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#460 by clongton on 03 Jan, 2020 13:38
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The US Navy usually has some ships in the Indian Ocean. There is the base at Diego Garcia. It may not be too big of a problem to have some ship nearby just in case.Right. Not all the way around. Safe abort to Indian Ocean.
Do they have rescue assets placed there or nearby? Wondering how long the capsule is good for if it aborts and has to await assets to travel some distance.
- Ed Kyle
Take a look at a map.
Diego Garcia is thousands of miles from the suborbital reentry landing zone.
The water there is warmer but it’s just as remote as the North Atlantic.
They must have some plan to drop rescue equipment and people but it could be a day to get anything there.
Does anyone have a reference on that?
The US Navy has a "Guardian Angel" program to rescue aborted commercial crew launches. It involves helicopter recovery if the IIP is 200 nm or less from the launch point or longer range C-17 aircraft from various points around the globe equipped with personnel and equipment to be dropped into the sea where the spacecraft is. It also involves making use of passing commercial shipping of opportunity to recover the crews. In almost no circumstances will recovery be as clean as we were used to seeing in the Apollo days and in a worst case scenario could leave aborted crews adrift for many days waiting for a ship, any ship, to show up to recover them. I would hope that one of those C-17 teams would be located in SW Australia, but I could find no reference to any Guardian Angel team locations.
https://www.nasa.gov/feature/rescue-operations-take-shape-for-commercial-crew-program-astronauts -
#461 by jstrotha0975 on 03 Jan, 2020 13:55
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The US Navy usually has some ships in the Indian Ocean. There is the base at Diego Garcia. It may not be too big of a problem to have some ship nearby just in case.Right. Not all the way around. Safe abort to Indian Ocean.
Do they have rescue assets placed there or nearby? Wondering how long the capsule is good for if it aborts and has to await assets to travel some distance.
- Ed Kyle
Take a look at a map.
Diego Garcia is thousands of miles from the suborbital reentry landing zone.
The water there is warmer but it’s just as remote as the North Atlantic.
They must have some plan to drop rescue equipment and people but it could be a day to get anything there.
Does anyone have a reference on that?
The US Navy has a "Guardian Angel" program to rescue aborted commercial crew launches. It involves helicopter recovery if the IIP is 200 nm or less from the launch point or longer range C-117 aircraft from various points around the globe equipped with personnel and equipment to be dropped into the sea where the spacecraft is. It also involves making use of passing commercial shipping of opportunity to recover the crews. In almost no circumstances will recovery be as clean as we were used to seeing in the Apollo days and in a worst case scenario could leave aborted crews adrift for many days waiting for a ship, any ship, to show up to recover them. I would hope that one of those C-117 teams would be located in SW Australia, but I could find no reference to any Guardian Angel team locations.
https://www.nasa.gov/feature/rescue-operations-take-shape-for-commercial-crew-program-astronauts
C-17 -
#462 by Comga on 03 Jan, 2020 14:19
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Wow, Chuck!
Talk about a full answer to my question!
Thanks
In response to yours, the article saysQuoteFor contingency landings outside of the 200 nautical mile-radius, a C-17 aircraft would deploy from either Charleston Air Force Base in South Carolina or Hickam Air Force Base in Hawaii, depending on the splashdown location, with the same type of team and equipment to execute rescue operations.
That says to me that there are only two C-17s
A subsequent question is whether they preposition one of them, say the one based on Hawaii, to some place like Australia to reduce response time to locations more than half the globe away from the other.
Antipodal to Charleston is 930 miles west of Perth, Australia and real close to the passive reentry area.
I would expect the Hawaii based C-17 to be propositioned there. -
#463 by woods170 on 03 Jan, 2020 15:05
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Right. Not all the way around. Safe abort to Indian Ocean.
Do they have rescue assets placed there or nearby? Wondering how long the capsule is good for if it aborts and has to await assets to travel some distance.
- Ed Kyle
CCP has a requirement for both providers that their capsules must be able to remain afloat (in calm to moderate sea states) for 24 hours after splash down (if I remember correctly). That is with hatches sealed and basic life support systems running.
