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Starship return payload capability
by
punder
on 24 Oct, 2020 17:38
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Apologies if already covered somewhere.
Starship probably has to be pretty light to avoid excessive heating on reentry, and the tanks are probably almost empty before the landing burn. So, with cargo in the nose, the CG must be shifted forward, and the more cargo onboard, the bigger the forward shift. I’m guessing this places limits on return payload mass, possibly much lower than the max payload to orbit.
Any ideas on what the limits might be?
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#1
by
Hauerg
on 24 Oct, 2020 17:41
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Unly number for return payload I ever read was „25t“ for a return flight from Mars.
But that was before they switched to steel IIRC.
Just ask Elon on twitter.
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#2
by
eriblo
on 24 Oct, 2020 19:09
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Two reasons to expect that Starship will be able to land with maximum payload:
1) They want to land the maximum payload possible on Mars (similar but not identical entry, more landing propellant).
2) If they can not land with full payload any mission with a sufficiently heavy payload that fails to deploy automatically becomes a total loss of payload and vehicle (unless it is in a stable orbit and you send a separate AAA Space Division Starship).
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#3
by
punder
on 24 Oct, 2020 19:26
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I would expect a Mars landing to be dominated, in terms of control, by RCS. But an Earth bellyflop is very much an aerodynamic thing. So CG has to matter. Of course I might be wrong!
That’s a good idea to just ask Elon on Twitter.
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#4
by
cdebuhr
on 24 Oct, 2020 20:06
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While is it obviously important to have sufficient attitude control authority during EDL, my understanding (FWIW, which might not be much) is that max payload down mass is more constrained by what the TPS can take. So to ask a complete question, you also need to specify both where you're landing and what the entry profile would be. A handful of options ...
Landing on Earth (in order of increasing difficulty):
1) LEO -> EDL - I think this was mentioned to have a max down mass of 50t, but don't ask where or when I heard that.
2) HEEO -> EDL - this should have a lower down mass capacity due to tighter margins on the TPS
3) LLO -> EDL - I guess this is really just a special case of #2
4) Mars-Earth slow transfer (MEST) -> HEEO via aero-capture > #2 (+/- additional aero-braking passes)
5) Mars-Earth fast transfer (MEFT) -> HEEO via aero-capture > #2 (+/- additional aero-braking passes)
6) MEST -> EDL
7) MEFT -> EDL
#7 is probably the worst case scenario for Starship landing on Earth, and may exceed TPS limits even with no down mass. This may be true of #6 as well. Note that once you're in HEEO, you can use as many aero-braking passes as you please to drop to LEO, but aero-capture is fundamentally a 1-pass operation. I seem to recall someone around here running the numbers and showing that aero-capture to HEEO from Mars is at least as hard on the TPS as LEO -> EDL.
A SImilar list applies to Mars landings:
1) LMO -> EDL
2) HEMO -> EDL
etc...
My recollection (I think from an official Elon statement at some time or other) is that Starship has a base specification of being able to land 100t on the surface of Mars. I believe this was assuming aero-capture into HEMO prior to EDL, specifically with the intent of staying within the margins of the TPS.
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#5
by
eriblo
on 24 Oct, 2020 23:15
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1) LEO -> EDL - I think this was mentioned to have a max down mass of 50t, but don't ask where or when I heard that.
I think the 50 t comes from "Typical return payload 50 t" in a slide from the 2017 presentation regarding the carbon fibre delta wing no flaps header tanks in aft methane tank BFR...
So somewhere around early bronze age level of relevance.
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#6
by
punder
on 24 Oct, 2020 23:22
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I tweeted the question to Elon.
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#7
by
cdebuhr
on 24 Oct, 2020 23:22
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1) LEO -> EDL - I think this was mentioned to have a max down mass of 50t, but don't ask where or when I heard that.
I think the 50 t comes from "Typical return payload 50 t" in a slide from the 2017 presentation regarding the carbon fibre delta wing no flaps header tanks in aft methane tank BFR...
So somewhere around early bronze age level of relevance.
Roger that - I think you might be right ... but I fall back on my standard "don't ask where or when" disclaimer!
So with respect to current down mass to Earth capabilities, My official position is "damned if I know". That said, I'd be surprised to see Elon back off anytime soon from the goal of 100t payload landed on Mars with each vessel.
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#8
by
equiserre
on 24 Oct, 2020 23:47
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Apologies if already covered somewhere.
Starship probably has to be pretty light to avoid excessive heating on reentry, and the tanks are probably almost empty before the landing burn. So, with cargo in the nose, the CG must be shifted forward, and the more cargo onboard, the bigger the forward shift. I’m guessing this places limits on return payload mass, possibly much lower than the max payload to orbit.
Any ideas on what the limits might be?
In one of the presentations there was a value of 50t downmass.
