Martian Rockets

JoshNH4H · 2013-11-24 14:05:24

At some point, it will be time to develop an indigenous launch capability on Mars. This wil ideally happen sooner rather than later, as the Martian colony begins to have the ability to produce material goods to sell to Earth. For the moment, we can ignore the question of what exactly these goods will be.

A more important question at this time is how we're going to send them there. Let's say that for the purposes of this thread we don't care what they are, only that their density is at least 1000 kg/m^3 and that we're looking for a launch vehicle that's initially in the 1 tonne class. Later, we might want to expand it, but by that time we'll probably have a Basalt Fiber space elevator going.

Anyway, Mars has an escape velocity of 5.1 km/s. Earth's escape velocity is 11.2 km/s and the delta-V required to get to orbit is about 9.25 km/s. This is a ratio of .83. Applying the same ratio to Mars implies a delta V to orbit of about 4 km/s, keeping in mind that unlike Earth atmospheric drag will be negligible. I realize that this method is not rigorous, but it's a close-enough kind of guess, barring more rigorous calculations.

Anyway, we have a rocket stage to design. An early colony will not be nearly so technologically advanced as we are here on Earth today, specifically when it comes to very fine manufacturing. This means that we can't get down to those 5 or 10% structural fractions we get here on Earth.

It's also quite possible that they will not have access to the same range of materials that we have here.

Basically, we want to manufacture a rocket that is simple to make but also as cheap as possible. I don't think that reusability is an option in the sense of landing and relaunching, but it might be possible to use it (refueling at phobos/deimos if possible) to send cargo to the Earth/Moon space.

As I see it, liquid fueled rockets (Probably CH4/LOX), Hybrid rockets (Probably some kind of organic polymer, maybe polyethylene and LOX), or solid rockets (Aluminium/Carbon and Ammonium Nitrate/Perchlorate) are all possibilities, but I'm not sure which would be the best.

Tom Kalbfus · 2013-11-24 19:25:34

How about bringing some industrial 3D printers to Mars, we print out the parts for the rockets out of local materials and then we assemble them into rockets?

JoshNH4H · 2013-11-24 20:33:51

Well, I can think of a couple issues.

1) Rockets are made of metal. 3D printers print plastic.
2) The ABS plastic used by 3D printers are made from monomers that would be highly nontrivial to synthesize on Mars.

As an added bonus, rockets are also quite large and therefore not easy to 3D print, plus 3D printers require some relatively fine manufacturing techniques to build.

I was thinking earlier that you could build tanks using cold or hot drawing in order to extrude the cylindrical portion of the tank as one piece. I don't know how much better this would be than doing it as a series of curved pieces of metal welded together.

RobertDyck · 2013-11-24 20:48:09

There are several different "3D printer" designs. One extrudes melted plastic, creating a very hollow thing out of plastic string. It's rigid, but lots of air gaps. Another uses powder: use an ink jet printer to apply glue, blow plastic dust and scrape off an even layer. Repeat. When finished, dig the finished part out of dry powder. This builds solid plastic parts, but they're just plastic powder held together with glue. There are 3D milling machines for wood or metal. They don't print, they're mills. And the last one is an actual 3D printer with metal. It blows metal powder onto the piece being assembled, and uses a laser to melt metal powder onto the partially assembled part. Again, there are air gaps between metal particles. And the metal is not at all tempered, it has horrible thermal stress.

Of these, the only one that produces metal parts capable of handling the stress of a rocket engine is the milling machine. That takes a solid block of metal and cuts away what isn't desired. Building a rocket engine requires a lot of different fabrication techniques. The best technique to fabricate the exhaust cone is a pressing sheet titanium alloy in a multi-tonne press. Spin sheet titanium to make the bell cone, then use a press to impress the channels. The inner cone is also spun, but left smooth. Then place brazing compound between the two, weld the outer seams. Then bake in an oven hot enough to melt the brazing compound.

Fabrication of functional mechanical parts is more involved than just pushing a button on a Star Trek replicator.

