Author Topic: Aerobraking Reusable Deep-Space Vehicles  (Read 16637 times)

Offline QuantumG

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Aerobraking Reusable Deep-Space Vehicles
« on: 10/11/2013 10:32 pm »
One of the many good ideas in the 1989 Space Exploration Initiative was the use of aerobraking to bring hardware back to a low Earth orbit space station where it could be checked out and reused. A significant limiting factor to this approach is that single orbit aerobraking is needed if crew are on-board, to prevent multiple passes through the Van Allen radiation belts.

The obvious question is, why keep the crew on-board? If the capsule separates from the reusable deep-space vehicle before aerobraking, it can reenter directly, leaving the empty deep-space vehicle to take as many passes as needed to brake into LEO.

I wonder what the best orbits would be for an architecture like that.

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Offline jongoff

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #1 on: 10/12/2013 05:19 am »
I wonder if you really need to limit it to just one pass through the Van Allen belts though. From what I heard, the dosage for the inbound and outbound pass on Apollo was quite a bit less than the dose from the rest of the mission. I wish I had some hard numbers though. Cause not having to lug around a reentry capsule everywhere would be good, and being able to reuse all of your in-space hardware would also be very very good. The in space hardware was actually a very large fraction of the cost of an Apollo mission, and if you could reuse it even a few times without a huge amount of refurbishment, it might help the costs of missions go down.

~Jon

Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #2 on: 10/12/2013 11:51 am »
I think it depends on how many braking passes are needed.  With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts.  The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.

If you're off beyond the magnetosphere, you're probably going to be getting a sievert or so every few months from GCRs.  So if the number of braking passes can be kept down to ten or so, my guess is that the van Allen Belts aren't going to be a major contributor to the overall dosage for the mission.  The thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.

Plus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs.  So shielding ought to be possible.  If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.

Offline mmeijeri

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #3 on: 10/12/2013 12:22 pm »
I wonder what the best orbits would be for an architecture like that.

Huntress et al proposed this in their exploration study: The Next Steps In Exploring Deep Space

IIRC they were talking about returning to L1/L2. As an incremental step towards this you could also do that propulsively, using just the energy advantage of cycling between high energy orbits and not aerocapture itself, then aerocapture into a high energy orbit and finally aerocapture into LEO. The last option would mean having to lift it back to L1/L2, so that doesn't seem attractive.
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Offline MP99

Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #4 on: 10/12/2013 12:31 pm »
I wonder if you really need to limit it to just one pass through the Van Allen belts though. From what I heard, the dosage for the inbound and outbound pass on Apollo was quite a bit less than the dose from the rest of the mission. I wish I had some hard numbers though. Cause not having to lug around a reentry capsule everywhere would be good, and being able to reuse all of your in-space hardware would also be very very good. The in space hardware was actually a very large fraction of the cost of an Apollo mission, and if you could reuse it even a few times without a huge amount of refurbishment, it might help the costs of missions go down.

If you're staging from EML, then the braking for a minimal capture (high apogee) vs targeting EML is going to be very similar.

Even better if the resulting orbit could target a ballistic / WSB capture direct to EML, but that seems to require an unrealistically precise result.

One thing that had never occurred to me until today ... how there's ~3km/s difference in entry speeds depending whether you enter on the side of the Earth that goes with Earth's rotation, or against it.

Cheers, Martin

Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #5 on: 10/12/2013 12:42 pm »
(40,000 km)/(86,400 s) = 0.46 km/s, so I think the difference is under 1 km/s, depending on latitude, of course.

Offline Hop_David

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #6 on: 10/12/2013 04:12 pm »
I think it depends on how many braking passes are needed.

I whomped up an Excel spreadsheet (attached).

Falling from EML2 and shedding .1 km/s each 200 km perigee, it takes 32 passes and 19 days to lower apogee to 300 km.

Same scenario but shedding .2 km/s each perigee gives 16 passes and about 11 days.

.5 km/s each perigee: 7 passes and about 8 days.

If you choose to shed 3.12 km/s in a single pass, it will take a little more than 6 days for the Hohmann orbit from EML2. If you want to use Farquhar's lower delta V route with a lunar gravity assist, time to first perigee is about 9 days.

Offline jongoff

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #7 on: 10/15/2013 07:32 pm »
I think it depends on how many braking passes are needed.  With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts.  The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.

If you're off beyond the magnetosphere, you're probably going to be getting a sievert or so every few months from GCRs.  So if the number of braking passes can be kept down to ten or so, my guess is that the van Allen Belts aren't going to be a major contributor to the overall dosage for the mission.  The thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.

Plus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs.  So shielding ought to be possible.  If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.

Thanks Proponent,

I was actually curious about this for two reasons. One, I have a depot-departure architecture I've been working on that would involve a few extra Van Allen Belt passes, and I was trying to figure out if this was a big hit to crew safety, or kind of in the noise. The good news is it looks more like the latter than the former.

As for aerocapture/breaking technologies, you need to be able to take enough of a bite off in one pass to go from your hyperbolic orbit into a highly elliptical orbit, which means you probably don't need a ton of passes to go from a HEO to LEO. Probably on the order of 3-4 passes, not dozens or hundreds. Knowing that I can afford to take things in a few passes safely is very good to know.

~Jon

Offline Hop_David

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #8 on: 10/16/2013 01:20 pm »
I think it depends on how many braking passes are needed.  With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts.  The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.

