Author Topic: MCT Speculation and Discussion Thread 4  (Read 611033 times)

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2620 on: 08/24/2016 01:55 PM »
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.

Are you sure? Wikipedia quotes 2,401 kg for the EDL system, and a 4.5m diameter heatshield, giving an area of 15.21 m^2. That's a β of 158 kg/m^2, although I've seen 110 kg/m^2 quoted elsewhere.
« Last Edit: 08/24/2016 02:13 PM by OneSpeed »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2621 on: 08/24/2016 04:31 PM »
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.

Are you sure? Wikipedia quotes 2,401 kg for the EDL system, and a 4.5m diameter heatshield, giving an area of 15.21 m^2. That's a β of 158 kg/m^2, although I've seen 110 kg/m^2 quoted elsewhere.

You're right about the 4.5m MSL shield, I was thinking of the Viking shields.

But the 2.4t mass is for the EDL system ONLY (skycrane, fuel, chutes, shield, backshell), and doesn't include the 900 kg rover itself. 3300 kg over 15.2 m2 gives a beta of 217 kg/m2

Edit: here's the link to JPL's listed masses for MSL... http://mars.jpl.nasa.gov/msl/mission/spacecraft/
« Last Edit: 08/24/2016 04:34 PM by envy887 »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2622 on: 08/24/2016 05:03 PM »
SpacecraftDiameter mArea m^2Volume m^3Mass mTBallistic coefficientSidewall angle °Peak gsEntry velocity km/s
BFS22380.13400035793030?412
Can you list the assumptions behind these values? The volume and mass seem rather high to me.

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2623 on: 08/24/2016 09:54 PM »
Can you list the assumptions behind these values? The volume and mass seem rather high to me.

I just took MikeAtkinson's suggestion of a 22m diameter, and worked back from the Red Dragon β and shape to derive the hypothetical mass and volume. There is a much more detailed analysis in L2.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2624 on: 08/25/2016 01:20 AM »
I don't find 22 m diameter to be plausible, it's far wider then even the widest speculated size of the BFR, but a vehicle 15 m in diameter with fold down flaps the same size as the current F9 landing legs which are 18 m long would give a 51 m diameter, and the legs only mass 2,500 kg on the F9R so this looks eminently practical and it far exceeds the diameter that a monolithic vehicle could be launched as. 

I'm also operating under the assumption of a 200 mt entry mass and a much more modest propellant quantity sufficient for 800 m/s.

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2625 on: 08/25/2016 04:40 AM »
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.

Are you sure? Wikipedia quotes 2,401 kg for the EDL system, and a 4.5m diameter heatshield, giving an area of 15.21 m^2. That's a β of 158 kg/m^2, although I've seen 110 kg/m^2 quoted elsewhere.

You're right about the 4.5m MSL shield, I was thinking of the Viking shields.

But the 2.4t mass is for the EDL system ONLY (skycrane, fuel, chutes, shield, backshell), and doesn't include the 900 kg rover itself. 3300 kg over 15.2 m2 gives a beta of 217 kg/m2

Edit: here's the link to JPL's listed masses for MSL... http://mars.jpl.nasa.gov/msl/mission/spacecraft/

I think I see the problem. β = mass / area * Cd, so we've both forgotten to divide by the drag coefficient, which for MSL is 1.4, giving 148 kg/m2. I've updated my table.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2626 on: 08/25/2016 02:38 PM »
I don't find 22 m diameter to be plausible, it's far wider then even the widest speculated size of the BFR, but a vehicle 15 m in diameter with fold down flaps the same size as the current F9 landing legs which are 18 m long would give a 51 m diameter, and the legs only mass 2,500 kg on the F9R so this looks eminently practical and it far exceeds the diameter that a monolithic vehicle could be launched as. 

I'm also operating under the assumption of a 200 mt entry mass and a much more modest propellant quantity sufficient for 800 m/s.

I think they could use shorter flaps as deployable drag and as variable lift control surfaces. Webbing has a lot of challenges, but interleaved rigid deployable flaps could easily double the diameter, say from 16m to 32m. That would only require flaps some 8m long.

Offline Xentry

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Re: MCT Speculation and Discussion Thread 4
« Reply #2627 on: 08/25/2016 03:04 PM »
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

Is there any particular reason why Mars atmospheric entry velocities would be as high as 9-12km/s for the Red Dragon and BFS? I would expect them to be on the same order of past Mars missions (that is, 5-7km/s)...

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2628 on: 08/25/2016 03:18 PM »
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

Is there any particular reason why Mars atmospheric entry velocities would be as high as 9-12km/s for the Red Dragon and BFS? I would expect them to be on the same order of past Mars missions (that is, 5-7km/s)...