JoshNH4H · 2013-11-24 20:59:27

I'm a bit skeptical when people say that 3D printing technology is a magic bullet that can accomplish any manufacturing task. That's not to say that it, and other forming techniques aren't useful but they can't do everything.

I am off the opinion that physical vapor deposition holdsa lot of promise asa manufacturing technique. It's somewhat like printing, but it would make virgin solid parts with few defects.

For Iron you could do chemical vapor deposition; I suspect that if Steel is desired it will be possible to find an organic compound that will decompose with the Iron Carbonyl.

GW Johnson · 2013-11-24 22:49:25

I to have heard there are 3-D printers that print metal. As best I know, it's basically a sintered-metal part. Mechanical properties of sintered parts are far inferior to castings, much less forged parts. We're not ready to print whatever we need. Still gonna need steel/aluminum/titanium mills for a while yet, I'm afraid.

GW

Terraformer · 2013-11-25 04:35:35

Well, at least making them reusable won't be difficult, so we don't need to make a rocket for each launch. I would go with MethLox, because we'll already have the ability to produce those two (we need them anyway, after all), and they offer a high enough performance that reusability is still an option. We're talking about a mass ratio of ~4, right, for a single stage? If we can't do it in a single stage, we can always use a popup first stage, which can do double duty as an exploratory vehicle when not needed. If that gives us over 1km/s, we can make the upper stage have a mass ratio of 2.5, and with 30% structure, that lets us take 10% as payload. So it will probably only be talking 2% overall, but once the craft are built, they should be pretty tough things and well able to handle a lot of launches.

Tom Kalbfus · 2013-11-25 19:17:44

GW Johnson wrote:

I to have heard there are 3-D printers that print metal. As best I know, it's basically a sintered-metal part. Mechanical properties of sintered parts are far inferior to castings, much less forged parts. We're not ready to print whatever we need. Still gonna need steel/aluminum/titanium mills for a while yet, I'm afraid.
GW

But were not going to have a Mars colony next year either, or the year after that either, by the time we do have a colony on Mars, I dare say those 3D will be considerably more advanced than they are today. An optimistic timeline for establishing a Mars colony would have it by the mid 2020s at the earliest, that is at least 10 years of further development of 3D printers!

Tom Kalbfus · 2013-11-25 19:20:23

Terraformer wrote:

Well, at least making them reusable won't be difficult, so we don't need to make a rocket for each launch. I would go with MethLox, because we'll already have the ability to produce those two (we need them anyway, after all), and they offer a high enough performance that reusability is still an option. We're talking about a mass ratio of ~4, right, for a single stage? If we can't do it in a single stage, we can always use a popup first stage, which can do double duty as an exploratory vehicle when not needed. If that gives us over 1km/s, we can make the upper stage have a mass ratio of 2.5, and with 30% structure, that lets us take 10% as payload. So it will probably only be talking 2% overall, but once the craft are built, they should be pretty tough things and well able to handle a lot of launches.

They will still need spare parts for servicing and maintenance, so we'll still need the printer, and the printer can print parts of Mars Rovers and Space Suits as well.

JoshNH4H · 2013-11-25 23:25:04

Terraformer-

While I agree that reusability is not notionally impossible, it probably won't be easy. My reasoning for this is simply to point to the state of reusability technology on Earth today, even with its massive manufacturing base. We haven't done it yet, so that means it will probably be beyond the capability of an early Mars colony.

If Phobos turns out to be chondritic, it should be possible to produce methlox fuel there, refuel, and do a burn to initiate descent. Alternatively, the low gravity implies that engine mass will be significantly smaller than it is here, making tankage proportionally larger. If the tanks are disposable but the rocket engines are not, and the payload gets dropped off in LMO, it should be a relatively light task for the engines to return.

For example, let's say you have methlox engines with a T/W of 50 [relative to Earth's gravity]. This is much lower than a typical kerolox engine on Earth, by a factor of about 2 (3 compared to the Merlin 1D engines). Let's say your mission delta-V is 4 km/s both ways. Let's say your tankage and "other stuff" masses 20% of its components. Let's say your desired Thrust-To-Weight ratio at liftoff is 2, in Martian gravity. Let's say your trajectory averaged exhaust is 3.7 km/s (Mars has a pretty negligible atmosphere, after all. This also makes a relatively low-pressure engine possible; Did you know that you can get SSME-like pressure ratios of 200:1 even if your chamber pressure is 1.2 atm? In a low-pressure environment, all sorts of things become possible.

