Author Topic: MCT Speculation and Discussion Thread 4  (Read 505640 times)

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2600 on: 08/22/2016 07:37 PM »
Does all-composite construction favor any particular architecture choices? SpaceX seems to put quite a bit of effort into TPS on the composite Falcon interstage compared to the Al-Li tanks. I wonder if they will need TPS basically everywhere on a composite booster and especially on an orbiter.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2601 on: 08/22/2016 08:15 PM »
Does all-composite construction favor any particular architecture choices? SpaceX seems to put quite a bit of effort into TPS on the composite Falcon interstage compared to the Al-Li tanks. I wonder if they will need TPS basically everywhere on a composite booster and especially on an orbiter.
Al-Li hates heat basically just as much, the difference is the interstage gets full blast from the upper stage.
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Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2602 on: 08/22/2016 08:33 PM »
Does all-composite construction favor any particular architecture choices? SpaceX seems to put quite a bit of effort into TPS on the composite Falcon interstage compared to the Al-Li tanks. I wonder if they will need TPS basically everywhere on a composite booster and especially on an orbiter.

Al-Li favours bi-conic fairing like shapes while composites could enable a much wider range of shapes. I've long thought that the BFS could have a semi-lifting body shape, one without winglets, but a non-circular cross section.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2603 on: 08/22/2016 11:30 PM »
Does all-composite construction favor any particular architecture choices? SpaceX seems to put quite a bit of effort into TPS on the composite Falcon interstage compared to the Al-Li tanks. I wonder if they will need TPS basically everywhere on a composite booster and especially on an orbiter.

The lower mass may allow double hulled designs on BFS (or fuel depots to limit boiloff), where the propellant tankage is surrounded by a vacuum space to the outer mold line.  Tank reinforcements and/or MLI could then be in the interstitial space between the shells.  Something like this is needed because CO2 (and H2O, of course) freezes out at lox/liquid methane temps, so the vehicle cannot sit on the Martian surface with fuel in the tanks for very long.  Design would be functionally similar to an LN2 storage Dewar.
« Last Edit: 08/22/2016 11:33 PM by AncientU »
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Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2604 on: 08/23/2016 01:31 AM »
Does all-composite construction favor any particular architecture choices? SpaceX seems to put quite a bit of effort into TPS on the composite Falcon interstage compared to the Al-Li tanks. I wonder if they will need TPS basically everywhere on a composite booster and especially on an orbiter.

Al-Li favours bi-conic fairing like shapes while composites could enable a much wider range of shapes. I've long thought that the BFS could have a semi-lifting body shape, one without winglets, but a non-circular cross section.

Richard Heidmann,

Bio....

French Mars Society

MCT concept....
« Last Edit: 08/23/2016 01:34 AM by docmordrid »
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2605 on: 08/23/2016 01:34 AM »
It's not going to have wings or land horizontally like that. I'd put a lot of money on that.
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Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2606 on: 08/23/2016 01:54 AM »
It's not going to have wings or land horizontally like that. I'd put a lot of money on that.

I'm more interested in the broad profile with a large TPS area than the engine placement and wings, which ISTM are of questionable utility in a thin atmosphere.

Think Max Fagin's retropropulsion thesis defense...

Horizontal entry, tail-landed (low c/g) uber-Dragon-ish split vehicle, with SD like pods in the upper 1/3-1/2 serving as LAS/ERV thrusters with the bottom, which contains the cargo, staying?
« Last Edit: 08/23/2016 02:01 AM by docmordrid »
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2607 on: 08/23/2016 02:05 AM »
Maybe. I think side entry is the most probable. I just don't think they'll use wings. I wonder if grid fins will make an appearance? May help a lot with the transition.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2608 on: 08/23/2016 02:11 AM »
Simply entering an atmosphere sideways as opposed to vertically is only going to approximately double the area at best and it requires the vehicle to be load bearing in a second direction, which is a significant mass penalty for a vehicle this large.

I suspect we will see the vehicle equipped with some form of radial expansion mechanism that give it a much larger cross sectional area and entry will be in a vertical orientation either base first or nose first to keep the forces of entry on the single strong vertical axis that is used during launch.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2609 on: 08/23/2016 02:26 AM »
Simply entering an atmosphere sideways as opposed to vertically is only going to approximately double the area at best and it requires the vehicle to be load bearing in a second direction, which is a significant mass penalty for a vehicle this large.

I suspect we will see the vehicle equipped with some form of radial expansion mechanism that give it a much larger cross sectional area and entry will be in a vertical orientation either base first or nose first to keep the forces of entry on the single strong vertical axis that is used during launch.
That's plausible as well. But a doubling or tripling of the area is not insignificant.
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2610 on: 08/23/2016 03:36 AM »
Simply entering an atmosphere sideways as opposed to vertically is only going to approximately double the area at best and it requires the vehicle to be load bearing in a second direction, which is a significant mass penalty for a vehicle this large.

