Landing on Mars

SpaceNut · 2012-08-13 12:01:38

Simular to the mass that the reference mission MarsDrive had been able to calculate in the DRM 2.5 document...which made use of a folding heat shield to increase the size and to block the upstream sensitvity to igniting the engines.....

http://www.marsdrive.com/Libraries/Down … .sflb.ashx

Rune · 2012-08-16 11:52:41

I AM going to answer to this, GW (and I expect I'll say mostly good things), but I'm afraid it will have to wait until I get back to Madrid, my own computer, and maybe a couple of exams.

Rune. Just logged in to let you all know... I will be back!

RobS · 2012-08-16 21:26:55

GW Johnson, I have a question about your Mars calculations: did you assume some lift from the capsule, which would keep the capsule flying closer to horizontal longer and lengthen your glide?

GW Johnson · 2012-08-18 07:18:38

RobS:

No, the calculation is too crude and primitive for that. Getting some L/D just makes things a tad better. My real point was to show that landing big things is feasible, even if you cannot take advantage of all the best tricks. I just go straight from plain vanilla ballistic hypersonics to rocket braking. There's no time to deploy a chute, much less have it do any good, coming out of entry that low.

The main angle-related troubles I can forsee are hitting the entry interface at too steep an angle. That's got to be very shallow (about a degree to a degree-and-a-half) or you'll whack the ground before entry is over. L/D 0.2 won't really change that very much. That sort of shallow entry is "guaranteed" from LMO with a min deorbit burn (you can screw it up with too aggressive a deorbit burn). Doing it from direct entry, it is very tricky to hit that corridor, just like it was coming back from the moon.

The models I am using are 2-D planar elliptic orbit to the interface altitude, followed by 2-D Cartesian approximation of entry down to Mach 3. That last is nothing but a spreadsheet "automation" of the old 1956 way of estimating velocity vs altitude from an exponential approximation to the density profile-with-altitude. I didn't like the closed-form heating estimates, so I re-did that part as an actual finite-difference "integration" in the spreadsheet. Doing a finite difference "derivative" in the spreadsheet matched the old closed-form peak gees estimate pretty well. Getting the dynamics about right but the heating wrong was pretty much the experience with this method back in 1956, with warheads and film capsules. I fixed the heating, so it works fairly well now. Convective heating only, though.

GW

RobS · 2012-08-18 14:56:13

Thanks, GW Johnson, for your quick reply. That's good news because lift obviously will help. I thought we heard somewhere that a Dragon capsule would dip low into the atmosphere, rise back to 45 kilometers altitude because of lift, then descend (I suppose rather vertically) to the surface. If you gain 20 kilometers, that really stretches out your landing process.

I want to ask you a different question, though: aerobraking into Mars orbit. How tricky is it? Let's say you need to burn off 4 kilometers per second (which I think is about right, from Earth). Or to take another example, let's say you came from Earth on a high-speed trajectory and hit the atmosphere at 15 kilometers per second, but want to retain 4 or 5 kilometers per second to go into orbit. How easy or difficult is that?

GW Johnson · 2012-08-19 14:27:10

RobS:

Aerobraking will always be tricky because the "window" requirements on trajectory angle will always be very tight.

What I have been doing with a 2-D Cartesian model for simple entry will never work for estimating aerobraking, because that requires a circular or spherical coordinate derivation. Mine is constant path angle 2-D.

I remember from the references where I got the simple model that aerobraking on Mars is descent to altitudes nearer 70 km, while aerocapture requires descent to altitudes nearer 25 km. While it's all thin-as-all-get-out densities, aerocapture densities are far higher. That's the Justus & Braun report on model atmospheres for Earth, Venus, Mars, Titan, etc, that Rune turned me onto.

With what I have, I just cannot quantify how much velocity you could dissipate, vs altitude & approach velocity.

GW

PS for the rest: I just posted some blunt capsule drag data over at "exrocketman" for Mercury and Apollo. Anybody have a good, traceable weight statement for Dragon?

http://exrocketman.blogspot.com

GW Johnson · 2012-08-19 17:36:27

I found some ballute stuff and posted it over on "exrocketman", too.