Anyway, what we end up with is that you need a mass ratio of 3 to achieve orbit. Your rocket needs to be 67% liftoff fuel, 1.6% engine, 13.4% tankage. This leaves 18% of GLOW as payload. However, a mass ratio of 3 would also be necessary in order to rocket-brake the engines all the way down to the ground. This would also necessitate some tankage. If you do out the math, you find that 7.3% of Gross Liftoff Weight has to go towards reusing the rocket engines. This leaves you with about 10% of the Gross Liftoff Weight as payload, reusable engines, and cheap throwaway tankage. As an added bonus this can be improved by dropping the tanks during flight, even though it's not necessary. I'd imagine that the tankage might have some value for construction of space infrastructure in Mars Orbit, and potentially elsewhere.

How does that sound?

GW Johnson · 2013-11-26 10:34:28

Here's a weird notion. I'm not at sure this is possible, but it might be.

Most plastics and elastomers are flexible at room temperature, but stiff when very cold. Make the tankage out of it, since we're talking methlox here, it's not extreme cryo like LH2. Warm up the empty tanks after the burn, fold them up, and stow them inside for the return in what is effectively a much smaller vehicle, for reuse.

You have to carry the mass of the "inflatable" tankage, yes, but you don't have to deal with a clumsy or vulnerable shape during entry.

Think a small capsule shape with the crew, cargo, some landing legs, and engines. Couple a bunch of these "inflatable" tanks to its "nose" for the ascent, and stow them inside for the descent. Some sort of rubberized canvas, multiple layered, perhaps?

GW

Last edited by GW Johnson (2013-11-26 10:35:39)

JoshNH4H · 2013-11-26 12:51:36

Well, I'm not sure the mass budget exists to do that on rocket power alone, unless we can skimp on tankage. Based on my calculations, (without orbital refueling or aerobraking) it's possible to make tankage reusable if the rocket structure (including the tanks and basically anything that's not an engine) masses 7% of the fuel mass or less.

I don't think aerobraking is a viable technology so soon, but it would of course make this calculation irrelevant.

GW Johnson · 2013-11-26 13:32:30

We're talking about travel between Mars's surface and orbit about Mars, or at least I thought we were. If you do not have to surround your tanks with structure, and you don't have to fit them inside any particular shape, I think your hard vehicle structure will be smaller and lighter than it otherwise would be. Your tanks are basically very lightweight inflatables, with a tad of tie-together hardware. I don't see why such tanks wouldn't fit in a very small mass budget, like 5-7%.

I was showing about 3.7 to 3.8 km/s for getting from the surface to orbit. Your figure of 4 is about right, and covers a de-orbit burn, and more. Once you stow the empty inflatable tanks inside, you have a small capsule shape with a heat shield, which can slow aerodynamically to about M3 at low altitude (5-10 km), which is about 0.7 km/s. Then just do retro thrust to landing. No more than about a km/s worth of delta-vee should cover that.

So we're talking about something on the order of 4.5 to 5 km/s total velocity increment for the round trip, a low-density ablative or refractory heat shield, landing legs, and inflatable tankage for most (but not all) of the propellant. For methlox, I can easily have a Vex near 3.1 km/s. At 5 required, I get a mass ratio of about 5. That's 80% propellant. If the inerts are around 10-15% (and with inflatables, this might be possible), you have 5-10% payload.

With LOX-LH2 it is even easier. Vex = 4.4 km/s is easy. MR= 3.1, which is 68% propellant. For 15% inerts, that's 17% payload. And I'm not at all sure that LOX-LH2 might not be the better choice, if a suitable inflatable can be devised. It's made from nothing but ice. All you need is a buried glacier and some solar electricity to make the gases. LH2 is tougher to liquify, though. And that super-cryo inflatable will be very tough to devise.