I suspect we will see the vehicle equipped with some form of radial expansion mechanism that give it a much larger cross sectional area and entry will be in a vertical orientation either base first or nose first to keep the forces of entry on the single strong vertical axis that is used during launch.

Plausible, certainly. But that's just trading structural mass for mechanism mass and additional complexity. They could also trade that mass for fuel and start SRP at a higher velocity.

Each approach has advantages that show in other parts of the mission, and it's hard to optimize without knowing the full mission profile. Particularly since more than one could be used.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2611 on: 08/23/2016 05:54 AM »
First, frictional dissipation of velocity using heat-shields, parachutes, HIAD, you name it, Always comes out more mass efficient then propellant, we would always choose to trade landing propellant for structural mass in the vehicle because the ratio is likely to be in excess of 10 to 1 in favor of the structural element over propellant.  Complexity is a downside, yes but if it means not having to be loaded with large amounts of propellant on atmospheric entry that is a big deal because that saves mass twice in two high delta v maneuvers, departure from Earth and departure from Mars.

Second SRP is only usable in the later parts of EDL after most velocity has already been lost through friction.  The problem is that the vehicle is going to have so high of a ballistic coefficient that it would strike the ground on mars before even reaching the speed with sufficient remaining altitude that SRP can handle, their must be something to lower the coefficient.

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2612 on: 08/23/2016 08:51 AM »
I suspect we will see the vehicle equipped with some form of radial expansion mechanism that give it a much larger cross sectional area and entry will be in a vertical orientation either base first or nose first to keep the forces of entry on the single strong vertical axis that is used during launch.

Sure, increasing Cd will help enormously with fuel conservation, but more is required. For the sake of argument assume your spaceship is travelling horizontally in the upper Martian atmosphere at 13km/s. Orbital velocity is 3.3km/s, so just to stay in the atmosphere, the ship needs to generate 3 Martian gs of negative lift.

So, it is not enough to expand the cross section radially, some sort of asymmetry needs to be introduced in order to create lift. What would be especially helpful would be if the Cl could be modulated. It will be extremely interesting to see how SpaceX solve this problem.
« Last Edit: 08/23/2016 07:05 PM by OneSpeed »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2613 on: 08/23/2016 12:40 PM »
First, frictional dissipation of velocity using heat-shields, parachutes, HIAD, you name it, Always comes out more mass efficient then propellant, we would always choose to trade landing propellant for structural mass in the vehicle because the ratio is likely to be in excess of 10 to 1 in favor of the structural element over propellant.  Complexity is a downside, yes but if it means not having to be loaded with large amounts of propellant on atmospheric entry that is a big deal because that saves mass twice in two high delta v maneuvers, departure from Earth and departure from Mars.
It's not entirely necessary to increase drag or ballistic coefficient for Earth return, so un-deploying a deployable drag device on Mars and then dragging it back to Earth is actually less efficient. The extra propellant capacity is actually useful for the return transit burns.

Quote
Second SRP is only usable in the later parts of EDL after most velocity has already been lost through friction.  The problem is that the vehicle is going to have so high of a ballistic coefficient that it would strike the ground on mars before even reaching the speed with sufficient remaining altitude that SRP can handle, their must be something to lower the coefficient.

Red Dragon will (try) to baseline a low Lift/Drag, high ballistic coefficient Mars entry. Assuming that succeeds in getting to the SRP regime with an appreciable amount of payload (i.e. it doesn't go "splat" first), than a larger vehicle with similar aerodynamic characteristics should be feasible.

That's where carbon fiber allows some architectural choices. With a lower density and much higher strength to weight ratio, it might be able to help get around the cube-square issues of scaling a vehicle up. If there are any mass savings, that translates directly to both a better L/D ratio and a better ballistic coefficient.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2614 on: 08/23/2016 01:13 PM »
Simply entering an atmosphere sideways as opposed to vertically is only going to approximately double the area at best ...

If the info in L2 is to believed (and I do not believe this bit of info), it is more like four times.

Drag is approximately proportional to area, but lift should be much higher for a (semi-)lifting body (i.e. one without wings/winglets) so the lift/drag should be better.

If my understanding of EDL is correct higher lift/drag can lead to both lower peak acceleration and lower speed before transition to retro-propulsion. Both very desirable characteristics.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2615 on: 08/23/2016 01:20 PM »
Second SRP is only usable in the later parts of EDL after most velocity has already been lost through friction.  The problem is that the vehicle is going to have so high of a ballistic coefficient that it would strike the ground on mars before even reaching the speed with sufficient remaining altitude that SRP can handle, their must be something to lower the coefficient.

Red Dragon will (try) to baseline a low Lift/Drag, high ballistic coefficient Mars entry. Assuming that succeeds in getting to the SRP regime with an appreciable amount of payload (i.e. it doesn't go "splat" first), than a larger vehicle with similar aerodynamic characteristics should be feasible.

