fuel you need to haul it and the more Delta-V you have left when you get to orbit.

By that argument, it would be smarter to take no LES tower at all.

If you take an LES, you're going for realistic safety options and/or historical accuracy. If your goal is simply maximizing efficiency at all expense because you're just playing a game and it doesn't really matter if anyone dies, leave the LES at home.

Edited by RoboRay
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The really unfortunate thing would be discarding your abort option before you have another abort option replace it, and then needing an abort option.

By that argument, it would be smarter to take no LES tower at all.

If you take an LES, you're going for realistic safety options and/or historical accuracy. If your goal is simply maximizing efficiency at all expenses because you're just playing a game, leave the LES at home.

As a general rule, I've found that that for most purposes, KSP wise, you don't really NEED a LES all that badly. On the vast majority of the manned rockets I've ever designed, the final powered stage was more than able to fulfill basic abort requirements in most circumstances. Even on ones where it wasn't powerful enough to immediately land on Kerbin, it WAS generally powerful enough to get the pod safely clear of the damaged rocket. With one exception: SRBs.

If your SRBs get loose, well, god help you. They are FAST when they're not attached to a rocket, far faster than any liquid based design I've yet built, and have a nasty tendency to curve inwards, presumably due to the radial decoupler unbalancing them. On a number of my designs this has resulted in broken-loose SRBs having a fair chance of hitting the uppermost stage, or on a few occasions even the pod itself(I remember one that skimmed right along the outside of the pod, taking one of the radial parachutes off but the pod surviving. Interestingly that SRB started on the opposite side of the rocket from the parachute it destroyed.) That was the one thing I really found a LET to be uniquely useful for: mostly outrunning rogue SRBs (even with 16 sepatrons for a Mark 1-2 pod, it STILL couldn't quite outrun them if they went straightish!)

Almost every launch failure I ever had after cutting the SRBs loose, decoupling the damaged sections was able to handle without a LET. Admittedly sometimes only with the use of Mechjeb's PANIC! button, which decouples far faster than can be done manually, (it's an interesting trick, that) but that was pre-action groups. In some cases I was even able to land the upper stage at the space center without even using the parachutes!

Edited by Tiron
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As a general rule, I've found that that for most purposes, KSP wise, you don't really NEED a LES all that badly.

No, not really, so long as your first stage doesn't tear itself apart with structural failures, you're probably going to be fine.

I'm assuming at some point, though, in Career Mode, random equipment failures will become a possibility. That's when it's going to get interesting... deciding on the fly whether an engine cut-out leaves you with enough thrust to reach orbit, designing craft to remain in-balance during engine-out scenarios, and giving real value to including abort systems to keep your highly-trained crew (and Bob) alive.

I use the NovaPunch LES tower, because nothing I've ever managed to cobble together provided sufficient TWR in a reasonable weight and part-count limit. The NovaPunch tower can pull a 3-man pod at 8 gees for one fairly light-weight part (two if you add a nosecone).

Edited by RoboRay
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No, not really, so long as your first stage doesn't tear itself apart with structural failures, you're probably going to be fine.

I'm assuming at some point, though, in Career Mode, random equipment failures will become a possibility. That's when it's going to get interesting... deciding on the fly whether an engine cut-out leaves you with enough thrust to reach orbit, designing craft to remain in-balance during engine-out scenarios, and giving real value to including abort systems to keep your highly-trained crew alive.

As long as your first stage doesn't have SRBs you're probably going to be able to use the last stage for an abort even on a early structural failure(if you're fast enough). At least enough of one to parachute to a landing. The key thing is that any engines still connected to the rocket can have the throttle cut, largely removing their threat.

And last I heard squad had said no to random failures, but I could be wrong I suppose (or they could change their minds). Maybe as an optional difficulty toggle, like the permadeaths, if they think it's worth it. Lack of them is one the main reasons LETs aren't as useful in KSP: almost all launch failures are going to be caused by design problems you're going to fix, and end up with an all but completely reliable launcher.

Another thing though, keep in mind that the main reason real rockets have had LETs is pad aborts. In real life, if a rocket goes up on the pad the explosion is going to be massive and completely destroy anything nearby (meaning the crew needed to be NOT nearby in a heartbeat). Explosions in KSP don't (yet) do any kind of damage to things nearby, so it's less important. Particularly if your topmost stage has a TWR greater than 1 on Kerbin at launch(if it doesn't, an LES may not be unwarranted).

Just as an example, the soviets tried to launch an N1 Moon rocket for the second time on July 3, 1969: a loose bolt got sucked into an oxygen pump just a few a seconds after liftoff, causing the pump to explode. The onboard computer then shut off 29 of the 30 engines. It fell back onto the pad, and became the largest non-nuclear explosion in human history. It took 18 months to repair the launch facility.

Apollo 11 launched on July 16.