Just some random, not-very-sane thoughts.

GW

Terraformer · 2013-11-26 16:50:07

You could possibly import engines from Terra, if we aren't using fully reusable craft by then. Plenty of discarded upper stages that are waiting to be stripped...

SpaceNut · 2013-11-26 17:11:29

I guess the way that I see the mars colony getting started with trade is to reuse the cargo landers, parts and pieces in such a way as to create a new rocket to get to earth.

JoshNH4H · 2013-11-27 00:43:32

GW-

My expectation was that aerobraking would be too hard to control with relatively simple technology. Maybe with a retro-fire it would be easier though.

I find it interesting that the person usually arguing for high structural mass fractions for reusable rockets is saying that 5% structure is fine. What's your reasoning?

Terraformer and Spacenut-

I was thinking we'd want to build the rocket engines on Mars if at all possible. Why import an item like that, which is fairly heavy and rather expensive to build?

Terraformer · 2013-11-27 03:27:31

I thought GW was arguing that it was okay *if you're using retrorockets*? Because they wouldn't take a beating every time you return.

The engines are already in space, why not send them over to Mars? Obviously this is assuming they're designed for it, but why not design our upper stages with recyclability in mind?

GW Johnson · 2013-11-27 10:58:38

This thing lost my response just as I finished typing.

OK, try again: I had in mind a sturdy, high-inert fraction capsule-shaped Mars ferry, just a lot smaller. It has conventional internal tanks for only the rocket-brake landing. The ascent propellant goes in very-low-inert (5%-ish) balloon tanks tied externally to the nose "somehow". Don't know how yet, or even that an elastomeric cryo-tank balloon is possible. But if it is, there's a huge advantage to it.

I don't see why such a balloon tank need be discarded after ascent. It is not that much weight to carry around. Warm it up and then fold and stow inside the capsule for the descent. On the surface, get it back out, reinflate it, and reload propellants for the next ascent. We're probably talking about multiple tanks of the same form factor, in a cluster. Might need a mast sticking out of the capsule's nose, I dunno.

Control during retro-braking descent is by simple attitude thrusters. Just use bigger ones to offset the larger disturbing forces. I'm not talking about the whole descent, just the final phase where you come out of hypersonics at local Mach 3 (0.7 km/s) around 5-10 km altitude. Thrusters like that could also do the de-orbit burn that starts the descent.

Retro thrust during descent is by the ascent engines, firing through open ports in the heat shield. If you seal the engine compartment, you stop the entry slipstream throughflow, other than a very minor pressurization transient as you descend. It would be easy to provide an offsetting gas flow into the compartment, if heat-sinking isn't enough to handle the pressurization intrusion of slipstream gas.

All just wild guesses.

GW

Last edited by GW Johnson (2013-11-27 11:03:20)

SpaceNut · 2013-11-27 21:58:08

The engines I was thinking of were part of the powered descent stage such as on the skycrane or lunar module which use Monomethylhydrazine

(MMH) is a volatile hydrazine chemical with the chemical formula CH3 (NH)NH2. It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as nitrogen tetroxide (N2O4) and nitric acid (HNO3).

Which if we are shipping some ammonia for the purpose of making fuel by electrolysis of it https://aiche.confex.com/aiche/2005/tec … P25627.HTM

Since there is some nitrogen in the atmosphere it can be used to suplement the amount of fuel that we can create.

JoshNH4H · 2013-11-27 23:44:15

Well, plugging back in, carrying back tanks that mass 5% of their contents will cut the payload by half compared to leaving the tanks in orbit. I'm not sure what the economics are, but fortunately it is something that could be transitioned to over the course of the rocket.

GW Johnson · 2013-11-28 10:43:34

If the payload fraction is big enough, losing some of it to reusable ascent balloon tanks could well be worthwhile. Elastomers will be hard to come by on Mars, at least initially. Does anyone know if there really is a suitable elastomer for LOX and LCH4? It doesn't have to bend while cold, just not spontaneously crack.