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#464 by Rocket Science on 03 Jan, 2020 16:59
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My comment is related to entry attitude control which is the discussion... They "are out" if you can't perform manual reversion as was available on all NASA capsules since Mercury either through control rods or RCS solenoids...
...in which case the thrusters aren't really out, which means that you're fine with an abort to orbit, because you can deorbit in addition to doing attitude control, so you're not stuck in a stable orbit.
Unless they have the ability to go RCS Direct as with the Apollo CM...Starliner seems to be a cross between Shuttle and Apollo, but just because either did something doesn’t say it’s optimal.
Is there an advantage to a capsule that is unstable rather than bring passively stable?
Remember that the issue we started with is whether a thrusters-out reentry was likely to succeed. Doesn't really matter if the computers are out if they can't command anything to do what they want.
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#465 by clongton on 03 Jan, 2020 18:34
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The US Navy has a "Guardian Angel" program to rescue aborted commercial crew launches. It involves helicopter recovery if the IIP is 200 nm or less from the launch point or longer range C-117 aircraft from various points around the globe equipped with personnel and equipment to be dropped into the sea where the spacecraft is. It also involves making use of passing commercial shipping of opportunity to recover the crews. In almost no circumstances will recovery be as clean as we were used to seeing in the Apollo days and in a worst case scenario could leave aborted crews adrift for many days waiting for a ship, any ship, to show up to recover them. I would hope that one of those C-117 teams would be located in SW Australia, but I could find no reference to any Guardian Angel team locations.
https://www.nasa.gov/feature/rescue-operations-take-shape-for-commercial-crew-program-astronauts
C-17
Thanks. Fixed it. -
#466 by TheRadicalModerate on 03 Jan, 2020 19:18
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...in which case the thrusters aren't really out, which means that you're fine with an abort to orbit, because you can deorbit in addition to doing attitude control, so you're not stuck in a stable orbit.
My comment is related to entry attitude control which is the discussion... They "are out" if you can't perform manual reversion as was available on all NASA capsules since Mercury either through control rods or RCS solenoids...
So you're just saying that the RCS can't be disabled through an automation or electronics failure, right? I think that gibes with what I'm trying to say, which is pretty simple: You're unlikely to have a successful reentry (i.e., wind up alive at the end of it) without active attitude control to guide the spacecraft and keep it in the corridor.
I know I'm sounding like a broken record here, but that still argues in favor of my main point, which is that Starliner's 72x181 passive reentry orbit is kinda silly, or at least a desperate last resort, if its sole purpose is to bring the crew back in the face of a complete absence of any way of generating delta-v, because the reentry probably won't work.
Instead, if Starliner were in a stable low orbit (say, 150x181), if the RCS system, computer-commanded or not, can still push hydrazine and NTO into RCS thrust chambers, it can likely manage the 25 m/s to deorbit and likely provide enough attitude control for a successful reentry. And if there's some fault that prevents that from happening, the crew is better off even if they're stranded in orbit, because then they at least have a chance to work the problem for more than 30-50 minutes before they die. Time is hope.
If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure. -
#467 by SWGlassPit on 03 Jan, 2020 19:54
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There's an awful lot of navel-gazing (getting wrapped around the axle, or whatever idiom you like) in this thread arguing over assumed reasons for a particular mission design as if they were authoritatively stated. Reading back through this thread, the passive abort idea appears speculative at best.
I recall it said (I think by Chilton at the press conference, though I may be mistaken) that the suborbital trajectory with Starliner performing circularization allows Starliner to burn-off the no-longer-needed abort propellant. -
#468 by TheRadicalModerate on 03 Jan, 2020 20:08
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There's an awful lot of navel-gazing (getting wrapped around the axle, or whatever idiom you like) in this thread arguing over assumed reasons for a particular mission design as if they were authoritatively stated. Reading back through this thread, the passive abort idea appears speculative at best.
