sevenperforce

The whole jointed-leg thing seems unnecessarily complicated. Why not just use conventional skids, but mount them on telescoping struts instead of fixed ones?

Upgrading kiln tech to enable better ceramics will almost certainly be simpler than trying to upgrade metallurgy to a comparable level. Plus, you can mix and match properties; you can use a ceramic inner layer for thermal properties with a steel outer layer for mechanical properties. A kludge, perhaps, but a workable one. We certainly wouldn't be using cryogens, so the goal is to build the SRB body with enough axial compressive strength to support the rocket in flight and enough skin tensile strength to contain the solid fuel combustion gases. The latter isn't as challenging as it seems, because the solid fuel itself will act as a heat sink and load distributor for the majority of the burn period. In fact, stage separation could be accomplished by merely making the upper portion of the booster body too weak to contain the combustion gases at the end of the burn. I'm thinking an internal wooden frame (wood is a good enough insulator to remain strong during the relatively brief burn, and its tensile, compressive, and torsion strengths will be better than metal for weight cost at this era), wrapped in thin beaten metal, with a few thicker metal bands wrapped around the outside. Getting gyros to work is hard, I agree.

Perhaps more critically, SEL2 is not actually stable. You either have to sit still with constant station-keeping, or you have to stay in a kidney-bean-shaped orbit that takes you above and below the ecliptic, exposing you to sunlight for most of the time.

Ceramic nozzle; banded metal for the body. Guidance is tricky, sure, but we could come up with something. Fuel chemistry is the main challenge.

Absolutely. Not to derail on the first reply, but if Saturn or Jupiter was teeming with microbial life right now, would we know? Jupiter's atmosphere is thousands of times the volume of our entire lithosphere; the odds seem fairly good that life could have arisen. But would we be able to see anything anomalous from here?

I can solve the rocket equation myself, but it's typically easier to just plug the values into something like this calculator (thus avoiding the potential for math errors). I'm not quite sure why you'd want to use escape velocity; the goal is merely to get to orbit. We can say 10.5 km/s to give ourselves a margin. Unfortunately, using the above calculator shows that with an ISP of 80, you would need over 655 tonnes of fuel for every kilogram of dry mass to get that kind of delta v. That's a mass ratio of 655000:1.

If ISP is too low, not even staging can save you. See what-if #24.

Because I was greedy and wanted a thread specific to my design. =P I'm not sure yours will be cheaper. Your mothership still needs a superstructure in order to support centrifugal gravity, and it needs some sort of basic shielding to protect the fuel tanks and the hab from micrometeoroid strikes. It won't cost significantly more to design your ship with the capacity to aerobrake. The aeroshell only limits geometry if you're intending to enter an atmosphere during a given mission. If your specific mission is limited to vacuum operations, then you can add whatever geometry you want -- auxiliary tankage, solar sails, massive amounts of cargo -- to increase delta V or to tow a new space station into orbit or whatever else you want to do. And once you reach your destination, you retain the flexibility to leave the vacuum-limited components in a high parking orbit and aerobrake your main ship. For each mission, the optimal landing configuration will be a function of the destination's surface gravity, escape velocity, atmospheric properties, and surface resources, plus the intended duration of your surface mission. There will be missions where it's cheaper overall to land the whole ship, there will be missions where it's cheaper to bring along a separate but reusable descent/ascent vehicle, and there will be missions where it's cheaper to bring along an expendable descent vehicle and a reusable ascent vehicle. And while the extra fuel to land on certain bodies might mass more than individual landers, fuel is cheap and simple and may be accessible on-orbit, while landers may not be any of those things.

That works to decrease overall launch costs, but it means you have to plan missions much further in advance and greatly limits flexibility. Having a landing-capable mothership in no way prevents you from bringing along one or more smaller landers. Nor does it prevent you from carrying additional fuel. The flying-saucer design in the other thread would probably be provided with a standardized on-orbit docking spire mating through the open center to which added cargo, added fuel reserves, and additional vehicles could be docked. I'm simply arguing that if you are using centrifugal gravity for your hab, then it makes more sense to rotate your entire mothership than to try and have a separately-rotating hab. And if you're rotating your entire mothership, then it makes sense to enclose your essential components -- the hab, the primary engines, and the primary fuel supply -- inside a single robust structure. And if you already have all those essential components inside a single robust structure, then it makes sense to make that structure an integrated aeroshell so that your mothership can aerobrake for 0 dV orbital insertion (for example, at Mars or Venus or Earth).

Hmm, good question. I don't know whether the Lissajous orbit around L2 would keep a spacecraft in Earth's shadow or not.