As for N2O4 and MMH, those are room-temperature storable, and we already have elastomers used as fuel bladders for them. The difference between that scenario and mine is we dispense with the surrounding metal pressure tank and run the bladder "naked" as a balloon. I'd bet they need to be composites: essentially rubberized canvas, to control shape.

Tanks like that would need to be pressurized somewhat, much like the stainless steel balloon tanks on Centaur. Then you withdraw propellant with a pump as you add pressurant gas to the expanding bubble on top of the propellant inside.

XCOR is demonstrating that the propellant pump can be positive-displacement piston technology, which has a far longer potential service life, as nothing is on the "hairy edge" the way turbine blading has to be.

I don't see why this reusable ferry can't be built almost right now, and simply used to make a bunch of landings on the first trip. It should go on the first mission, and be left there for subsequent missions and base personnel to use.

GW

SpaceNut · 2013-11-28 14:16:29

I am thinking that the reason for the spontaneous cracking has to do with the temperature differential from the fuel temp to that of the outside, basic laws of contraction and expansion taking place under some inside pressure is enough to cause the cracking. Usually even a composite tank has an inner shell of aluminum to help the outer composite hold shape as it is made of composites of carbon and fiber glass. One place I worked had had composite tanks that were the size of a divers and it would hold 5000 psi of o2 for extened use.

JoshNH4H · 2013-11-28 14:19:32

How about PVC with some plasticizers added?

GW Johnson · 2013-11-30 09:57:26

PVC/plasticizers? I dunno. I'm not that good with the plastics and elastomers. Metals, wood, and organic composites I have experiences with. But I do know a little about aircraft fuel bladders, and the bladders used in liquid-fueled missiles. Those are various rubbers, including both a polyurethane and neoprenes. They work with what we consider to be "room temperature liquids". Under military specs, that's as bad as -65 F to 145/160 F.

If the Mars ferry used NTO-MMH or similar "storable liquids", the tank bladder materials we have right now could be part of the balloon tank we are talking about here. I'd use the tank bladder material as an inner liner layer, with a "rubberized canvas" outer layer, perhaps of neoprene and kevlar, or even just cotton. It would probably need an outer fabric shell with some distributed heating elements built-in, so that it could be maintained warm enough to fold up for stowage without cracking.

If we use cryopropellants, then we'll need to develop a combination of presently-unknown materials that would work very cold. Most of the elastomers that we have undergo glass transition at too high a temperature even for LOX.

I don't know much beyond that, so I cannot judge whether a cryo-propellant balloon tank could be developed in time for a Mars mission in the 2020's or 2030's.

GW

JoshNH4H · 2013-12-01 00:00:40

PVC with plasticisers is the stuff they make air mattresses out of, I believe. I'd imagine its low temperature properties are poor, as with other polymers (no matter how much hard-to-manufacture plasticiser you add), but it is foldable at room temperature.

The issue with MMO/N2O4 is production: We haven't determined whether or not there are recoverable nitrates on Mars, so in the interim we have to expect that they're not there. While the Haber-Bosch Process to make Ammonia is pretty well-established and will be absolutely necessary for a Martian colony, things like Hydrazine are harder to make, not even to speak of N2O4.

Hydrazine is made using the Olin-Raschig Process, which requires NaClO and Ammonia, and has a couple of toxic intermediates. It also uses Aniline to separate the water from the Hydrazine.

N2O4 is produced by the oxidation of Nitric Oxide. Nitric Oxide has to be produced by the oxidation of Ammonia above a Platinum catalyst at 800 C.

The fact that the two are hypergolic means that it's not surprising that production and handling are annoying and moderately difficult.

I think what I'm saying is that, if fuel costs are a significant portion of rocket cost, it will be desirable to use something like Methlox over hypergols, even if the rocket engine gets a bit more complicated, because it requires an ignition source.

Here's an idea I've always wondered about: a liquid fueled (for example methlox) engine that uses a Hydrazine monopropellant as an ignition source. Rather than the solids they use in rocket engines today, this would be limitlessly restartable. The monopropellant technology is fairly simple.

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Martian Rockets

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