I recall it said (I think by Chilton at the press conference, though I may be mistaken) that the suborbital trajectory with Starliner performing circularization allows Starliner to burn-off the no-longer-needed abort propellant.
But that doesn't make any sense either. If Starliner needs to burn off prop, they can do it from a stable orbit simply by engaging in a less-optimal set of phasing burns on the way to ISS. It doesn't matter how heavy the vehicle is until it needs to dock.
I completely agree that it makes massively more sense, given all the prop they have, for the Starliner OMACs to do the apogee burn than it does to delay Centaur separation and rely on it to do a restart. But that just begs the question of why they need an apogee burn in the first place. -
#469 by Rocket Science on 03 Jan, 2020 20:12
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No... I am addressing the fail-safe manual entry only, simple as that......in which case the thrusters aren't really out, which means that you're fine with an abort to orbit, because you can deorbit in addition to doing attitude control, so you're not stuck in a stable orbit.
My comment is related to entry attitude control which is the discussion... They "are out" if you can't perform manual reversion as was available on all NASA capsules since Mercury either through control rods or RCS solenoids...
So you're just saying that the RCS can't be disabled through an automation or electronics failure, right? I think that gibes with what I'm trying to say, which is pretty simple: You're unlikely to have a successful reentry (i.e., wind up alive at the end of it) without active attitude control to guide the spacecraft and keep it in the corridor.
I know I'm sounding like a broken record here, but that still argues in favor of my main point, which is that Starliner's 72x181 passive reentry orbit is kinda silly, or at least a desperate last resort, if its sole purpose is to bring the crew back in the face of a complete absence of any way of generating delta-v, because the reentry probably won't work.
Instead, if Starliner were in a stable low orbit (say, 150x181), if the RCS system, computer-commanded or not, can still push hydrazine and NTO into RCS thrust chambers, it can likely manage the 25 m/s to deorbit and likely provide enough attitude control for a successful reentry. And if there's some fault that prevents that from happening, the crew is better off even if they're stranded in orbit, because then they at least have a chance to work the problem for more than 30-50 minutes before they die. Time is hope.
If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure. -
#470 by SWGlassPit on 03 Jan, 2020 20:16
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There's an awful lot of navel-gazing (getting wrapped around the axle, or whatever idiom you like) in this thread arguing over assumed reasons for a particular mission design as if they were authoritatively stated. Reading back through this thread, the passive abort idea appears speculative at best.
I recall it said (I think by Chilton at the press conference, though I may be mistaken) that the suborbital trajectory with Starliner performing circularization allows Starliner to burn-off the no-longer-needed abort propellant.
But that doesn't make any sense either. If Starliner needs to burn off prop, they can do it from a stable orbit simply by engaging in a less-optimal set of phasing burns on the way to ISS. It doesn't matter how heavy the vehicle is until it needs to dock.
I completely agree that it makes massively more sense, given all the prop they have, for the Starliner OMACs to do the apogee burn than it does to delay Centaur separation and rely on it to do a restart. But that just begs the question of why they need an apogee burn in the first place.
And I think that comes back down to the fact that the trajectory chosen is depressed relative to the normal, lofty Atlas-Centaur trajectory in order to ensure abort survivability at all ascent phases. -
#471 by edkyle99 on 03 Jan, 2020 20:28
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If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure.
ULA catalogs Atlas 5-N22 capability to a 407 x 407 km x 51.6 deg circular orbit to be 13,250 kg. They would have margin even beyond that. My guess is that AV-080 could have boosted OFT directly to a circular, though probably lower than 400 km, orbit if desired. The difference between the planned orbit and a circular 181 km orbit is only 32 m/s delta-v. That's less than 300 kg of Centaur propellant. I've read somewhere that Centaur had more than a tonne of propellant left at insertion.
- Ed Kyle -
#472 by Comga on 03 Jan, 2020 20:56
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There's an awful lot of navel-gazing (getting wrapped around the axle, or whatever idiom you like) in this thread arguing over assumed reasons for a particular mission design as if they were authoritatively stated. Reading back through this thread, the passive abort idea appears speculative at best.
