Concept of a single core MCT launcher
The shuttle
One might think that the launcher architecture does not directly impact the design of the shuttle, but this is not the case. The single core configuration does in fact necessarily implies that the ship be at the top of the rocket (above the second stage). As a result, the aerodynamic shape of the latter should not have a significant bearing surface, which would force making the rocket too unstable to remain controllable. The monstrous tail surfaces that the designers of the DynaSoar project had to provide their launcher with in order to stabilize it, is an interesting illustration of this problem!
Artist rendition of a Dyna-Soar launch.
This constraint will result in a severe limitation in terms of reducing the ballistic coefficient (Mass / (CxSurface)), detrimental in terms of the moderation of flows and loads on the thermal protection that is to be fully reusable and, especially in terms of atmospheric deceleration ability, leading to an increase in the mass of propellant dedicated to the final braking.
The large mass of the shuttle during the Martian atmosphere entry phase (over 200 tons), prohibits a capsule shape, which should be equipped with an oversized shield. Nor could a capsule provide the minimum volume to accommodate 100 passengers (the absolute minimum per person is estimated at 10 m3). This leads us to retain a cylindroconical lifting body formula, close to that considered for DRA5, with a diameter (12 m) and a length (39 m) allowing lodging passengers, tanks and a cargo bay.
In this version of a single core launcher, we chose as the only option for the return trip the « conjunction » scenario trajectory, in a Hohmann mode (with a payload limited to 5 tons). However, the choice of this scenario implies that the shuttle will have to stay 18 months on the surface of Mars before taking off for the return leg of the trip, and that, therefore, it will not be available for the launch window following its own; in other words it would only be used once every 52 months! This is not consistent with the ambitions of the SpaceX management, for whom the MCT must eventually induce a commercial business … That is why, in the section dedicated to the multi core version, we will discuss the possibility of an immediate take-off after landing, for a return in an « opposition » mode, even though it would be more expensive as far as the propellant mass (and takeoff mass) is concerned. We will see that, in this case, it is the return leg which constrains the size of the shuttle tanks.
But in the present case, it is the outward leg that constrains the sizing. This is explained by the fact that (incidentally in both cases) the analysis of the performances and of the launch vehicle recovery capabilities, leads to assign to the shuttle itself the burden of supplying the ΔV injection for the Earth to Mars transfer orbit, but also the final propulsion effort for reaching the parking orbit (on LEO). To this end, we put aside a propellant mass of 400 tons; this facilitates the return of the burnt stages to the Earth surface, in particular by limiting the speed at the end of the combustion of the second stage.
As far as the propulsion of the ship is concerned, we kept the ventral engine option only, with a « flat » Martian landing and take-off. Taking into account the presence of these engines and the limitations of winged area, it is possible to consider a final terrestrial landing not in a gliding mode but in a propelled « helicopter » (hovering) mode (which also allows to lighten the landing gear).
Taking into account the shape constraints, the necessity to minimize the ballistic coefficient which a priori is a disadvantage in this configuration, the different volumes to lodge into the shuttle (at least 1500 m3 for the passengers) and the resulting centering balance problems, we devised a layout project. Further, extrapolating the data estimated in the previous study, we devised a mass estimate that we checked by computing the masses of propellant needed to provide all the propulsive maneuvers of the outward leg (final LEO injection, transfer injection, maneuvering in the Martian environment, final braking during EDL).