The mothership can certainly tow along any number of other vehicles. Of course, those vehicles will need to be launched separately from Earth, which is a substantial cost. Logistics starts to play a part, however, when you look at the necessary requirements for your mothership. It already needs to have enough structural integrity to provide artificial gravity by rotation. Thus, slapping a lightweight aeroshell around it so that it can aerobrake for orbital insertion from a transfer won't really incur much additional weight cost. While towing along a separate descent/ascent vehicle is fairly reasonable, it doesn't really make sense with landings on airless worlds. It is cheaper to simply carry extra fuel to deorbit/reorbit the entire ship them it would be to carry a separate landing vehicle and ascent vehicle. So it only makes sense to equip the mothership with powerful enough engines to support descent and ascent on airless worlds all by itself. And once you have both aerobraking capability and lunar SST-LEO capability, you might as well give it Martian landing/ascent capability.

A fascinating possibility. What kind of ISP could that develop, and how would you stage/contain it?

Well, as I explained in the other thread, once you are talking about multiple interplanetary missions you really want more flexibility. Unless you have dedicated orbital space ports at every possible destination, then the spaceship has to bring along a lander of some kind. You can only aerobrake on Earth, Mars, and Titan. Thus, for any realistic mission flexibility, it will usually make more sense to equip the mothership for powered landing and ascent instead of dragging a bunch of other spacecraft along, which would themselves need to be launched from Earth to resupply for every individual mission.

I can't see how they could get away without a reentry burn. Otherwise that booster will smack into the atmosphere like an egg hitting concrete.

And the ballistic performance of a cylindrical rocket is anything but simple.

So what's the most likely failure mode? Breakup during re-entry, or a missed/hard landing?

Reusability development probably has something to do with it. LH2 destroys tanks. It would require a separate engine and an added fuel type when they are trying to streamline operations by doing everything with a single engine and single fuel type. Finally, its bulk would make the upper stage ungainly.

Odds on a successful landing tonight?

Yeah, being in GEO isn't going to help because solar wind hits a spacecraft in all directions, not just out, due to the solar mag. field. More importantly, "geostationary" isn't stationary. It's geostationary. You're still orbiting; you just happen to be orbiting with the same period as the planet, so the planet keeps the same face to you all the time. With Earth, a geostationary orbit is an orbit with a period of 24 hours approximately 6.6 Earth radii away, meaning you're in Earth's "shadow" only 10% of the orbit, or about 140 minutes each day. If you want to use Earth as a shield for part of the time, then stick to LEO, where you are in Earth's shadow for almost half of each orbit.

RPM is the primary factor in play for whether a crew will become disoriented in centrifugal gravity, and an RPM of 2 is well within acceptable limits. There are a lot of logistics considerations surrounding the design of an interplanetary spaceship. There are lot of variables that depend heavily on the mission profile, and if you want to build an extended-persistence reusable spaceship, it needs to be capable of accommodating a wide range of mission profiles. Unless you have a fully operating orbital spaceport at every possible destination (which won't happen for a LONG time, if ever), then you will need to bring along a lander. Perhaps you can expect that certain destinations, like Mars, will eventually be able to provide launch services to return a crew capsule to orbit; if this is the case, then you can get away with bringing along nothing more than an aerobraking descent capsule. But aerobraking is only useful on Mars, Earth, and Titan, so a mission to any other world will need to bring along its own descent engine and fuel supply. The lander will probably need enough fuel for ascent as well, since you can't really expect to have refueling capacity on most worlds. This means your mothership needs additional fuel to tow the lander, plus heavier engines to compensate for the increased mass. If the lander is going to stay down for any appreciable period of time, it also needs its own separate hab and life support, which means more dead weight, more fuel, and even bigger engines on your mothership. Since resupply would primarily be via Earth launch, this has a tendency to make your costs skyrocket, since the landers and their engines and their fuel PLUS the added fuel for the transfer vehicle all have to be orbited before each mission. To save immediate launch costs, you'll design each lander for each specific mission, which means they might as well be expendable, which means more down time and fewer overall missions. It rapidly becomes apparent that for the case of something like a mission to Ceres or the Moon, you're really going to come out a lot better if you use just slightly larger engines and simply land the whole ship. Do that, and you no longer have to worry about the lander with its separate engine, separate fuel supply, separate life support system and hab, and separate structure. Resupply is simply a matter of orbiting fuel and supplies, standardizing and streamlining the resupply process. This is good, because larger engines are also important if you have even the slightest inclination of ever doing extended missions. One major point of an interplanetary spaceship with artificial gravity is to allow missions beyond the asteroid belt; this means it will need to carry more supplies and more fuel (all probably mated externally), all requiring more thrust if you want to get moving with any sort of haste. But here's where designing for mission flexibility starts to drive you toward something like my design. If your mission profile for a lunar landing requires you to land and take off, then you really need enough takeoff thrust to be able to make the trip all the way back to LEO. With your "high-gear" specific impulse of 520 seconds, getting the 4.8 km/s of dV for the LLO-LEO transfer will require a propellant mass fraction of 61%. Getting from the lunar surface to LLO with your low-gear specific impulse of 478 s, then, requires a propellant mass fraction of 29%. Since the dimensions of the vehicle are set by the need for artificial gravity, and these generally dictate dry mass, getting off Luna with a standard gee of acceleration will require a liftoff thrust of 49.3 MN. Since it turns out that 49.3 MN is more than enough for a powered takeoff from the Martian surface into LMO, it only makes sense to give it an aerodynamic shell and aerobraking capacity, even if that wouldn't be the typical mission profile. And a craft with an aerodynamic shell and aerobraking capacity which can SSTO from Mars can most certainly manage an empty-tank propulsive landing on Earth.