GW

http://exrocketman.blogspot.com

GW Johnson · 2012-08-28 18:58:37

OK, guys, I have revisited the chemical Mars lander problem under different assumptions regarding ballistic coefficient scaling versus lander mass, and further, I took it to rough dimensions and volumes, not just rough weight statements. I got 3 men and 5 tons of supplies and equipment to the surface in a lander massing 60 metric tons at entry, along with an ascent vehicle (as dead-head payload) whose crew cabin is also an abort capsule, either in descent or ascent. The same 60 ton (at entry) lander without men or an ascent vehicle delivers over 28 metric tons of cargo to the surface, even very low-density cargo. There's over 170 cu.m usable cargo volume available.

These are one-shot vehicles, not reusable. Storable propellants MMH and NTO. I included a cosine factor for 10 degree engine cant, to get flowfield stability firing retro thrust into the oncoming supersonic stream. No retro thrust during hypersonic entry from LMO, but no aero-decelerator, either; there's just not enough time to deploy one, much less have it do any good. End of hypersonics, just drop the heat shield, and fire up the rockets. Direct rocket braking to touchdown. One neat little nuance: I used the engines of the ascent vehicle as my descent engines, just sucking from descent propellant tanks.

I posted this stuff as a technical article over at "exrocketman", dated today (8-28-12), and titled "Manned Chemical Lander Revisit". Search/screen keywords are "Mars" and "space program". Have fun. There's a bunch of stuff I've posted over there. Feel free to use any of it.

GW
http://exrocketman.blogspot.com

PS -- hey Rune! This latest one is the one you really want to examine critically. Turns out landing big things on Mars is not so very hard after all. I think you'll really like what I did.

RGClark · 2012-08-29 23:50:28

GW Johnson wrote:

OK, guys, I have revisited the chemical Mars lander problem under different assumptions regarding ballistic coefficient scaling versus lander mass, and further, I took it to rough dimensions and volumes, not just rough weight statements. I got 3 men and 5 tons of supplies and equipment to the surface in a lander massing 60 metric tons at entry, along with an ascent vehicle (as dead-head payload) whose crew cabin is also an abort capsule, either in descent or ascent. The same 60 ton (at entry) lander without men or an ascent vehicle delivers over 28 metric tons of cargo to the surface, even very low-density cargo. There's over 170 cu.m usable cargo volume available.
These are one-shot vehicles, not reusable. Storable propellants MMH and NTO. I included a cosine factor for 10 degree engine cant, to get flowfield stability firing retro thrust into the oncoming supersonic stream. No retro thrust during hypersonic entry from LMO, but no aero-decelerator, either; there's just not enough time to deploy one, much less have it do any good. End of hypersonics, just drop the heat shield, and fire up the rockets. Direct rocket braking to touchdown. One neat little nuance: I used the engines of the ascent vehicle as my descent engines, just sucking from descent propellant tanks.

There has been some recent work that you can reduce boil-off of cryogenic propellant greatly by using a sun shield:

A STUDY OF CRYOGENIC PROPULSIVE STAGES FOR HUMAN EXPLORATION
BEYOND LOW EARTH ORBIT.

With the sun shield, internal analysis by United
Launch Alliance predicts that the average propellant
boil-off losses will be reduced by a factor of two.
Based on their estimate of 0.03%/day for the short
term CPS design, the long term CPS with sun shield
will have a boil-off rate of roughly 0.015%/day per
day.

http://www.sei.aero/eng/papers/uploads/ … y-revC.pdf

Over a 200 day flight to Mars this would be only 3% lost to boil off. What would be the gross mass of your lander using LH2/LOX propellant under that scenario?

Bob Clark

Last edited by RGClark (2012-09-03 17:21:39)

GW Johnson · 2012-09-01 11:07:02

@ Bob Clark:

Honestly Bob, I dunno how it would size out on LH2-LOX. I'd probably start at the same gross entry mass, and adjust the weight statements for less propellants and more cargo mass, due to the higher Isp. The volumes would be quite different, especially for the LH2, so there'd be a lot less available volume within the descent shape, and the ascent vehicle would be a lot longer at the same diameter. Whether that would seriously change the bi-conic descent shape is anyone's guess.

GW

GW Johnson · 2012-09-03 12:35:06

We seem to be getting spammers again, at least in this thread. Anyway, I looked at reconfiguring my chemical Mars designs to work at Mercury. Turns out not to be all that hard. Cargo payloads are reduced a bit in favor of extra descent propellant on an airless world, but I can delete the heavy heat shield. Posted today at "exrocketman". Have fun.