It turns out that the payload should be reduced to 95 T (Mars at perihelion)..
| DATA | |
| Length (m) | 39 |
| Width (m) | 12 |
| Height (m) | 12 |
| Payload mass, outward leg (T) | 95 |
| Payload mass, return leg (T) | 5 |
| Tanks structural coefficient | 0.035 |
| Propergol mass coefficient margins | 1.033 |
| Complement shuttle prop for LEO injection (T) | 400 |
| MANEUVERS (km/s) | |
| ΔV outward leg (Hohmann, Mars at perihelion) | 3.46 |
| ΔV return leg (Hohmann, Mars at aphelion) | 6.52 |
| ΔV maneuvers on Mars orbit | 0.15 |
| ΔV EDL braking & « Helico » | 0.60 |
| ΔV Return to Earth & landing maneuvers | 0.60 |
| MASS ITEMS (T) | |
| Equipments | 12.0 |
| Engines : 4 x 83 Tthrust | 3.5 |
| Structures & Thermal Protection | 45.0 |
| Tanks | 29.6 |
| Dry Mass | 90.1 |
| TOTAL MASS (T) | |
| In LEO Parking | 630.9 |
| During Earth to Mars transfer | 241.5 |
| at entry of the EDL process | 224.6 |
| Taking off from Mars | 565.3 |
| PROPELLANT MASS (T) | |
| For transfer injection E to M | 389.3 |
| For Mars orbit maneuvers | 16.9 |
| For EDL | 39.5 |
| For return flight | 550.1 |
| For return to Earth maneuvers (incl. landing) | 16.9 |
Features of the shuttle for a single core launcher version
Schematic shuttle layout with a single core launcher (pre study, length shortened to 35 m, pax volume limited to 1000 m3).
The required performance in LEO is 631 tons tons (with contribution of the shuttle propellant). Warning: this may be achieved only in the most favorable conditions of transfer:
- Hohmann trajectory;
- Mars at perihelion at the time of arrival.
- And with a lower payload mass (minus 5 tons).
It seems right, for the purpose of this feasibility study, not to consider tougher assumptions aiming at shortening the travel time; a 6 months trip would require an additional ΔV of 880 m/s which would cut the payload, all things being equal, down to just over 60 tons… Since we are speaking of a “mass” transport scenario of passengers, we may assume that the interior design will allow a level of protection against radiations resulting in individual doses consistent with that which their long stay on Mars will make them endure anyway.
This being said, it is necessary to consider performing a launch at each window, even in the worst case, when Mars is, upon arrival, near its aphelion. In this latter case, computation shows that the payload goes from 95 tons down to 80 tons.
The 225 tons mass at EDL entry induces (in a hypersonic mode and a high-incidence, Cx = 0.8)) a ballistic coefficient of the order of 600 kg/m², that is to say of the same order as that of the Space Shuttle. This is not a good result; we will see that, in this respect, the multi core version is to be preferred.
Tanks capacity must reach 846 tons (sum of the outward trip items). This large quantity contributes to the choice of a 12 m diameter (which will also be that of the second stage of the launcher), knowing that, in order to ensure a good emptying within the non axial propulsion configuration chosen, nearly spherical tanks are preferable.
The interface with the launcher allowing, it would be interesting to flatten the (hot) belly of the shuttle, so as to increase the effective braking surface.
General view of the shuttle, in Mars orbit.
Embarquement pour Mars – Préface Thomas Pesquet
Webinar #2 APM sur STARSHIP disponible en replay
RSS
A very interesting analysis. But I sincerely doubt SpaceX would go the route of sandwiching their passengers in amongst all that rocket fuel. So far everything that they have done has been safety first, re-usability second, cost third, and everything else following on from there.
A single core seems highly improbable, but multiple cores joined like Falcon Heavy (maybe even in a 5, or 9 core config) seems more likely. It maintains their philosophy of doing everything like a production line.
But even multi-core doesn’t preclude the placing of a large craft on the top.
And have you considered that landing of payload my not require the whole thing to return to earth? Payload capsules could be left behind (even maybe acting as shelters) which would reduce the return mass considerably.
I think you need to seriously consider the cost of the relative parts to see which bits they will return and which will get left behind. It’s not all or nothing here.
Anyway, fascinating analysis. Thanks.
hi,
*passengers safety: in any case, as there is no possibility to install a launch escape system, it could prove necessary to ferry the passengers with « taxis » after the mammoth launcher has placed the MCT in LEO.