As to BFS Musk has said that he wants shorter trip times, so shorter trip times equates to higher Km/sec entry velocity
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2629 on: 08/25/2016 03:44 PM »
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

Is there any particular reason why Mars atmospheric entry velocities would be as high as 9-12km/s for the Red Dragon and BFS? I would expect them to be on the same order of past Mars missions (that is, 5-7km/s)...

As to BFS Musk has said that he wants shorter trip times, so shorter trip times equates to higher Km/sec entry velocity

Not necessarily. You just need more fuel to slow down before you get to entry.

Offline Jimmy Murdok

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Re: MCT Speculation and Discussion Thread 4
« Reply #2630 on: 08/25/2016 04:30 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.
I agree with you and when I saw the "Battlestar Galactica" french design... :o Should be much more simple.

What if the actual configuration of F9 is already testing many parts of the final design:

- Landing a 9 motor configuration. Or at least vertical landing similar to what they are mastering.
- During launch, the abort unit is a "Super Dragon" with similar shape to the V2. The trunk include the living quarters and services.
- In case of main stage issues during EDL, the "Super Dragon" could detach and follow similar trajectory of Red Dragon. Survival in the capsule until new ascent spaceship arrives.
- The legs have double function: semi expanded work as a frame for a deceleration fabric that increase the surface. (If active hydraulics can also provide control).
- The legs do provide enough clearance for relaunch: no need to move the rocket to vertical position. Just land on top of a leveled clean pad.
- The plan in September would not look that crazy (except for the scale). We have seen landings and the dragon abort.
- The deceleration system can be scale tested in F9 with practical output: reduce landing fuel and stress of the engines. 

See the graphics attached. Need much more development but tomorrow I work early.
I´m happy to modify if good suggestions and/or share the models.

Edit:typo
« Last Edit: 08/25/2016 05:09 PM by Jimmy Murdok »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2631 on: 08/25/2016 05:15 PM »
Overall, some really cool ideas, but you could work on how they are configured. Do you have any masses for any of the vehicles, or required fuel masses calculated?

- Landing a 9 motor configuration. Or at least vertical landing similar to what they are mastering.
How does this monster get to LEO? 9 Raptors are VERY underpowered for Earth launch at sea level, but rather overpowered for anything else.
Are you planning on having those 9 engines facing into the orbital reentry blast? They won't like that, unless they are firing the whole time... and firing the whole time uses too much fuel.
That lower section should have more engines, and just be a stage 1 booster. It only does sub-orbital EDL and doesn't need much shielding. Move the deployable heat shield up the the Super Dragon Trunk. The Trunk and Super Dragon are the unit that then would go to orbit, and with the deployable shield for reentry at Mars and Earth.
Move the "SuperDraco" pods down to the trunk sidewall and fire them nearly straight back, between the landing legs. They have to be able to fire with the shield folded or deployed. That's how it will get from staging to orbit/TMI (with the shield folded) and that's how it will do supersonic retro-propulsion (with the shield open).

Quote
- During launch, the abort unit is a "Super Dragon" with similar shape to the V2. The trunk include the living quarters and services.
How much mass are you putting through abort? What engines would be used for abort? IFAIK SpaceX is not working on a engine that would be suitable to abort that large a capsule from Earth launch.
Also, why do you need to abort the whole thing? MCT can easily carry a 7 man Dragon 2 (or later a hypothetical ~25 passenger, ~6 meter Dragon 3) on top, which can abort on SuperDracos and shuttle back to Earth immediately. Since ~4 refueling launches are needed per Mars injection, just send 1/4 of the people up on every launch.

Quote
- In case of main stage issues during EDL, the "Super Dragon" could detach and follow similar trajectory of Red Dragon. Survival in the capsule until new ascent spaceship arrives.
I can see where EDL abort capability would be nice, but I can't see any way it's practical. No Earth return craft has ever had that, for good reasons. Any EDL failure that's not survivable on a spacecraft probably isn't survivable in the abort capsule either. There's really no reason to separate the Super Dragon from it's Trunk. Land them together, launch them together, and abort them together if needed.

Quote
- The legs have double function: semi expanded work as a frame for a deceleration fabric that increase the surface. (If active hydraulics can also provide control).
You don't need grid fins for Mars, and behind a heatshield they won't do anything useful on Earth, so move teh heatshield up to the Trunk. Where does the fabric go when the legs are folded up? Is it stowed? Can it be re-stowed automatically on Mars for relaunch?


Quote
- The legs do provide enough clearance for relaunch: no need to move the rocket to vertical position. Just land on top of a leveled clean pad.
A separate "cargo bay" isn't practical; the cargo bay and hab should be the same size and shape to fit in the same mold lines. Almost everything will need to be shipped pressurized, and the pressure vessel should to be removed at Mars to serve as storage/habitat. There's no point in returning your shipping container to Earth.
« Last Edit: 08/25/2016 05:25 PM by envy887 »

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2632 on: 08/25/2016 06:16 PM »
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

Is there any particular reason why Mars atmospheric entry velocities would be as high as 9-12km/s for the Red Dragon and BFS? I would expect them to be on the same order of past Mars missions (that is, 5-7km/s)...

