Author Topic: Back-up plan for Apollo  (Read 16805 times)

Offline Glom

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Re: Back-up plan for Apollo
« Reply #15 on: April 07, 2012, 10:23:28 AM »
It's hard to imagine after 30 years of Space Shuttle that a launch vehicle could lose an engine and the missions just shrugs it off.

Offline raven

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Re: Back-up plan for Apollo
« Reply #16 on: April 07, 2012, 11:46:39 AM »
It helps that it was a centre engine I imagine, correct me if I am wrong, so there was no loss of symmetry of thrust. An outer engine failure would have created some massive torque.

Offline Donnie B.

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Re: Back-up plan for Apollo
« Reply #17 on: April 07, 2012, 06:23:04 PM »
It helps that it was a centre engine I imagine, correct me if I am wrong, so there was no loss of symmetry of thrust. An outer engine failure would have created some massive torque.
The S-II J2 failures on Apollo 6 were two adjacent outer engines.  The ICU still managed to get the S-IVB into orbit... in a most unconventional manner.  It was actually thrusting backwards at engine cutoff.

(I hope I'm remembering this correctly -- it's been a while since I read up on it.)

Though that mission was a "partial success", it was only such because it was unmanned.  If it had been crewed it would certainly have ended in an abort.


Offline Donnie B.

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Re: Thought experiment for an ultra-cheap earth escape trajectory
« Reply #18 on: April 07, 2012, 06:25:56 PM »
What this really says is that the most energy-efficient way to escape the earth is on a hyperbolic trajectory with a perigee as close to the earth's center as possible. Since, sadly, the real earth is unreasonably dense even if not completely solid, you will have to fly a less energy-efficient trajectory from your surface launch site to your departure hyperbola, joining it at the first post-perigee point above the amosphere. Your subterranean perigee is then of no consequence since you won't be coming back to fly through it. But if you are required to complete one or more parking orbits before departure, then a subterranean perigee is a definite problem. You'll have to spend some of your launcher capability on raising it to a safe level -- capability you would have preferred to spend on raising your apogee.

Clearly, the solution is to convert yourself into neutrinos, and launch yourself right though the Earth.

Offline Donnie B.

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Re: Back-up plan for Apollo
« Reply #19 on: April 07, 2012, 06:37:22 PM »
The mission itself would have been quite a challenge. If the Saturn V were launched first, its S-IVB would have to wait some time for the the Saturn IB to bring the crew.

This was addressed in the backup plan proposal.  By removing the LES, the S-V could carry extra cryogenics.  This increased the endurance to 10 hours in orbit.

That's still a tight timeline, though, which suggests that they might have chosen to launch the crew ferry mission first.  That had some extra mission risk -- if you launch the lunar stack first and it fails, you don't launch the crew at all.  Of course, if you launch the crew first and then lose the S-V you can still do an orbital mission of some kind; and if the S-V launch succeeds your timing requirements are more relaxed.

It's a wild thought, though -- imagine a manned liftoff from one pad while the big Saturn was already sitting on a nearby pad, all set to go (or vice versa).  Shades of (dare I say it) Armageddon!

Offline ka9q

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Re: Thought experiment for an ultra-cheap earth escape trajectory
« Reply #20 on: April 07, 2012, 08:13:17 PM »
Clearly, the solution is to convert yourself into neutrinos, and launch yourself right though the Earth.
Sounds good at first, but it won't really work even though, as far as we know, at least some neutrinos have rest mass and are subject to gravity. My model put all the earth's mass at its center so that you could fall down an enormous gravity well, pick up a large velocity, and then exploit the Oberth effect with a puny rocket. What seems to be "something for nothing" really comes from having carried a chunk of propellant deep into a gravity hole and dropping it there so I don't have to carry it back out. I get its released gravitational potential energy.

But the real earth distributes its mass throughout its interior. If the mass were uniformly distributed, gravity would fall linearly as you approach the center instead of increasing with the inverse square of your distance to a hypothetical black hole at the center.

In reality the earth's density is not uniform; it has liquid and solid iron cores that are even denser than iron on the surface because they're under extreme compression. So gravity actually reaches a peak at the surface of the outer core of 10.62 m/s^2 (about 1.08g) before dropping to zero at the center. But this still wouldn't give the dramatic effect of my black hole at the center.

Offline ka9q

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Re: Back-up plan for Apollo
« Reply #21 on: April 07, 2012, 08:16:25 PM »
It's a wild thought, though -- imagine a manned liftoff from one pad while the big Saturn was already sitting on a nearby pad, all set to go (or vice versa).  Shades of (dare I say it) Armageddon!
Or Skylab...?

Offline ka9q

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Re: Back-up plan for Apollo
« Reply #22 on: April 07, 2012, 08:20:51 PM »
It helps that it was a centre engine I imagine, correct me if I am wrong, so there was no loss of symmetry of thrust. An outer engine failure would have created some massive torque.
As long as the remaining engines can generate net thrust through the center of gravity, there's no problem. That means having gimbals that go far enough. This would increase the angle of attack, and that might be a problem during atmospheric flight but not in vacuum.

As I recall, at least some of the Saturn V launches canted the four outer F-1 engines outward slightly after tower clear to improve the IU's ability to compensate were one of the engines to fail. Since liftoff acceleration was barely greater than 1 g I don't think the Saturn could tolerate an engine failure immediately after liftoff, but if it happened later when acceleration had increased and less burn time would be lost on the failed engine, it might well have been able to compensate. I don't know what the angle of attack would be, this is absolutely critical in the max-Q region but you get out of that as quickly as you enter.