I recall it said (I think by Chilton at the press conference, though I may be mistaken) that the suborbital trajectory with Starliner performing circularization allows Starliner to burn-off the no-longer-needed abort propellant.
Burning off the prop may be “how” but it can’t be “why”
That is, Starliner can use the prop that is no longer needed to raise the perigee, but there are other ways to get rid of it.
(We saw on the display on the back wall of the control room that Starliner was burning off lots of fuel with no real change in trajectory.
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But to design the entire launch to be short enough velocity to be absolutely dependent on powering up the capsule within a few minutes to do the orbit insertion burn?
This sounds like one of those jokes where each step is logical but the result is absurd. -
#473 by Comga on 03 Jan, 2020 20:59
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If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure.
ULA catalogs Atlas 5-N22 capability to a 407 x 407 km x 51.6 deg circular orbit to be 13,250 kg. They would have margin even beyond that. My guess is that AV-080 could have boosted OFT directly to a circular, though probably lower than 400 km, orbit if desired. The difference between the planned orbit and a circular 181 km orbit is only 32 m/s delta-v. That's less than 300 kg of Centaur propellant. I've read somewhere that Centaur had more than a tonne of propellant left at insertion.
- Ed Kyle
Yes
Someone said a ton and a half. (They had it in kg.)
This is consistent with standard Atlas V operations. -
#474 by cwr on 03 Jan, 2020 21:05
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If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure.
ULA catalogs Atlas 5-N22 capability to a 407 x 407 km x 51.6 deg circular orbit to be 13,250 kg. They would have margin even beyond that. My guess is that AV-080 could have boosted OFT directly to a circular, though probably lower than 400 km, orbit if desired. The difference between the planned orbit and a circular 181 km orbit is only 32 m/s delta-v. That's less than 300 kg of Centaur propellant. I've read somewhere that Centaur had more than a tonne of propellant left at insertion.
- Ed Kyle
My recollection [but I can't quote where] is that ULA announced the reason for the sub-orbital insertion
was so that they could burn off the fuel ear-marked for an abort burn and hence not carry it around throughout
the mission.
Carl
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#475 by 1 on 03 Jan, 2020 21:10
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Sorry if I'm being confusing. The issue is whether a thrusters-out reentry is likely to succeed. You can be all lined up perfectly at entry interface--which is what will happen with a passive abort--but you still need some guidance to stay in the corridor in the atmosphere, because air makes guidance errors pile up a lot faster than vacuum does. If those errors result in going too steep or too shallow , there's a problem.
Bolded part is where I find your statements unclear. Specifically, I suspect you're worried about issues that can only materialize from much higher energy trajectories. Sent you a PM yesterday to try to get a better feel for where you envision the danger to be; would be perfectly happy to continue discussion there for the time being.
I find the tumbling issue mentioned by envy887 to be more concerning, although I take that article with a grain of salt since it doesn't go into much technical detail and some reporters tend to over-sensationalize. -
#476 by TheRadicalModerate on 03 Jan, 2020 21:51
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If the assertion above is correct, then we get to the final piece of my argument: If passive reentry is silly in an of itself, then Boeing's choice of 72x181 isn't solely for its passive abort capabilities. Instead, it's more likely that they simply didn't have enough delta-v in the DEC, after flying the depressed trajectory to keep ascent fully abortable, to reach a stable orbit without the perigee-raising burn 20 minutes after insertion, and they then chose 72x181 as the best of a variety of suborbits because it at least provided some slim hope of reentry success, even if it wasn't likely, in the event of a total propulsion failure.
ULA catalogs Atlas 5-N22 capability to a 407 x 407 km x 51.6 deg circular orbit to be 13,250 kg. They would have margin even beyond that. My guess is that AV-080 could have boosted OFT directly to a circular, though probably lower than 400 km, orbit if desired. The difference between the planned orbit and a circular 181 km orbit is only 32 m/s delta-v. That's less than 300 kg of Centaur propellant. I've read somewhere that Centaur had more than a tonne of propellant left at insertion.
