Author Topic: BFS and 3-stage re-entry for Earth return  (Read 3170 times)

Offline alexterrell

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BFS and 3-stage re-entry for Earth return
« on: 03/08/2018 04:59 pm »
It may be that return from Mars to Earth Surface imposes significant engineering challenges on the heat shield, which are not experienced by return from Low Earth Orbit.

Assuming a "delta-V" reduction of 8km/s for a return from LEO, we get the following approximate specific energies that need to be dissipated by the re-entry process:

From             Delta V (km/s)   E (MJ/kg)
Mars Transfer     11.8          69.62
GTO                    10.5          55.125
LEO                      8             32

Would a three step re-entry be effective:
1. Aerocapture on return with a target orbit of something like a GTO. Go too high, and the orbit takes too long, but a GTO would give about about 10.5 hours for the heat shield to cool. This process sheds 14.5MJ/kg.
2. Another aerocapture at perigee, aiming to lower orbit to something akin to LEO. This process sheds 23MJ/kg. There may be an option at this point to have an apogee burn and to remain in Low Earth Orbit.
3. Finally, reentry from LEO, to shed 32MJ/kg. (Assuming the landing burns don't change this too much).

Before all this, the BFS will have shed at least 20.5 MJ/kg on landing at Mars. Given at that point it will have a full payload, the heat shield load could even be higher, so it might be worth doing the same at Mars. But as far as I'm aware, no one has attempted Aero-capture at Mars.

Would this be a sensible approach, to enable the heat shield to be designed for 32MJ/kg (plus margin) at a time? From reading about Skylon and Dragon, I get the impression that a LEO (32MJ/kg) reentry is a lot easier to design for than a 70MJ/kg re-entry.

Offline speedevil

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Re: BFS and 3-stage re-entry for Earth return
« Reply #1 on: 03/08/2018 05:24 pm »
It may be that return from Mars to Earth Surface imposes significant engineering challenges on the heat shield, which are not experienced by return from Low Earth Orbit.

In principle, yes, this might work - it depends on your assumptions on heat-shield performance.
Hypersonic aerodynamics are hard.
There are other related options - for example, topping up the tank in LEO moderately helps because you now have a 20 ton lighter craft, which can even kill 500m/s with the fuel it would have otherwise used landing.
Refuelling in GTO would let you kill a _lot_ of incoming velocity, if your heatsheild was somewhat damaged.

If you actually believe fuel is cheap, and launches are reliable, approaches all the way up to 'catch unmanoevering tank containing crew' are plausible.
It was specifically said at IAC that earth entry will wear the heatshield.
« Last Edit: 03/08/2018 05:28 pm by speedevil »

Offline alexterrell

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Re: BFS and 3-stage re-entry for Earth return
« Reply #2 on: 03/09/2018 09:46 am »
In a Q&A Musk said:
Quote
The propellant tanks need to be cylindrical to be remotely mass efficient and they have to carry ascent load, so lowest mass solution is just to mount the heat shield plates directly to the tank wall

So one issue might be that after a reentry, heat will transfer from the heat shield to the tank, and start to boil the cryogenic fuel. That is not a problem if by the time the heat comes through the vehicle is landed on Mars or Earth. But it could be a problem for an aerocapture manoeuvre.

Some active cooling of the inside of the heat shield could negate this issue.

Offline livingjw

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Re: BFS and 3-stage re-entry for Earth return
« Reply #3 on: 03/09/2018 10:58 am »
There won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.

John

Offline jpo234

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Re: BFS and 3-stage re-entry for Earth return
« Reply #4 on: 03/09/2018 11:28 am »
There won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.

John

This is what Elon said during the latest Reddit AMA:
Quote
The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline robert_d

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Re: BFS and 3-stage re-entry for Earth return
« Reply #5 on: 03/09/2018 12:52 pm »
It may be that return from Mars to Earth Surface imposes significant engineering challenges on the heat shield, which are not experienced by return from Low Earth Orbit.



I always wondered about this regarding a sustainable Moon architecture and the Space Shuttle. The two main issues are that crewed vehicles are almost insanely complex and expensive versus uncrewed; thus failure to re-use would be nearly criminal. And two, there may be a need to return substantial payload to Earth or at least LEO from the Moon or Mars, making re-entry harder. At the time I was thinking that at least the ascent stage of a lunar lander could be returned to LEO for refurbishment. Back then it was the Shuttle tile system, and whether a vehicle could use it in either a two stage re-entry or aerobraking into some useful LEO. Are there issues with multiple passes through the Van Allen belts? Where is the breakeven vs propulsive maneuvers? Does lunar (or Martian) ISRU fundamentally change the costs? None of this makes sense in a capsule based return system because of the short duration the capsule alone can sustain. But a full-up spaceship can stay up long enough to shed the heat of the first pass. Does the concept point to a true 'space station/space dock' where a Mars return vehicle would be refit and resupplied without ever returning to the surface? Are fueling and Human sustainment incompatible in the same station?
   Amazing though that these issues are in serious play for future architectures.   
« Last Edit: 03/09/2018 05:40 pm by robert_d »

Offline livingjw

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Re: BFS and 3-stage re-entry for Earth return
« Reply #6 on: 03/09/2018 08:31 pm »
There won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.