As to BFS Musk has said that he wants shorter trip times, so shorter trip times equates to higher Km/sec entry velocity

Not necessarily. You just need more fuel to slow down before you get to entry.

Not a great mission profile wasting valuable rocket equation exponential propellant when the atmosphere gives the delta V to you if you are very clever in your spacecraft design and guidance.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Jimmy Murdok

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Re: MCT Speculation and Discussion Thread 4
« Reply #2633 on: 08/25/2016 06:37 PM »
Overall, some really cool ideas, but you could work on how they are configured. Do you have any masses for any of the vehicles, or required fuel masses calculated?
The main objective of the post was to get feedback about the concept of decelerator deployed with legs. Done all the design after work. Many artistic licenses.  If you want to feed with data, I´m happy to modify and improve. Everything is parametric.

Quote
How does this monster get to LEO? 9 Raptors are VERY underpowered for Earth launch at sea level, but rather overpowered for anything else.
It´s a second stage BFS. And one stage to orbit form Mars + earth injection.

Quote
Are you planning on having those 9 engines facing into the orbital reentry blast? They won't like that, unless they are firing the whole time... and firing the whole time uses too much fuel. ´
Was thinking on extensions on the low part of the legs that cover the external engines when deployed. Depends on how many engines are needed in total and how many only for the EDL?  9 were random to visualize the concept (an octaweb would simplify their design).

Quote
That lower section should have more engines, and just be a stage 1 booster. It only does sub-orbital EDL and doesn't need much shielding. Move the deployable heat shield up the the Super Dragon Trunk. The Trunk and Super Dragon are the unit that then would go to orbit, and with the deployable shield for reentry at Mars and Earth.
Move the "SuperDraco" pods down to the trunk sidewall and fire them nearly straight back, between the landing legs. They have to be able to fire with the shield folded or deployed. That's how it will get from staging to orbit/TMI (with the shield folded) and that's how it will do supersonic retro-propulsion (with the shield open).
Need to analyze this input

Quote
How much mass are you putting through abort? What engines would be used for abort? IFAIK SpaceX is not working on a engine that would be suitable to abort that large a capsule from Earth launch.
Also, why do you need to abort the whole thing? MCT can easily carry a 7 man Dragon 2 (or later a hypothetical ~25 passenger, ~6 meter Dragon 3) on top, which can abort on SuperDracos and shuttle back to Earth immediately. Since ~4 refueling launches are needed per Mars injection, just send 1/4 of the people up on every launch.
Those huge super dracos are artistic. Could be a much smaller capsule powered by SD like your 6m Dragon 3. But I expect there will be an abort function during launch and EDL.
 

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2634 on: 08/25/2016 07:44 PM »
The main objective of the post was to get feedback about the concept of decelerator deployed with legs. Done all the design after work. Many artistic licenses.  If you want to feed with data, I´m happy to modify and improve. Everything is parametric.
I can certainly provide some calculations. It would be helpful to know some of the dimensions you have modeled, such the main diameter of the stage.

The deployable decelerator is a great idea, although I would suggest you decouple it from the landing legs, for a couple reasons: 1) a Mars lander will be rather short and squat, so it's legs can be mostly under it (look at the leg concepts for Red Dragon). 2) you want to be able to stow the decelerator while keeping the legs out... can't have that thing hanging out when you launch from Mars, since it will make a great drag device going the other direction as well 8) The frames used for deployment can still look a lot like the F9 legs, but don't need to rotate all the way to the ground.

Quote
It´s a second stage BFS. And one stage to orbit form Mars + earth injection.
That good, but it's quite over sized for that. A 15m diameter subcooled methalox tank holds about 180 tonnes of propellant per meter of length, so for the 1000t or so of propellant it takes to put 100t of payload through TMI and EDL, you only need some 5 to 6 meter long tanks. The tanks on BFS are going to be pretty close to spherical and 10 to 12m diameter, not a long cylinder as you show. The long cylinder is exactly what the booster stage will look like.

Quote
Was thinking on extensions on the low part of the legs that cover the external engines when deployed. Depends on how many engines are needed in total and how many only for the EDL?  9 were random to visualize the concept (an octaweb would simplify their design).
Depending on how massive it is, it will need somewhere between 2 to 6 vacuum Raptors on the second stage. Retropropulsion requires that the engines be as far outboard as possible, and if they are set back from the heatshield around the edges they don't need a cover - the bow shock will carry the hot plasma out and around them. Look at the SuperDracos on Dragon 2. That's exactly where you want to put your engines for EDL... or pretty much like this:

Quote from: envy887
Move the "SuperDraco" pods down to the trunk sidewall and fire them nearly straight back, between the landing legs. They have to be able to fire with the shield folded or deployed. That's how it will get to orbit/TMI (with the shield folded) and that's how it will do supersonic retro-propulsion (with the shield open).