Bob - this is the same one-shot chemical design. MMH-NTO storables, very lightweight tanks in terms of inerts.

I will be looking at our moon next. But I have the expectation that configuration variations of exactly the same hardware set will work at destinations further than we can yet send men: all the way out to Titan.

GW
http://exrocketman.blogspot.com

RGClark · 2012-09-03 17:13:45

GW Johnson wrote:

We seem to be getting spammers again, at least in this thread. Anyway, I looked at reconfiguring my chemical Mars designs to work at Mercury. Turns out not to be all that hard. Cargo payloads are reduced a bit in favor of extra descent propellant on an airless world, but I can delete the heavy heat shield. Posted today at "exrocketman". Have fun.
Bob - this is the same one-shot chemical design. MMH-NTO storables, very lightweight tanks in terms of inerts.
I will be looking at our moon next. But I have the expectation that configuration variations of exactly the same hardware set will work at destinations further than we can yet send men: all the way out to Titan.
GW
http://exrocketman.blogspot.com

What kind of launch vehicle are you using to lift them to LEO?

Bob Clark

GW Johnson · 2012-09-03 18:08:03

Bob:

Gotta be lifted in pieces that will fit on top of things like Delta-IV, Atlas-V, Falcon-9, and (soon) Falcon-Heavy. This is on-orbit docked module assembly, maybe a bit more. It would really help if we had a supple MCP suit to wear for this work. And we could have one soon, because we had one working as a feasibility demo back in 1969.

GW

Russel · 2012-11-27 05:12:37

Ever so slightly off topic, since this is about getting off Mars, but..

What is the minimum piece of hardware you need to get 2-6 people off the surface of Mars and into Mars orbit?

I originally thought about re-using the Dragon or something similar. But the problem there is the dead weight. So with methane/LOX you'd be launching with the better part of 20 tonnes.

Then I realised that there just isn't the space for tankage inside that thing so I put the whole idea aside for a while.

Recently I came back and in a moment of not-having-a-life I started to think about this again.

Here's 3 approaches.

One is basically no capsule at all. Just a fairing and a seat sitting on top of a platform with rocket motors. You'd be relying entirely on your spacesuit for life support. Nice thing is it cuts the fuel needed to a third. Yes, it has obvious safety concerns.

Another was more or less what I've seen before. A cradle of sorts onto which the Dragon sits, with its own motors - able to provide enough Delta-V but then able to re-land itself. I stole this from what I believe is a SpaceX idea to have an adjunct to the Dragon that itself is useful for small hops around the planet.

Third approach is simply a big brother to the Dragon with enough tankage to go into orbit and/or return to the surface or at least abort from most of the way.

Any thoughts on which of these takes your fancy?

BTW, seriously considering using CO/O2 fuel for the sake of simplicity (on the assumption that cheap/light solar cells are on the books)

louis · 2012-11-27 14:25:39

I have suggested the cradle concept before now.

I think it depends what we are trying to do. Ideally we should keep the lander-ascent vehicle as light as possible and deliver surface supplies/hab etc independently.

Russel wrote:

Ever so slightly off topic, since this is about getting off Mars, but..
What is the minimum piece of hardware you need to get 2-6 people off the surface of Mars and into Mars orbit?
I originally thought about re-using the Dragon or something similar. But the problem there is the dead weight. So with methane/LOX you'd be launching with the better part of 20 tonnes.
Then I realised that there just isn't the space for tankage inside that thing so I put the whole idea aside for a while.
Recently I came back and in a moment of not-having-a-life I started to think about this again.
Here's 3 approaches.
One is basically no capsule at all. Just a fairing and a seat sitting on top of a platform with rocket motors. You'd be relying entirely on your spacesuit for life support. Nice thing is it cuts the fuel needed to a third. Yes, it has obvious safety concerns.
Another was more or less what I've seen before. A cradle of sorts onto which the Dragon sits, with its own motors - able to provide enough Delta-V but then able to re-land itself. I stole this from what I believe is a SpaceX idea to have an adjunct to the Dragon that itself is useful for small hops around the planet.
Third approach is simply a big brother to the Dragon with enough tankage to go into orbit and/or return to the surface or at least abort from most of the way.
Any thoughts on which of these takes your fancy?
BTW, seriously considering using CO/O2 fuel for the sake of simplicity (on the assumption that cheap/light solar cells are on the books)

Russel · 2012-11-27 23:20:30

Has anyone considered taking seed hydrogen to Mars in the form of liquid water?

