Author Topic: Apollo and aerocapture  (Read 6129 times)

Offline jhoblik

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Apollo and aerocapture
« on: 05/08/2009 05:32 am »
I read today Popular mechanics article about Apollo 11. I was surprise about how they slow aircraft on the way back to Earth, it looks like aerocapture.
Kraft: .. because velocity was so high 30000 feet/s... we did get them into the atmosphere, skip it out to kill some velocity, and then bring it back again......
Description sounds like aerocapture. Is it true I never heard about it.

Offline Ben the Space Brit

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Re: Apollo and aerocapture
« Reply #1 on: 05/08/2009 09:12 am »
It came as a surprise to me when I first learnt of it a few years back. 

I was a bit of a HSF amazing people in my misspent youth and read a lot of Project Apollo-era NASA publicity materials (collected by my mother, who was around at the time).  Of course it was PAO fluff but it was still technically involved enough that it surprises me that there was no mention of a 'skip' re-entry profile.  The impression that was given was a direct re-entry from return orbit.  Of course, most of it dated from around the era of Project Gemini.  I still laugh at the silver 'accordian joint' spacesuits that they presented as the Project Apollo EVA suits; Hell, a lot of the artwork got the shape of the Saturn-V wrong - Apollo CSM as wide as the S-IVB! So, it is possible that lots of the details hadn't been ironed out by that point.

The big difference between skip and aerocapture was that this manoeuvre was just to burn off sufficient energy to get the forces and heat experienced during re-entry down to survivable levels.  Aerobraking, in other words. 

True aerocapture is fine-tuned as far as possible to put the vehicle into something resembling an orbit.  I know that this is used by unmanned probes to Mars, has been proposed for a Titan orbiter and has also been proposed by the DIRECT team for their manned Mars misison profile.
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Offline ugordan

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Re: Apollo and aerocapture
« Reply #2 on: 05/08/2009 09:23 am »
True aerocapture is fine-tuned as far as possible to put the vehicle into something resembling an orbit.  I know that this is used by unmanned probes to Mars, has been proposed for a Titan orbiter and has also been proposed by the DIRECT team for their manned Mars misison profile.

Aerocapture is not used by Mars probes or anywhere else at this point. What recent Mars orbiters do is use aerobraking to slowly bleed off eccentricity of the initial capture orbit obtained by a propulsive engine burn.

Aerocapture you speak of would imply direct insertion into an orbit, which would conceivably be raised in periapsis at the first apoapsis and indeed there were proposals for Titan, Neptune, etc. missions.
« Last Edit: 05/08/2009 10:07 am by ugordan »

Offline Xentry

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Re: Apollo and aerocapture
« Reply #3 on: 05/08/2009 10:17 am »
I read today Popular mechanics article about Apollo 11. I was surprise about how they slow aircraft on the way back to Earth, it looks like aerocapture.
Kraft: .. because velocity was so high 30000 feet/s... we did get them into the atmosphere, skip it out to kill some velocity, and then bring it back again......
Description sounds like aerocapture. Is it true I never heard about it.

The Apollo Entry Guidance was designed to perform both direct and skip-entries (but not aerocapture). However, the entry modes related to skip entry were really never used - this is because, especially in really long skip entries, serious range prediction errors would occur, originating from the simplifications made to the equations of motion so that the algorithm could be stored onboard the Apollo computer. Also, the control method they use is not really suited to achieve the proper exit conditions in the first entry.
Due to these problems, both the guys at NASA JSC and at Draper Laboratory (who designed the original guidance) have modified part of the original Apollo algorithm to include precise numerical propagation methods, which will be used in the Orion vehicle, and allow really long skip entries (>10000km).
Still, as far as I know there is no plan to include an aerocapture algorithm in the Orion guidance.

Offline Patchouli

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Re: Apollo and aerocapture
« Reply #4 on: 05/12/2009 03:11 am »
OT but wasn't aerocapture also proposed for lunar shuttles or ferrys?

It seems aerocapture could in theory also be used to allow vehicles incapable of a direct lunar reentry such as those equipped with a thermal soak/radiative type heat shields to preform a lunar mission.

Though it seems only a vehicle with a fair amount of built in delta V can perform aerocapture since it seems the perigee would have to be raised after capture.

The Apollo CM couldn't do it no translational control the capture will just be a long skip but an HL-20 or DCY could.

I do have a sim of the ESAS CEV and it has translational control in just the command module but I doubt the real Orion has this.
« Last Edit: 05/12/2009 03:19 am by Patchouli »

Offline Spacenick

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Re: Apollo and aerocapture
« Reply #5 on: 05/12/2009 06:31 pm »
Hmm I think aerocapture doesn't make much sense for a capsule atleast not for one with a service module as it is pretty incapabel without it, on the other hand a vehicle like HL-20 could benefit from aerocapture a lot provided this technique can be made reliable enough.

I wonder whether an HL-20 could actually perform a lunar return by utilizing skip reentry or even aerocapture...

« Last Edit: 05/12/2009 06:33 pm by Spacenick »

Offline StarStuff

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Re: Apollo and aerocapture
« Reply #6 on: 05/18/2009 06:04 pm »
Apollo 11 did a skip entry to land beyond the bad weather at the original landing site. Collins talked about it during the debriefing. The AGC apparently used the P65 and P66 programs that were not used by a non-skip entry. No record of trajectory data was made, and that may be part of the reason why many say the skip entry was never performed by Apollo.