- Ed Kyle
Sounds right, and that's something I mentioned when I started picking at this up-thread.
But it's not quite that simple, for a few reasons.
First and foremost, that 407x407 orbit assumes the usual lofted trajectory, which the N22 can't fly and meet the continuous abort requirement for a crewed trajectory. Hence the depressed trajectory. That depression comes at a pretty hefty cost in performance, if for no other reason than you're going to be spending a lot more delta-v in gravity drag to keep the lower-staging DEC from falling back into the atmosphere before Coriolis effects transfer tangential velocity into enough radial velocity to hold the silly thing up. Beyond that, when you're counting on effects like that, you usually have to do weird things to circularize, which in turn cost even more delta-v.
When I did the computation (it's up-thread), I came up with about 1.5 t of residual prop, which does indeed sound like more than enough to raise the perigee. But there are two things that impact the planning for the use of that:
1) Whatever ullage is needed to keep the turbopump inlets sufficiently covered and pressurized.
2) The actual flight performance reserve, which may be more uncertain with the weird trajectory than it would be if they were engineering it from the 3𝛔 distribution of previous flights--because there are no DEC flights, to say nothing of DEC flights flying depressed trajectories, to use as a statistical universe.
Finally, there's the insertion anomaly. From published sources, we know that the apogee kick is 40 seconds, occurring about 20 minutes after the DEC MECO. That makes the insertion anomaly about 90º.
Now, it is absolutely true that, because even a 72x181 orbit is essentially circular, you could just spend another 25 m/s of delta-v after the nominal MECO and wind up with a 150x181 orbit, easey-peasey. But if you did that, you'd also shift the argument of periapse by 90º. I don't know if that's a problem for phasing or not, but shifting it back to where it is for the insertion costs about 105 m/s. So you're at a 130 m/s total, which should only cost 120 kg of prop--but that's not nothing, depending on the required residuals and the FPR, which are going to be higher for a crew-rated flight than for an ordinary payload.
It's close. It looks to me like they ought to be OK (which is what I said up-thread), but it's close. Maybe it's too close for a human-rated trajectory.
So, again, I'm left with no definitive explanation: the passive abort doesn't make a lot of sense to me (and I realize it does to a lot of people, which is why I've been picking at it). The "burn off the extra prop" argument doesn't cut it either. But it does seem as if there ought to be enough performance left to go to a higher perigee.
So I'm still stumped, or at the very least still lacking the "aha!" explanation that makes this all a slam-dunk. -
#477 by TheRadicalModerate on 03 Jan, 2020 22:08
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Sorry if I'm being confusing. The issue is whether a thrusters-out reentry is likely to succeed. You can be all lined up perfectly at entry interface--which is what will happen with a passive abort--but you still need some guidance to stay in the corridor in the atmosphere, because air makes guidance errors pile up a lot faster than vacuum does. If those errors result in going too steep or too shallow , there's a problem.
Bolded part is where I find your statements unclear. Specifically, I suspect you're worried about issues that can only materialize from much higher energy trajectories. Sent you a PM yesterday to try to get a better feel for where you envision the danger to be; would be perfectly happy to continue discussion there for the time being.
I find the tumbling issue mentioned by envy887 to be more concerning, although I take that article with a grain of salt since it doesn't go into much technical detail and some reporters tend to over-sensationalize.
I'm gonna keep this in-line, since it still seems germane to the trajectory and orbit design, which seem on-topic to me.
No, you can't skip off the atmosphere and wind up in an orbit that's so high that you'll run out of consumables before you come back to perigee, as you can from an Apollo-style TEI. But you can get things out of control enough that you exit the atmosphere and wind up reentering again, a few minutes later.
The problem is how you reenter again, at what angle, and with what energy. I'm not competent to figure this out, but just from an arm-wave standpoint, exiting the atmosphere should look an awful lot like an impulsive maneuver near perigee, which should raise the apogee from your last reentry trajectory, albeit with less energy. So: lower perigee, lower apogee, but more eccentricity.