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Very true. I've actually done pad-aborts in KSP, when a craft started crumbling when a support stand broke as the physics loaded. :)

But I do try to play with realistic safety concerns, so I include an LES on manned flights. I also do a roll and pitch maneuver right off the pad, for range-safety purposes (so any debris will fall into the ocean instead of back down onto the launch site).

Edited by RoboRay
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Very true. I've actually done pad-aborts in KSP, when a craft started crumbling when a support stand broke as the physics loaded. :)

But I do try to play with realistic safety concerns, so I include an LES. I also do a pitch and roll maneuver right off the pad, for range-safety purposes (so any debris will fall into the ocean instead of back down onto the launch site).

Actually that last is going to be a thing for all of us at some point I'm pretty sure: The ability for KSC facilities to get damaged and cost money to be repaired is on the planned features list.

As for 'realistic safety concerns', if you mean you're treating the explosions like they do damage, well, okay. Otherwise that alone doesn't really require a special LES (unless you have a low TWR upper stage.)

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The Abort group can't shut down liquid engines on sections that have broken free of the spacecraft, so a high-gee powered escape (rather than tumbling through the debris cloud and hoping nothing hits the pod) is always desirable to me. My orbiters generally have very low TWRs to maximize delta V, pretty much always less than 0.5 gee, making an 8 gee LES option nice to have for outrunning lower stages (or even SRBs) when the fecal matter impacts the rotary air impeller.

Edited by RoboRay
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Don't jettison it based on your altitude... jettison it based on when you no longer need it.

Apollo dropped it much higher, during the second stage, once they were high and fast enough that the S-IVB and CSM's SPS motor could manage an Abort to Orbit if the S-II failed during the burn.

In the event that the S-IVB also failed and the craft was still to low and slow for the CSM to reach orbit alone, the SPS would be fired to maneuver away from the S-IVB and the SM would be jettisoned from the CM for landing.

I understand. In KSP rockets don't blow up and destroy the command module so its main mission is to give attitude and distance to the rocket so we can deploy parachutes. Then solid booster has burned up and you are high enough to use parachutes even in worst case scenario of spinning rocket you can jettison it.

4-5 km should be enough.

It's also important to kill all engines then using the escape function. then it will work well also without the escape tower.

I never use them on small reliable rockets however the kill engines and separate pod is also useful, most used during landings if I have to lite fuel for powered landings.

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The Abort group can't shut down liquid engines on sections that have broken free of the spacecraft, so a high-gee powered escape (rather than tumbling through the debris cloud and hoping nothing hits the pod) is always desirable to me. My orbiters generally have very low TWRs to maximize delta V, pretty much always less than 0.5 gee, making an 8 gee LES option nice to have for outrunning lower stages (or even SRBs) when the fecal matter impacts the rotary air impeller.

I honestly can't think of an instance where I've had a broken-loose liquid section prevent me from safely landing the pod without an LES. I've seen times where it made it a lot hairier than it might have otherwise been, but it still worked. It's the SRBs that are *really* dangerous.

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I honestly can't think of an instance where I've had a broken-loose liquid section prevent me from safely landing the pod without an LES. I've seen times where it made it a lot hairier than it might have otherwise been, but it still worked.

Sure, but the point of safety systems is to eliminate or mitigate the possibility of something bad happening. When "it got hairy" it could have gotten bad. It didn't, this time, but that doesn't mean the collision won't ever happen. With an LES that can outrun every other engine on the ship, that risk is close to eliminated.

I spent 16 years working in aviation safety (12 years as a part-time aspect of my job, and 4 years full-time), so risk-management techniques are just second nature to me. I treat KSP spacecraft with the same kind of concerns I became accustomed to applying to aircraft in the real world.

Risk-management can be looked at as a pretty simple system...

  • Consider what can go wrong.
  • Determine the worst-possible outcome if it does go wrong.
  • Devise a means of eliminating (or at least minimizing) that hazard.
  • Consider any impairment of the mission by that means.
  • Implement the means if impairment of the mission is less significant than the worst-possible outcome of the hazard.

An LES tower, if it's lightweight and high-powered, is very challenging to disregard under that type of analysis.

Is it really necessary for KSP? No. It's pretty hard to kill the crew in a launch-failure. But, in the real world, nobody would take that kind of chance. And I try to keep things realistic.

Edited by RoboRay
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I usually ditch mine with the lighting of the second stage. At least that's the way I set it up with the Storax Anacostia 7 rocket (the treehugger Apollo-style Duna mission I've got in progress); with the Castle Romeo 7 (Munar Apollo-style), I forgot to ditch the thing entirely until I was in orbit...

All stock; just a decoupler, a modular girder adapter and twelve Sepratrons tied in with the Abort action group (the decoupler in the action group is the one holding the CM to the rocket; there's a second decoupler that will jettison the assembly from the stack). A second action group jettisons the tower post-abort and a third activates the descent chutes.

I have pictures of the system in action, come to think of it:

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Damn good thing I put it on the Storax Anacostia - the second and third launch attempts suffered RUVD during the first stage and I'd have lost my three musketeers for sure without it.

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