They didn't like using the AGC for primary guidance with only a limited backup capability on-board to manually fly the skip and without communications with ground stations at that phase of the flight.

It's not just that new algorithms are better. It's also that modern spacecraft can carry redundant backup computers and they can communicate via satellites.

Offline Jim

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Re: Apollo and aerocapture
« Reply #7 on: 05/18/2009 06:09 pm »
Skip and extending the entry are two different things

Offline StarStuff

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Re: Apollo and aerocapture
« Reply #8 on: 05/18/2009 06:49 pm »
They extended over 400 miles which must have included up-control and Keppler phases in the trajectory. That is not a skip?


Offline DaveJes1979

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Re: Apollo and aerocapture
« Reply #9 on: 05/18/2009 09:20 pm »
They extended over 400 miles which must have included up-control and Keppler phases in the trajectory. That is not a skip?



400 miles is well within the downrange capability of any reentry vehicle with Apollo's hypersonic L/D. 

Apollo's reentry profile is classified as a 'double-dip' reentry, which is best thought of as an aborted skip trajectory.  My old Spacecraft Dynamics textbook (Wiesel) puts it like this: 

The first phase of the reentry was flown with the life vector up and was a skip trajectory...the Apollo capsule would have left the atmosphere still above escape speed if the skip have been flown to completion....The solution is to roll the vehicle over so the lift bector points down, thus using the lift to help hold the vehicle in the atmosphere while it completes its deceleration.

Offline StarStuff

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Re: Apollo and aerocapture
« Reply #10 on: 05/18/2009 10:37 pm »
I'm defining skip in terms of the AGC programs. Going directly from P64 to P67 is a non-skip entry. The sequence P64-P65-P66-P67 is a skip entry.

Publications from Draper Labs define skip as a lofting where the spacecraft gains altitude, it does not have to leave the atmosphere to be a skip.

Both definitions seem to be consistent.

Collins said in the debriefing that they went 1500 miles downrange which I interpret to be the low end of a skip entry. He spoke of a roller coaster EMS trace: "It really climbed for altitude after the initial pulse and hung way up there high."

So I'm assuming the AGC was in it's skip mode. And that is what contradicts all the people, including Draper Labs, who said the AGC skip mode was never used.

So I'm still confused by it all.

Offline Jorge

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Re: Apollo and aerocapture
« Reply #11 on: 05/18/2009 10:43 pm »
I'm defining skip in terms of the AGC programs. Going directly from P64 to P67 is a non-skip entry. The sequence P64-P65-P66-P67 is a skip entry.

Publications from Draper Labs define skip as a lofting where the spacecraft gains altitude, it does not have to leave the atmosphere to be a skip.

Both definitions seem to be consistent.

Collins said in the debriefing that they went 1500 miles downrange which I interpret to be the low end of a skip entry. He spoke of a roller coaster EMS trace: "It really climbed for altitude after the initial pulse and hung way up there high."

So I'm assuming the AGC was in it's skip mode. And that is what contradicts all the people, including Draper Labs, who said the AGC skip mode was never used.

So I'm still confused by it all.


The Apollo 11 mission report states that upcontrol (P65) was used but makes no mention of kepler (P66). That would be the case if, as Dave says, A11 did a double-dip ("aborted skip"). It appears that some sources don't consider it a "skip" unless there's a kepler phase in there.
JRF

Offline StarStuff

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Re: Apollo and aerocapture
« Reply #12 on: 05/18/2009 11:33 pm »
OK. Now I get it!

The Kepler phase was referred to as the skipout-type trajectory. Kepler was only required for downrange greater than 1800 nautical miles. Without Kepler, the up-control lofting was minimal and range was about 1400 to 1800 nautical miles. (Those numbers are based on the Apollo 11 speed and flight-path angle.)

In order to be a skip trajectory, the load factor has to drop below 0.2 g which results in a double entry.
« Last Edit: 05/18/2009 11:58 pm by StarStuff »

Offline Patchouli

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Re: Apollo and aerocapture
« Reply #13 on: 05/18/2009 11:39 pm »
Hmm I think aerocapture doesn't make much sense for a capsule atleast not for one with a service module as it is pretty incapabel without it, on the other hand a vehicle like HL-20 could benefit from aerocapture a lot provided this technique can be made reliable enough.

I wonder whether an HL-20 could actually perform a lunar return by utilizing skip reentry or even aerocapture...



Most capsules have no translational control without their service module Dragon is unique in that most of it's service module is part of the reentry vehicle.
Plus they also dump their radiators with the SM though you probably could use a sublimator as backup cooling.

As for the Hl20 performing a lunar mission with aerocapture it's probably possible a lifting body was seriously considered for both Apollo and the CEV.

What type of profile and how long the Kepler element likely would depend on loading and TPS material.

You would need to give the TPS enough time to reject part of the heat load from the first skip if it's a thermo soak TPS.
If it's an ablative I guess it would have it's own issues.
I read that the old carbon phenolic supposedly did not like long duration low level heat pulses due to it going out of ablation and getting too much thermo soak.
I'm not 100% sure but I think many ablatives don't like getting cold they can crack which was a big worry with Apollo 13 if they dumped it's dead service module.
Not sure if PICA has any issues like that.
« Last Edit: 05/18/2009 11:49 pm by Patchouli »

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