Terraformer · 2012-11-30 09:57:09

The trouble with that is that water is only about 11% hydrogen by mass, so most of what you're taking will be oxygen. Which you can already get there.

Russel · 2012-11-30 22:08:16

I know it appears daft, but its really trading landed mass off against security.

In this case water will store indefinitely. Whilst (deep) cryogenics are pretty well understood, its still another part that can fail.

The bigger picture is using a propellant that is only part methane, mostly liquid CO so you get the benefit of density whilst getting higher isp. You're still using ISPP Oxygen for the bulk of the oxidant.

In that context you only need a few tonnes of water, the hydrogen can be extracted as-needed, and you're not stuck with large volume tankage either.

And if you really want to you can treat the Oxygen that comes with the water as a bonus - yet another backup system.

I wonder if Mars needs more marshmallows?

Last edited by Russel (2012-11-30 22:10:30)

Russel · 2012-12-01 06:58:15

This starts with a question.

Imagine a landing and ascent vehicle for Mars. The core is a conventional blunted cone capsule. Attached to the docking ring at the top is a framework that extends beyond the slipstream of the capsule. On the ends of the framework are tankage and rocket motors. Suitable heat shielding is then applied to protect the tankage/plumbing. In addition there may be light weight mesh structures designed to add drag.

So basically you're aiming for a low(er) ballistic coefficient vehicle. Not a glider by any means, but a step above just a regular capsule.

The intention here is to simply slow the vehicle enough for a propulsive final descent.

Now, the question is, given that you've built something that's fairly ugly and definitely not streamlined, how much does this matter on ascent? Presumably there is a fuel penalty. You've got to find a compromise between gravity losses and aerodynamic losses. But presumably also, once you're past the first few Km of atmosphere you can accelerate more or less how you like. Anyhow the question is, has anyone given this thought?

Ok, now lets consider this further. We're building a fairly hefty craft. Gross mass in the order of 40 tonnes, fully loaded. Able to deliver a mass of about 6 tonnes into high Mars orbit.

If I have my sums right it will be able to transport a crew into high orbit, or abort from any point in the process. Just. It will also make a pretty decent surface hopper.

One tricky bit - its LOX/CO powered. The trade-off here is "wasting" more propellant against a simpler and more efficient production process. After all, the one thing that comes for free is the Martian atmosphere, and LOX/CO is as simple as you can get without abandoning ISPP altogether.

The tankage allows for an excess of oxygen to be carried into high orbit - minus a crew of course. About 6-8 tonnes worth.

Now, imagine that we've sent a space hab to Mars well in advance of a manned mission.

On top of that we send at least one such lander. Plus a very minimal ISPP plant that only knows how to make Oxygen/CO

Yep, I'm revisiting my fuel ferry concept. However now that I've had the chance to step away from it and come back and think about it, I think I like it better this way.

This time there will be no hydrogen transferred to the surface. Instead, the space hab is itself carrying the necessary methane (about 8 tonnes worth). Additionally the space hab carries its own propulsion unit, but scaled to the task - so maximum acceleration is in the order of 1m/s/s - about that of a commuter train. Which makes for a reasonably light propulsion unit.

You also have the freedom here to go for a non-cryogenic fuel, even old fashioned RP1 or some other storage-friendly liquid.

What you save here, is about 32 tonnes (or more actually since you can also stock up on breathing oxygen) which translates to about 130 tonnes saved in LEO terms.

It would take about 6 return journey's for the fuel ferry to completely refill the oxygen tank on the space hab. But the risks involved are mission critical not life critical. You more than likely would like some redundancy which could be easily provided - landing two complete fuel ferries and support systems from the word go.

Before any human leaves Earth you have one fully fueled space hab waiting in high Mars orbit.

This time you bring another mars lander/ascent vehicle, this time fully configured for crew. As well as the second space hab. That way you've always got one fully fueled spare in orbit.

Eventually as you develop confidence in the hardware you can avoid sending a new lander/ascent vehicle with every mission.

The space habs are recycled as well. In theory they could last several missions with refurbishment.

If you really wish to be frugal you could avoid taking a separate crew capsule on the journey and simply aerobrake into Earth orbit, then dock with the final landing capsule in orbit.

In this way the entire process can run on chemical fuels, and use at most a Falcon Heavy plus a bit of in orbit refueling.