*placing the craft on the top: as I wrote in the report this poses the dilemma of choosing between a low ballistic coefficient (to improve EDL and thermal conditions) and a lighter vessel (with no wings at all) ; I choose the first option, as I thought it was a condition for a really reusable Thermal Protection. I may be too pessimistic.
*not returning everything on Earth: many people think even not returning the shuttle on Earth and refueling it on LEO… In this case, we certainly come to a less gigantic launcher, but on the cost of multiple launchs and augmented operations complexity.
Thank you for your contribution.
Richard
You start with the assumption that the Mars landing craft must have a minimum habitable volume of 10m^3 per person. Dragon v2 has a habitable volume of 10m^3 for 7 people. Why would the Mars landing craft be any bigger than necessary to accomodate 100 people crammed together in their seats? This landing craft could be a capsule shape. I think the MCT architecture will go something like this… 100 person capsule is launched into LEO where it is docked with (3) BA-330’s and a Mars TMI stage. With a 6 month voyage to Mars, everyone will have about 10 m^3 of space. Once at Mars, the capsule separates and performs EDL. The BA-330’s head back to Earth on a free-return trajectory. A capsule on Mars could launch and rendevous with the BA-330’s if anyone wants to go home.
That makes a lot of sense. I think a permanent spacecraft (as in, a spacecraft that never reenters the atmosphere), is very likely. The use of BA330, if bigelow is successful, is almost a given. Isn’t 10m^3 a little cramped for a month long voyage ? I’m trying to remember how big a dragon is, and I wouldn’t like to live inside one for month. But space is a hard place, so it might do.
Also consider that this is 10m^3 of 3D space in microgravity. Every wall and ceiling is another potential floor. Take a look above you at the ceiling (assuming you’re indoors) and imagine what you could do with all that unused real estate if gravity weren’t in your way. With that in mind 10 m^3 might even be too much room.
10 m3 is not for EDL but for the long transfer journey. It’s very minimum ; 15 m3 preferable (and possible in this design).
Landing 100 pax / 100 T total P/L within a capsule is not feasible ; the b allistic coeff would be very high, with inefficient deceleration and very high thermal conditions. You need to present a large surface to the atmosphere. Furthermore, you need another different vehicle to come back !… The idea of this study was to have only one piece of HW (the shuttle) and to not throw away anything (fully reusable). That said, this principle leads to an over-sized launcher, and raises the question of the reusabillity period (52 months except if it can afford an « immediate » (opposition type) return.
Your proposal of a BA330 on a free return trajectory is seductive. But it looks like the cycler concept, which Elon Musk has explained why it will not be his choice.
You forgot about in orbit refueling,
as per Musks comment:
« I mean, if you do a densified liquid methalox rocket with on-orbit refueling, so like you load the spacecraft into orbit and then you send a whole bunch of refueling missions to fill up the tanks and you have the Mars colonial fleet – essentially – that gets built up during the time between Earth-Mars synchronizations, which occur every 26 months, then the fleet all departs at the optimal transfer point. »
You are righ ; I get a lot of commentaryabout that. But you have to understand that I wanted to look to this extreme solution of only one launcher. As you saw, the result is decouragingly gigantic. So yes, refueling could be a solution, but it would also be more difficult to get low travel price if you have a « bunch » of launches! Also, transefering cryogens in microgravity is not simple ; I would rather consider plugging interchangeable tanks.
Thanks!
Interchangeable tanks have several advantages; they avoid the complications of zero-g pumping, they could be collected in orbit while the expensive parts (the engines) of the tanker returns for more, and if removable at martian gravity, they could be trucked between the landing pad and the fuel factory, allowing these to be located safely apart from each other.