I find the Wikipedia article confusing. They refer to a "safe zone" between 2 and 4 earth radii. But the article's graphics indicate high flux at 3 earth radii for protons more than 1 MeV and at about 1.4 earth radii for protons more than 400 MeV.

It seems to me the 400 MeV or more protons are the biggest concern. The others are easier to shield against. If so, it seems the worst orbit would be one with a 1.4 earth radii apogee.

The thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.

A 6678x15000km orbit would have a period of about 3 hours. A perigee speed of about 9.2 km/s, about 1.3 km/s over a circular orbit.

Goff's saying he wants to circularize over 3 to 4 passes which seems to indicate he's hoping to lose 1 km/s each perigee pass. If so, the astronauts wouldn't have to endure more than 1 or 2 orbits with apogees in bad spots.  With two bad apogees, maybe 3 to 4 hours in high radiation zones. At .1 Sv per hour, that'd be .3 to .4 Sv. The Wikipedia article says 50 mSv is the annual dose set by the U.S. Atomic Energy Commission for people who work with radioactivity. 


Plus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs.  So shielding ought to be possible.  If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.

The 400 MeV protons are more like GCRs.
« Last Edit: 10/16/2013 01:27 pm by Hop_David »

Offline MP99

Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #9 on: 10/17/2013 10:55 am »
(40,000 km)/(86,400 s) = 0.46 km/s, so I think the difference is under 1 km/s, depending on latitude, of course.

Sorry, yes - you're right. Was confusing 1000's of km/h with km/s. 3.6x, of course.

cheers, Martin

Offline simonbp

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #10 on: 10/17/2013 04:05 pm »
Well, if you are bringing crew down from L2, I'm not sure why they wouldn't just directly reenter and land. Non-crew vehicles could take much slower trajectories (i.e. a low delta-v WSB transfer that puts them in a high Earth orbit that can be gradually aerobraked down.
« Last Edit: 10/17/2013 04:05 pm by simonbp »

Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #11 on: 10/17/2013 07:06 pm »
Well, the disadvantage of bringing the crew all the way back to Earth is that then you need to lug a re-entry capsule all the way to Mars (or wherever) and back.  Maybe that would be the easiest thing to do.  On the other hand if you've got cheap transport of humans between Earth and LEO, as we might hope one day to have, then aerobraking into LEO could look more attractive.

Offline QuantumG

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #12 on: 10/17/2013 08:31 pm »
I think you'll want that anyway, for fast aborts.

There's really no need for a lifeboat to be all that heavy.
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Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #13 on: 10/18/2013 11:07 am »
Why couldn't you do a fast abort with aero-braking instead of re-entry?
« Last Edit: 10/18/2013 11:08 am by Proponent »

Offline QuantumG

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #14 on: 10/18/2013 11:13 am »
Just saying it'll always be faster to do direct reentry.
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Offline Joel

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #15 on: 10/18/2013 11:27 am »
What is the fundamental reason why multiple passes through the atmosphere is necessary in the first place? Is it the heat load, limits of the actuators, limits on the control algorithms, something else?

Offline QuantumG

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #16 on: 10/18/2013 11:33 am »
The more energy you want to dissipate, the deeper into the atmosphere you have to go. At some point you go so deep you don't come back out.
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Offline Joel

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #17 on: 10/18/2013 11:50 am »

The more energy you want to dissipate, the deeper into the atmosphere you have to go. At some point you go so deep you don't come back out.


Now that's what I don't understand. Can't you dissipate more delta-v by spending more time in the atmosphere - by controlling the L/D - instead of diving deeper into it?

Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #18 on: 10/18/2013 02:55 pm »
What is the fundamental reason why multiple passes through the atmosphere is necessary in the first place? Is it the heat load, limits of the actuators, limits on the control algorithms, something else?

Actually, I think we're getting little confused here (and it's partly my fault).  We started by talking about aerobraking, which means, as I understanding it, is using the atmosphere to slow down without re-entering.  Then we got on to the topic of, let's call it magnetospheric braking, which more or less means use of the earth's magnetic field to brake.  One of the good things about magnetospheric braking is that you can turn it off anytime you want to, regardless of altitude, and you can do it at relatively high altitudes.  You could, for example, use a perigee of 400 km for all magnetosphereic braking passes and settle in to a 400-km circular orbit without burning any fuel at all (or hardly any).  To get to 400 km circular with aerobraking, on the other hand, you'd have to use a lower perigee for aerobraking, and then propulsively raise your perigee  to 400 km.

You could do all your aerobraking in one pass, but then your spacecraft is going to need a pretty hefty heat shield.

As for using the L/D ratio, I'm sure it's possible, but now you need not just a craft capable of handling a lot of aero heating, but one with a lot of lift.  You might need wings, for example.  Do you really want to lug those around solar system?
« Last Edit: 10/18/2013 03:56 pm by Proponent »

Offline Proponent

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Re: Aerobraking Reusable Deep-Space Vehicles
« Reply #19 on: 10/18/2013 03:09 pm »
Just saying it'll always be faster to do direct reentry.

True, but the difference in time to return is a couple of days at most, if we can manage 1 km/s or so of deceleration on each pass.  On an interplanetary mission, a "fast" abort is likely to take weeks anyway.
« Last Edit: 10/18/2013 03:11 pm by Proponent »

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