Over to you guys..

Whip me! Beat me! Force me to use C++

Last edited by Russel (2012-12-01 08:02:29)

Russel · 2012-12-01 07:36:45

http://spacecraft.ssl.umd.edu/publicati … hieldx.pdf

Interesting read..

Russel · 2012-12-01 07:47:08

Here's another design factor I've not seen widely considered.

Would you rather

a) Aerobrake into high Mars orbit, then commence entry and landing.

b) Land directly.

Point is a) has its hazards - you'd need to be pretty confident in your navigation

And b) has it hazards - you come in faster meaning all else being equal you end up with less margin for error in final descent

I wonder if anyone's thought about this?

With reference to the mission idea I posed above. You get a choice, provided you're travelling with a fresh landing/ascent vehicle. If you don't you've only got one choice - aerobrake and catch up with your descent vehicle in orbit. Personally, I'd have to be convinced that either my landing vehicle can handle the higher speed with plenty of margin, or that someone's made aerocapture foolproof.

Of course, its possible that you get an option there too. If your approach into aerocapture isn't good, or your fuel isn't where it should be, you've got a window into which you can jump into the landing vehicle. It may even be possible to abandon the space hab, and use the remaining fuel to aerocapture, orbit then descend in the landing vehicle.

Btw, the above considerations is part of why I like missions like the above - there's always another workaround if at all possible - apart from getting hit by an asteroid.

(Yes, I was an asteroids addict once.. )

GW Johnson · 2012-12-01 09:30:42

For landing big items on Mars, I like entry from low orbit better than direct entry or entry from a high orbit. This is because the entry angle is enforced to be very shallow and the speed at interface minimum, with no risk of ricochet off into space. Shallow entry angle and minimal interface speed lets the hypersonics slow you down to around local Mach 3 at altitudes you can cope with, even at truly enormous ballistic coefficients. Up around 1000 kg/sq.m. Entry speeds that way are around 3.5 km/sec. The local M3 speed is around 0.7 km/sec.

For the numbers I have been running, I get a Mach 3 terminal altitude (at a very shallow flight path angle) in the neighborhood of 5 km for craft massing 60 metric tons. That's too low for a chute or ballute (or any other aero decelerator) to work, but it is low enough for simple rocket braking to be quite practical. All that is needed is supersonic retro thrust. NASA looked at this in the wind tunnel about 1960, but has since forgotten all about those results. It's an ancient NASA TN cited in Hoerner's old "drag bible" ca. 1965, which shows the data. Retro thrust reduces supersonic drag coefficient by about half. So, plan your retro thrust as if you were landing in vacuum, and you'll have a small kitty of extra emergency propellant. Which is the smart thing to do, anyway.

There is a plume stability issue: an axial retro plume does not "know" which way to bend when it reverses into the slipstream. So it flip-flops around. That induces side forces and moments that could tumble the craft out of control. The "fix" is multiple thrust nozzles, each canted a bit off-axis. It will be stable because each plume "knows" which way to bend. We've already seen this with the thrusters on shuttle, the X-15, and a few SR-71's so equipped. We're about to see it again when Spacex puts the big Super-Draco thrusters on their Dragon capsule.

In short, there's no technical reason this cannot be done. It ought to be tried in the next unmanned Mars probe lander, before we commit to landing men with it. That would be the smart way to do it, but it requires breaking with traditional practices. Tradition dies hard. Landing big things on Mars is not nearly as hard as been recently ballyhooed. But it does require doing something different than the same aeroshell and chute family we have used since Viking.

As for the cradle idea, what do you do about crew bailout if there is an ascent or descent failure? The separable crew capsule idea can be made to handle that as a one-shot "ejection-seat" item. Just a question, not a claim that it cannot be done. I have never before considered hypersonic bailout in nothing but a suit. I dunno whether it can be made to work or not, here or Mars. The Baumgartner jump recently does suggest there might be hope.

GW

Last edited by GW Johnson (2012-12-01 09:56:51)

Russel · 2012-12-01 20:03:08

In no particular order..

The baseline here is a fully pressurised capsule with its own abort capability. Much like the proposed Dragon/Super Draco. Since the cradle is sized to deliver a reasonable sized payload in a reasonable number of trips (that means to me about 6-8 tonnes per trip) its not that hard in its crewed version to give the capsule a reasonable amount of fuel, and even a backup parachute.