I made a spreadsheet calculating the range of CM locations and approximated the Center of Pressure with the projected areas at an AoA of 90 deg (when falling vertically). When you trim the flaps for max forward CM, anything more than 50t wipes out your pith up control authority, so it roughly matches the published value.
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#9
by
punder
on 25 Oct, 2020 15:14
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I made a spreadsheet calculating the range of CM locations and approximated the Center of Pressure with the projected areas at an AoA of 90 deg (when falling vertically). When you trim the flaps for max forward CM, anything more than 50t wipes out your pith up control authority, so it roughly matches the published value.
Fantastic, that’s the kind of thing I was looking for!
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#10
by
Twark_Main
on 25 Oct, 2020 19:38
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1) LEO -> EDL - I think this was mentioned to have a max down mass of 50t, but don't ask where or when I heard that.
I think the 50 t comes from "Typical return payload 50 t" in a slide from the 2017 presentation regarding the carbon fibre delta wing no flaps header tanks in aft methane tank BFR...
So somewhere around early bronze age level of relevance.
We don't even need to resort to circumstantial reasoning. We got an update from Elon just two weeks later invalidating that number:
The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk. In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.
https://old.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodcg22/Curious that the math still seems to support a 50 t downmass limit.
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#11
by
eriblo
on 25 Oct, 2020 21:14
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Apologies if already covered somewhere.
Starship probably has to be pretty light to avoid excessive heating on reentry, and the tanks are probably almost empty before the landing burn. So, with cargo in the nose, the CG must be shifted forward, and the more cargo onboard, the bigger the forward shift. I’m guessing this places limits on return payload mass, possibly much lower than the max payload to orbit.
Any ideas on what the limits might be?
In one of the presentations there was a value of 50t downmass.
I made a spreadsheet calculating the range of CM locations and approximated the Center of Pressure with the projected areas at an AoA of 90 deg (when falling vertically). When you trim the flaps for max forward CM, anything more than 50t wipes out your pith up control authority, so it roughly matches the published value.
This is very sensitive to the aerodynamic assumptions.
Doing the same analys assuming on fael097's SN8 profile with a constant Cd (i.e. just using projected area at 90 deg AoA) for the two extreme cases (aft fins fully deployed and forward fins fully tucked in and vice versa) I get a CoP shift from ~20 m to ~24 m height (from the aft end). If you match the CoM for a 150 t "empty" landing mass to the first case you can add ~44 t in the middle of the payload bay (at 37 m height) before you exceed the forward limit in the second case (with no margin). I guess this is similar to what you got.
But if you instead assume that Cd for the fully deployed fins and the area between them increases by 50% compared to the cylinder/tucked fins (equal to or less than the standard Cd difference between a flat plate and a cylinder) the CoP range doubles to ~18 m to ~26 m and you can now fall evenly with ~104 t of payload at 37 m.
So I would say that you would need a more detailed analysis before ruling out landing with a full payload.
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#12
by
equiserre
on 25 Oct, 2020 21:51
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Apologies if already covered somewhere.
Starship probably has to be pretty light to avoid excessive heating on reentry, and the tanks are probably almost empty before the landing burn. So, with cargo in the nose, the CG must be shifted forward, and the more cargo onboard, the bigger the forward shift. I’m guessing this places limits on return payload mass, possibly much lower than the max payload to orbit.
Any ideas on what the limits might be?
In one of the presentations there was a value of 50t downmass.
I made a spreadsheet calculating the range of CM locations and approximated the Center of Pressure with the projected areas at an AoA of 90 deg (when falling vertically). When you trim the flaps for max forward CM, anything more than 50t wipes out your pith up control authority, so it roughly matches the published value.
This is very sensitive to the aerodynamic assumptions.
Doing the same analys assuming on fael097's SN8 profile with a constant Cd (i.e. just using projected area at 90 deg AoA) for the two extreme cases (aft fins fully deployed and forward fins fully tucked in and vice versa) I get a CoP shift from ~20 m to ~24 m height (from the aft end). If you match the CoM for a 150 t "empty" landing mass to the first case you can add ~44 t in the middle of the payload bay (at 37 m height) before you exceed the forward limit in the second case (with no margin). I guess this is similar to what you got.
But if you instead assume that Cd for the fully deployed fins and the area between them increases by 50% compared to the cylinder/tucked fins (equal to or less than the standard Cd difference between a flat plate and a cylinder) the CoP range doubles to ~18 m to ~26 m and you can now fall evenly with ~104 t of payload at 37 m.
So I would say that you would need a more detailed analysis before ruling out landing with a full payload.
Agree 100%. Very interesting, I had only increased the Cd for the flaps and not the area in between.
Of course I don´t have the knowledge to go farther, my point is that whenever thinking about downmass, this is the frame of thought: flaps need vary the CP along a bigger range than the CM. CM from max downmass and empty moves about 3m (I get 24m and 21m from the bottom)
Flaps need to trim out this range of variation, and have enough left for control.