Quote
But I expect there will be an abort function during launch and EDL.
Launch? Yes. EDL? I don't think there are a lot of scenarios where that's survivable with any form of abort. Maybe on early missions where you can fit the crew in a Dragon on top, but certainly not at the colonization level. Some risks are just inherent with no practical means for abort, and EDL has always been one of those. By the time they get past the "3.7m or 6m Dragon on top" type of missions, the architecture will have flown thousands of entries and have a very high confidence in reliability, 99.9% or greater.
« Last Edit: 08/25/2016 07:45 PM by envy887 »

Offline Nilof

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Re: MCT Speculation and Discussion Thread 4
« Reply #2635 on: 08/25/2016 10:02 PM »
I'm not sure about the whole super-dragon thing and some of the other details, but that leg + decelerator combined design is really clever.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2636 on: 08/25/2016 10:38 PM »
My first memory of a deployable decelerator like that was the TGV  MICHELLE 2 Ansari X-Prize entrant.
« Last Edit: 08/25/2016 10:49 PM by docmordrid »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2637 on: 08/26/2016 01:38 AM »
Yes something like that, though I expect a much shorter vehicle then that depiction or the one Murdock mocked up, the fundamentals of the deployment are what I was envisioning.  A base first entry is the most likely entry orientation but I'm not  entirely ruling out nose first entry.  In the base first configuration I would expect a heat-shield to cover most of the vehicle base with immovable engines nozzles flush to the heat-shield and located around the perimeter. 

Using the ribs as landing legs is not something I'm comfortable endorsing, the leg system needs to absorb shock which is a very different function from the ribs of a decelerator which need to be stiff, if the system were only used once some kind of crumple system might work but reusability is a must and bringing the ribs down in contact with the surface presents a danger to the fabric as well as complicates unloading and launch.  A more likely leg configuration is like that on Dragon v2, telescoping out from heat shield with a segment of the shield forming the foot.

Lastly I should note that I see the whole side of the vehicle folding down like a petal not just a set of singular ribs that are visible on the outside of the vessel, when the system is stowed it is all under a smooth shell which covers the whole vehicle.  This allows delicate systems intended for use in space to be stowed under the petals, such as radiators and solar panels, I see solar being on the underside of the petals and radiators on the sides of the vehicle body, this allows the vehicle to fly through space with the nose pointing sun-ward which will illuminate the panels and shade the radiators.

Lastly some estimates on mass, as noted earlier the leg system on F9 masses 2500 kg, carbon fiber fabric 5 layers thick totaling 1 kg/m^2 and covering the space between a 15 m base and a 51 m total diameter would be 1800 m and kg bringing total mass to 4.3 mt, a very modest amount that would be far lighter then carrying extra propellant.  The propellant needed for just the last bit of deceleration and up to landing is going to need to do around 800 m/s based on extrapolation from Red Dragon, and that's going to come out to around 20 percent propellant fraction minimum.  I'm estimating 200 mt at entry and 40 mt of landing propellant.

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2638 on: 08/26/2016 08:39 AM »
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

Is there any particular reason why Mars atmospheric entry velocities would be as high as 9-12km/s for the Red Dragon and BFS? I would expect them to be on the same order of past Mars missions (that is, 5-7km/s)...

As to BFS Musk has said that he wants shorter trip times, so shorter trip times equates to higher Km/sec entry velocity

Not necessarily. You just need more fuel to slow down before you get to entry.

Not a great mission profile wasting valuable rocket equation exponential propellant when the atmosphere gives the delta V to you if you are very clever in your spacecraft design and guidance.

Never said it was a good idea, just that it was a possible idea, as the previous post needed correction. Although depending on how good your craft design is, you MIGHT need to slow down to get to a survivable entry speed, if you want fast transit times.

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #2639 on: 08/26/2016 11:27 AM »
Lastly some estimates on mass, as noted earlier the leg system on F9 masses 2500 kg, carbon fiber fabric 5 layers thick totaling 1 kg/m^2 and covering the space between a 15 m base and a 51 m total diameter would be 1800 m and kg bringing total mass to 4.3 mt, a very modest amount that would be far lighter then carrying extra propellant.  The propellant needed for just the last bit of deceleration and up to landing is going to need to do around 800 m/s based on extrapolation from Red Dragon, and that's going to come out to around 20 percent propellant fraction minimum.  I'm estimating 200 mt at entry and 40 mt of landing propellant.

Reducing dry mass will have priority since the architecture is constrained by the return leg. The main benefit of something like HIAD would be its lower mass compared to a mid L/D aeroshell.

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