John

This is what Elon said during the latest Reddit AMA:
Quote
The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.

I took his statement to mean during the 3-4 month coast period after TMI. I doubt having zero tank pressure during reentry would be structurally stable. Tanks can be re-pressurized prior to Mars atmosphere entry.

John

Offline jpo234

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Re: BFS and 3-stage re-entry for Earth return
« Reply #7 on: 03/09/2018 09:06 pm »
There won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.

John

This is what Elon said during the latest Reddit AMA:
Quote
The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.

I took his statement to mean during the 3-4 month coast period after TMI. I doubt having zero tank pressure during reentry would be structurally stable. Tanks can be re-pressurized prior to Mars atmosphere entry.

John

I agree. Just wanted to add context.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline Slarty1080

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Re: BFS and 3-stage re-entry for Earth return
« Reply #8 on: 03/23/2018 02:04 pm »
I’m not knowledgable on re-entry dynamics but isn’t there some half way house between a high g deep re-entry and a low g shallow re-entry? As the BFS can generate lift as well as drag can’t it “bob” (for want of a better word) in the top layer of the atmosphere? When it’s too high and in danger of leaving the atmosphere – increase the angle of attack to increase drag and decrease lift. When it’s too low and g forces are getting too high – decrease the angle of attack to reduce the drag and increase lift…

Just curious, is this just wrong from a physics perspective, is it in the too difficult/dangerous box or is it doable? I think the plan was to do something vaguely similar at Mars if I remember the video correctly.
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Offline speedevil

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Re: BFS and 3-stage re-entry for Earth return
« Reply #9 on: 03/23/2018 02:12 pm »
I’m not knowledgable on re-entry dynamics but isn’t there some half way house between a high g deep re-entry and a low g shallow re-entry? As the BFS can generate lift as well as drag can’t it “bob” (for want of a better word) in the top layer of the atmosphere? When it’s too high and in danger of leaving the atmosphere – increase the angle of attack to increase drag and decrease lift. When it’s too low and g forces are getting too high – decrease the angle of attack to reduce the drag and increase lift…

BFS is a terrible, terrible glider.

Yes, you can generate lift, but only at the expense of very considerable drag.
This means that 'fancy' manoevering at the edges of the atmosphere are limited to the time before you slow down enough that the effective gravity left over after you take off the effect of the orbit you would be in without the atmosphere gets high enough that you stall.

It can do a very shallow reentry up until you get to perhaps 5km/s or so, at which point it's coming in nomatter what you do.

Offline Slarty1080

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Re: BFS and 3-stage re-entry for Earth return
« Reply #10 on: 03/23/2018 06:53 pm »
I’m not knowledgable on re-entry dynamics but isn’t there some half way house between a high g deep re-entry and a low g shallow re-entry? As the BFS can generate lift as well as drag can’t it “bob” (for want of a better word) in the top layer of the atmosphere? When it’s too high and in danger of leaving the atmosphere – increase the angle of attack to increase drag and decrease lift. When it’s too low and g forces are getting too high – decrease the angle of attack to reduce the drag and increase lift…

BFS is a terrible, terrible glider.

Yes, you can generate lift, but only at the expense of very considerable drag.
This means that 'fancy' manoevering at the edges of the atmosphere are limited to the time before you slow down enough that the effective gravity left over after you take off the effect of the orbit you would be in without the atmosphere gets high enough that you stall.

It can do a very shallow reentry up until you get to perhaps 5km/s or so, at which point it's coming in nomatter what you do.

Sounds like a plan. Do a very shallow 5-10min reentry with fancy manoevering to get from 11km/s down to 5km/s or so and then just re-enter. If this is possible why do people worry about coming in at too high a speed? Is it that the "fancy manoevering" is in the too difficult/dangerous pile?
My optimistic hope is that it will become cool to really think about things... rather than just doing reactive bullsh*t based on no knowledge (Brian Cox)

Offline speedevil

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Re: BFS and 3-stage re-entry for Earth return
« Reply #11 on: 03/23/2018 07:03 pm »
Sounds like a plan. Do a very shallow 5-10min reentry with fancy manoevering to get from 11km/s down to 5km/s or so and then just re-enter. If this is possible why do people worry about coming in at too high a speed? Is it that the "fancy manoevering" is in the too difficult/dangerous pile?

It is technically problematic for reasons skipped over above.

A slower reentry may mean more of the heat from reentry goes into your vehicle, not into heating air.
It may mean that your insulation fails, or has to be thicker, as the heat has gotten underneath it with the prolonged heating.

It requires very good knowledge of your vehicle aerodynamics, which is much less of a problem admittedly now than back when the shuttle was about to fly.

It needs very good knowledge of the atmosphere to plan your trajectory, which is not a problem for earth, but may be a problem in some other cases.

Exact reentries, and which is best depend on details of the heat shielding, shape of the vehicle, atmosphere you're going into, safety margins, if you plan to reuse your craft, ...


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