Edit to add: Remember the IU deliberately shut down the inboard engine before the four outboard engines. I'm sure the IU was programmed so that if an engine had already failed, the IU would not shut down another one, and this would probably compensate for much of the lost performance. Maybe even completely compensate if the engine failure happens late enough.

« Last Edit: April 07, 2012, 08:23:19 PM by ka9q »

Offline Donnie B.

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Re: Thought experiment for an ultra-cheap earth escape trajectory
« Reply #23 on: April 07, 2012, 08:22:06 PM »
Clearly, the solution is to convert yourself into neutrinos, and launch yourself right though the Earth.
... But this still wouldn't give the dramatic effect of my black hole at the center.

Spoilsport.  I guess I'll have to find a different application for my Reversible Neutrinoizer.

Offline raven

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Re: Back-up plan for Apollo
« Reply #24 on: April 07, 2012, 10:46:13 PM »
Thanks everyone for the replies on my statement. I learned another bit of Apollo history. Thanks!

Offline Glom

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Re: Re: Back-up plan for Apollo
« Reply #25 on: April 08, 2012, 03:24:55 AM »
It's a wild thought, though -- imagine a manned liftoff from one pad while the big Saturn was already sitting on a nearby pad, all set to go (or vice versa).  Shades of (dare I say it) Armageddon!
Or Skylab...?

Thank you. And Skylab had the sense to not put the two space vehicles right next to each other.

Offline Bob B.

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Re: Back-up plan for Apollo
« Reply #26 on: April 09, 2012, 02:26:20 PM »
In reality the earth's density is not uniform; it has liquid and solid iron cores that are even denser than iron on the surface because they're under extreme compression. So gravity actually reaches a peak at the surface of the outer core of 10.62 m/s^2 (about 1.08g) before dropping to zero at the center. But this still wouldn't give the dramatic effect of my black hole at the center.

Where did you find that 10.62 m/s2 number?  A couple years ago I modelled Earth's interior to estimate gravity versus depth and I arrived at a number very close to that right at the surface of the outer iron core.  I didn't expect such a result when I started, so it's good to see data that appears to confirm my numbers.



Here's the original thread in which this was discussed:
http://apollohoax.proboards.com/index.cgi?board=theories&action=display&thread=2964&page=3
« Last Edit: April 09, 2012, 02:30:43 PM by Bob B. »

Offline ka9q

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Re: Back-up plan for Apollo
« Reply #27 on: April 11, 2012, 07:24:08 AM »
Where did you find that 10.62 m/s2 number?
Via Google, of course. I didn't hang onto the link, so if you published it on the web, your result might have been the one that I found. So don't use it as confirmation. :-)

I do remember the person did a fairly straightforward if brute-force numerical integration, working through the earth as a series of concentric shells, each shell having a uniform composition that changed with depth. I had remembered seeing something like this result a while ago, but I wasn't sure of the actual acceleration value.
« Last Edit: April 11, 2012, 07:29:36 AM by ka9q »

Offline ka9q

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Re: Back-up plan for Apollo
« Reply #28 on: April 11, 2012, 07:28:07 AM »
The ICU still managed to get the S-IVB into orbit... in a most unconventional manner.  It was actually thrusting backwards at engine cutoff.
Are you sure about that? It's hard for me to conceive of a reason why the IU would flip the stack around. That would only happen if there had been an overspeed, but this was an underspeed.

I do remember reading that in the LM guidance equations it was possible for the LM to invert during descent if one of the phases (probably the braking phase) had been carried too far, because it was targeted for a specific point in space and normally switched to the next program (probably the approach phase) before actually reaching it. Remember, computers (like many humans) lack any notion of common sense.


Offline Donnie B.

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Re: Back-up plan for Apollo
« Reply #29 on: April 11, 2012, 09:05:33 PM »
The ICU still managed to get the S-IVB into orbit... in a most unconventional manner.  It was actually thrusting backwards at engine cutoff.
Are you sure about that? It's hard for me to conceive of a reason why the IU would flip the stack around. That would only happen if there had been an overspeed, but this was an underspeed.

You made me go and look it up!

This is regarding Apollo 6, the second all-up unmanned test of a Saturn V.  The quote is taken from Apollo: The Race To the Moon by Murray and Cox, one of my favorite references -- it's more focused on the flight controller side of things than the astronauts', and has an excellent chapter on A13 (among many other things).

Quote
After the two [S-II] engines had gone out, the vehicle had maintained a pitched-up attitude known as "chi-freeze" for far longer than it would have under ordinary circumstances.  "Well, the S-IVB lit up," [FIDO Jay] Greene recalled, "and the first thing it said was, 'Omigod, I've got too much altitude.'  And so it pointed its nose straight at the center of the earth."  This battle between the guidance system and the gimbal limits on the engine continued for about eighty seconds, with Greene getting closer and closer to calling an abort of his own.  When the S-IVB finally gave up trying to get to the altitude it wanted, it had a flight-path angle that was unacceptably low.  "So then the little devil said, 'Well, this is bad, I've got to pick up the flight-path angle,' so it started pitching up, and as it started pitching up it said, 'Now I'm overspeed,' so it actually went into orbit thrusting backward."

Nothing like a little IU anthropomorphizing to brighten a FIDO's day.
« Last Edit: April 11, 2012, 09:10:18 PM by Donnie B. »