Richard,
I’ve seen a few attempts at modeling this before, but yours is particularly impressive and the first to show the effects of the MCT’s dry mass. This is particularly useful for me, as we have no lack of people at Nasaspaceflight.com convinced that the most logical thing to do is to fuse the second stage and the MCT lander together. The argument they make is based on economics and not mass optimization, and I believe more than a few would change their minds if they could see how heavy the MCT would need to be to make it back from Mars. You should mention to your translator, Pierre, that the term « ton » in North America means 2,000 pounds, not 1,000 kilograms. To avoid any confusion I suggest using the term « tonne », which is the correct term in all English-speaking countries for the unit you mean.
It was mentioned that your reason for this approach was the fact that “I wanted to look to this extreme solution of only one launcher”. While I appreciate the simplicity of single reusable launch to Mars, there are a number of issues besides the fact that Spacex mentioned in-orbit refueling.
–Explosion Risk: A 2,750 tonne Soviet N-1 moon launcher exploded seconds after lift-off, resulting in a 7 kiloton blast that was the largest non-nuclear explosion in human history. You propose a 13,700 tonne rocket, which would mean you would have to sell the public on allowing a rocket able to explode with a force over 2X that of the Fat Man atomic bomb to launch anywhere near them. A smaller rocket would attract less public opposition and be far less dangerous and costly in event of failure.
–Engine Count: The only LV in history to have 30 engines failed in all 4 of its launches, though obviously Spacex are working on a 27-engine rocket. You propose that the risk of a 61-engine first stage is acceptable. I think the risk of engine failure leading to a catastrophic failure is too high, even with the more benign nature of the full-flow staged combustion cycle. A more realistic alternative is a 27-Raptor first stage, which would still enable a 5200 tonne RLV capable of pulling off your Mars mission in 3 launches.
–Economics: Your rocket’s first stage is 20 meters in diameter while the second stage is 12 meters. This requires two sets of tooling, which would add enormous cost. The ideal solution would be two stages with the same diameter, which would let you build them with the same set of tooling. This would make your RLV much less expensive.
–Lack of LAS: If you are going to launch 80-100 people at a time, chances are the US government will insist on there being some way to save the crew if the rocket were to explode at launch. You should probably add LAS engines to reflect this reality.
One possible reason why Spacex’s figures are so hard to attain is because you have used an entirely conventional approach. You probably did not hear that Spacex are considering solar electric propulsion in addition to the chemical engines to up payload and trim transit time. Some calculations on our site showed that the MCT could plausibly generate more than twice the power of the ISS, which would make this a viable option. Hope that helps and thanks again for the superb work!
Why assume the lander and rocket would transport 100 at a time? I imagine a large space station that you could attach many landers to and load it up over many launches to fill this colossal space station and use it as a back and forth transport ship
I heard Musk say that they will probably use a vehicle in constant transit within the Earth-Mars system (transporter) while using tanker launches to refuel the cycler. Can`t remember the source.
I haven`t heard anything regarding EDL but their current interest in hypersonic reentry profiles would indicate that whatever tech they use to land, will not be centered around inflatable decelerators nor cranes (due to high mass of the vehicles)
I guess we`ll have to hold our breath until September 2016 when the architecture is unveiled.
Just happened across your site and I really enjoyed reading your analysis and proposals.
My own opinions are :
1. Booster necessary to lift heavy hardware to Earth orbit.
2. Earth – Mars (maybe Moon use too) Ferry, the MCT
3. Mars SSTO for transfer of material and people between orbit/ground. CH4+O2 engines
4. Nuclear plant on Mars for in situ electrical and CH4 generation.
5. Some kind of reusable entry skin/bladder/container with engines that once the payload is extracted it is folded up and shipped back to Earth for refilling. Very low volume/mass.
Also the first few rockets would carry a constellation of combined Weather, Communication and GPS satellites to place in orbit around Mars.
With the payloads being talked about the MCT could probably deliver all this in once shot.
Investigate a Phoboa and Diemos tankage / rendezvous location as well.