I guess you'd have to think through where and under what circumstances the cradle itself might become a dead weight and how to maneuver from under it.

I'm a bit reticent to consider a crew travelling on a bare cradle with no more than (perhaps) a faring. It would be an interesting trip, especially if you're chasing the space hab into high orbit.

As far as bailout goes. I think that's possible but you don't get a lot of time to make decisions. You'd need both a reasonable chute, and a personal rocket pack to actually land, methinks. Baumgartner would be jealous that's for sure.

Going back to plume stability. There's a bunch of questions here. First, at what speed ranges are we dealing with exactly. Where does local Mach 3 put you? Closer to yep, we can do this easily? Or closer to OMG that aint gonna work?

With the cradle I was considering a slight angle - around 10-15% for the thrusters. Why? Just gut feeling. Some ideas I have floating in the back of my head involve thrusters that can be maneuvered, along with the arm holding them - which dovetails into ideas that are related to packaging. Its possible the capsules angled thrusters may play a useful role. So you can steer a nozzle to suit the phase of the flight.

Does anyone know when SpaceX will test its Dragon/Super Draco in real world hypersonics? Or if they even plan to do so, thoroughly? By that I mean starting the Super Draco even at speeds that aren't really necessary to prove safety of human flight, but which reveal more data about what goes on in hypersonics.

Going back to the 5Km altitude for a heavy craft. How sensitive is that figure - what's the error bounds given every conceivable variable such as atmospheric variability. And where does that place you on Mars. I'm pretty sure its doable. You're going to run into the same problem I have which is where are the nozzles, and how are they configured or deployed (wrt to the shield) and how are they covered, if at all. After all, with a 60 tonne craft you're also dealing with fairly sizeable motors.

Here's a really goofy concept. Suppose you have a nozzle where the edge of the nozzle is aligned with the surface of a heat shield, in the middle of the shield. Leaving aside the question of whether entry will bother a nozzle at all, what if you were to pump a small quantity of fuel through the nozzle (no ignition). Would that have a cooling effect?

As you know I'm reluctant to land people with big cargo. I'd rather land them in a smaller craft, with lots of margin. Having decoupled the crewed landing from the larger items - presumably habitat - then I don't mind if the larger items are slightly riskier.

And speaking of robotic missions, I think its also an opportunity to test ideas like the cradle too.

Going back to large craft, since you're relying heavily on a gentle landing from low orbit that commits you to prior aerobraking, does it not? How confident do you feel about that problem being solved? The point I'm making here is the bigger craft with the higher ballistic coefficient also has the greatest challenge doing a satisfactory aerobrake.

Last edited by Russel (2012-12-01 20:03:59)

Russel · 2012-12-01 20:23:24

One other lingering thought..

If you're at 5Km and you're travelling down at 700m/s, what rate of deceleration do you need?

My calculation puts it at 50m/s/s

So, some serious thrust, no?

louis · 2012-12-02 09:31:01

I'm no expert but I've always been attracted by the idea of orbital deceleration (combined with retro rocket). So what if you linger in orbit for a month? I doubt that will make much difference to anything. Also, it is good because you can make a weather assessment - with direct entry you might have to commit to landing in a dust storm which can't be ideal.

Russel wrote:

Here's another design factor I've not seen widely considered.
Would you rather
a) Aerobrake into high Mars orbit, then commence entry and landing.
b) Land directly.
Point is a) has its hazards - you'd need to be pretty confident in your navigation
And b) has it hazards - you come in faster meaning all else being equal you end up with less margin for error in final descent
I wonder if anyone's thought about this?
With reference to the mission idea I posed above. You get a choice, provided you're travelling with a fresh landing/ascent vehicle. If you don't you've only got one choice - aerobrake and catch up with your descent vehicle in orbit. Personally, I'd have to be convinced that either my landing vehicle can handle the higher speed with plenty of margin, or that someone's made aerocapture foolproof.
Of course, its possible that you get an option there too. If your approach into aerocapture isn't good, or your fuel isn't where it should be, you've got a window into which you can jump into the landing vehicle. It may even be possible to abandon the space hab, and use the remaining fuel to aerocapture, orbit then descend in the landing vehicle.
Btw, the above considerations is part of why I like missions like the above - there's always another workaround if at all possible - apart from getting hit by an asteroid.
(Yes, I was an asteroids addict once.. )

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