My understanding is that Red Dragon will have g-forces which are too high for humans (especially those that have spent 3+ months in zero-g). So it is not just possible to scale up Red Dragon (to about 22 m in diameter).

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2616 on: 08/23/2016 05:04 PM »
Red Dragon's gee forces are not too high. Compare a Soyuz ballistic entry after 6 months in orbit.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2617 on: 08/24/2016 06:47 AM »
Red Dragon's gee forces are not too high. Compare a Soyuz ballistic entry after 6 months in orbit.

Those forces are not nominal and are routinely cause the cosmonauts to black out, your always siting superlatives and extremes as if they are usable as nominal conditions but that is flawed because their would be no margin left for any off nominal event.

A radial area expansion system dose not rule out lift generation, in fact it makes it easier to control the asymmetry of the vehicle and control the amount of lift generated which is highly advantageous given the variable density of mars atmosphere.  A lifting body or bi-conic will generated high lift but at the cost of reducing drag as these bodies to not enter fully horizontally and while the lift is needed for g-force reduction from what I've read you lose too much drag which raises the ballistic coefficient back into dangerous territory.  A fairly simple radial expansion in the form of fold down petals and connecting carbon fiber webbing between them could easily triple the vehicle radius to 40 meters which would give a comparable ballistic coefficient to the MSL.

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2618 on: 08/24/2016 01:01 PM »
I suspect we will see the vehicle equipped with some form of radial expansion mechanism that give it a much larger cross sectional area and entry will be in a vertical orientation either base first or nose first to keep the forces of entry on the single strong vertical axis that is used during launch.

Regarding lift generation, I had assumed from this post that you expected the entry to be in a purely vertical orientation. I agree that a radial area expansion system could generate lift, but to do so, the ship would need to pitch off axis by some amount. Still, the stress on a blunt body shape would be significantly less than that experienced by a biconic or lifting body, or indeed any shape that would be dynamically modelled by a pinned beam.

Red Dragon's gee forces are not too high. Compare a Soyuz ballistic entry after 6 months in orbit.

Those forces are not nominal and are routinely cause the cosmonauts to black out, your always siting superlatives and extremes as if they are usable as nominal conditions but that is flawed because their would be no margin left for any off nominal event.

A radial area expansion system dose not rule out lift generation, in fact it makes it easier to control the asymmetry of the vehicle and control the amount of lift generated which is highly advantageous given the variable density of mars atmosphere.  A lifting body or bi-conic will generated high lift but at the cost of reducing drag as these bodies to not enter fully horizontally and while the lift is needed for g-force reduction from what I've read you lose too much drag which raises the ballistic coefficient back into dangerous territory.  A fairly simple radial expansion in the form of fold down petals and connecting carbon fiber webbing between them could easily triple the vehicle radius to 40 meters which would give a comparable ballistic coefficient to the MSL.

Regarding ballistic coefficients, I've attached a table that compares several designs, including a hypothetical BFS with the same ballistic coefficient as Red Dragon.

SpacecraftDiameter mArea m^2Volume m^3Mass mTBallistic coefficientSidewall angle Peak gsEntry velocity km/s
MSL4.515.21?3.314840155.8
Soyuz2.23.843789747.6
Cargo dragon3.710.75107.2515154.57.6
Red dragon3.710.751010715157.49.6
BFS22380.13400035771530?412

Firstly, MSL is the only spacecraft in the list that has actually landed on Mars. For me, the standout difference is its ballistic coefficient, at least 3 times less than all the others. Of course Soyuz and Cargo Dragon only need to perform EDL on Earth, so a higher β makes sense. I've simulated the Red Dragon EDL profile, and I believe that on a good day it will work. The problem is the day to day variation in the density of the upper Martian atmosphere. On a bad day, the density might be halved, making EDL much more difficult and dangerous. As a risk mitigation strategy, I'd suggest increasing the BFS Cd and Cl, giving greater control authority, and minimising peak g forces.

The other standout difference is entry velocity. Heating increases as the cube of velocity, so, for the same β, the thermal difference between MSL and BFR is a factor of 9. Again, lowering the β of the BFR to perhaps 400kg/m^2 would help. A higher entry velocity also magnifies any trajectory deviation, and hence landing positional error, so maximising control authority is critical.
« Last Edit: 08/25/2016 04:24 AM by OneSpeed »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2619 on: 08/24/2016 01:30 PM »
...easily triple the vehicle radius to 40 meters which would give a comparable ballistic coefficient to the MSL.

How much mass are you figuring at EI? MSL had 3300 kg at EI behind a 10.75 m2 heatshield, or 307 kg/m2, and it slowed to below 450 m/s before deploying chutes.

If BFS masses 240,000 kg at EI (100t payload, 80t vehicle, 60t EDL propellant), then it only needs a 15.8m radius to match MSL's ballistic coefficient.

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