Since we're interested in the no-attitude-control case here, that sounds an awful lot like you're going to burn up on the next entry. At the very least, your angle of incidence will be steeper. Whether that's offset by the fact that you killed some energy the first time through, I can't say. I'll bet I still couldn't say if I had access to the full suite of orbit and reentry design tools, either, because it's an inherently uncertain proposition: Did the guidance get out of control early in the entry, with a lot of energy, or late, when a lot of it had bled off?
NB: While envy's linked news blurb was indeed designed to be general-public dramatic rather than informative for space nerds, and it was talking about a case where there was no automated GN&C at all, the thrusters-out case is exactly the same. If the GN&C can't command the vehicle to change its attitude, it is no longer guidable, even if it's still in control.
None of this improves my confidence that an entry with no attitude control has a high probability of survival. -
#478 by 1 on 03 Jan, 2020 23:47
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I'm gonna keep this in-line, since it still seems germane to the trajectory and orbit design, which seem on-topic to me.
Well, my concern was that this could be more general info related rather than Starliner-specific, but I don't mind continuing here if the mods don't.The problem is how you reenter again, at what angle, and with what energy.
Your angle should be nearly identical, and your energy should be reduced. A capsule has relatively low L/D; it can't drastically change its trajectory in the upper atmosphere. You'll bleed off much more energy than you successfully "reassign", so to speak. More on that below.exiting the atmosphere should look an awful lot like an impulsive maneuver near perigee, which should raise the apogee from your last reentry trajectory, albeit with less energy.
No. Interacting with an atmosphere is an energy dispersive maneuver. It will slow your capsule, and almost certainly lower your apogee. The very idea of brief interactions with the atmosphere forms the foundation for concepts such as aerobraking.that sounds an awful lot like you're going to burn up on the next entry.
Disagree. Sounds an awful lot like you'll be coming in on a ballistic trajectory, likely with no hop, which is what has historically happened to capsules that have lost control.At the very least, your angle of incidence will be steeper.
You can't say this with any confidence. Drastically changing your angle of incidence is akin to drastically changing your inclination. Easy to do from an Apollo-like return trajectory, but very energetically expensive from low orbit or suborbital trajectories. More likely that your new angle of incidence will be extremely close to your original one, as the overwhelming majority of your energy is due to horizontal velocity.If the GN&C can't command the vehicle to change its attitude, it is no longer guidable, even if it's still in control.
My concern with the tumbling isn't guidance. My understanding is that the capsule should be able to return without it; albeit off target. My concern is that tumbling could end up exposing the hatch/sidewalls/(parachutes compartments?) etc to much higher mechanical and thermal loads than they're designed for. Soyuz has partial shielding along the walls and top of the reentry module. I don't know if that's true for Starliner or not, or how much punishment Starliner can take in the wrong orientation.
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#479 by Comga on 04 Jan, 2020 00:07
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I have a serious question:
From the standpoint of the rocket equation, it is inefficient to bring modest Isp fuel, like that for the OMAC engines, using high Isp fuel like the LH2/LOX for Centaur.
Would it be possible to burn, say, a pair of 1,500 lb thrust OMAC engines to augment the 45,000 lb thrust RL-10 pair?
This could get rid of the unwanted fuel mass and save enough prop for the Centaur to do a deorbit burn. (and wind up with the same fuel margin as the reference mission)
Lockheed looked into using the LAS engine on Orion to augment the SLS. However, being solid launch abort engines, they could not be throttled, and the resulting massive thrust would have caused such a change in load paths that enormous structural redesign would have been required. In the end this idea was dropped.
But a pair of OMAC engines would be <7% of the thrust of the DEC, and the load on the Centaur would always remain positive, in compression.
The LAS engines are live, of course, before and during launch, but does this mean the OMAC engines are enabled or disabled? Do they fire during an abort for control or not?
The fuel becomes excess after MECO, where the LAS doesn’t have to start at a standstill, plow through the air at MaxQ, it outrun a nearly empty first stage. Before dropping the aeroskirt seems to be the perfect time to burn it thru the OMACs.
)