The HAB will be surrounded by tanks, but it won't be touching them. No way for the heat in the HAB to enter the tanks. And it wouldn't matter anyway if the fuel is dense enough to be liquid at room temperature.

I was going to go with nuclear power. Power efficiency at high thrust wouldn't be quite as good as with an NTR, but it's a lot safer, and could be used in-atmosphere without giving anyone a hernia.

That's a shame. Someone should design an electrostatically-neutral ion thruster with a propellant that could fire in-atmosphere and could readily interact with diatomic nitrogen, so that the nitrogen could absorb at least some of its kinetic energy.

Using LOX augmentation with an NTR in a LANTR design can approximately triple the T/W ratio while only reducing the specific impulse by around 30%. Moreover, the low T/W ratios of NTRs are typically the result of using liquid hydrogen; a denser fuel (necessary in order to fit inside the internal fuel tanks) reduces ISP but does wonders for thrust. A pebble-bed NTR has a T/W ratio of around 20 using liquid hydrogen. Running the NTR on hydrazine instead should yield performance approximately comparable to using liquid ammonia, for a T/W increase of 252%. Injecting LOX downstream should be done with a hydrogen:oxygen mass ratio of 1:4.83. Since hydrazine is 12.5% hydrogen by weight, the hydrazine:LOX ratio needs to be approximately 3:2, so this would mean the propellant flow would be 45% H2/LOX. Each kilogram of H2/LOX has a kinetic energy of 9.84 MJ, while each kilogram of propellant coming out of the NTR has a kinetic energy of 13 MJ. Do the math, and optimized LOX injection ends up increasing your T/W ratio by an additional 168% while only decreasing specific impulse by about 8%. So we're looking at an engine T/W ratio of around 82:1. At that specific impulse, SSTO requires a GLOW of 6,335 tonnes. Lifting off vertically, with a gee of acceleration, is going to require 124 MN of thrust, corresponding to a mass of 26 tonnes for each engine. More than I would want; doable, but overkill. The ship would likely need a reaction wheel in order to spin up without burning propellant. I wonder...if you used an electromagnetically-powered centrifugal impeller located inside the central column (near the bottom) to suck in and compress air and then add it to your propellant stream, would the added weight cost of the ducting be balanced out by sufficiently greater thrust augmentation and improvement in specific impulse? The impeller could serve as the reaction wheel in space. Another option would be to put a fat booster inside the central column and use it to zero out gravity drag while (smaller) main engines did the horizontal impulse burn; the booster would drop out and return to the launch site on its own. The crew cabin is 27 meters in diameter; you can get over half a gee with a nice leisurely 2 RPM. Nothing to worry about there. Using the command ring as a LAS is a stretch, but that's hardly a necessary element. The purpose of enabling Earth launch is that I just don't want to go through the painful process of trying to assemble this thing in space. I suppose that strap-on boosters could be used for the initial Earth launch, although the vehicle won't mate to them very easily in any sort of aerodynamic configuration. If you eliminate the requirement of Earth launch, then your thrust requirements drop pretty substantially. For landing and return, you don't really have to worry about the landing side of things because the landing thrust requirements are far lower than the launch requirements. Takeoff from Mars requires a GLOW of 2,900 tonnes, corresponding to a liftoff thrust of just 37 MN; with this arrangement each nuclear reactor only weighs 7.6 tonnes. And that's plenty of thrust to make a propulsive Earth landing with fuel = bingo; it's also more than enough thrust to fly directly from the lunar surface to LEO. Well, yes. Thrust vectoring FTW.

Added benefit: you don't have to worry about filtering out methanol. What kind of ISP would an ethanol/methanol/iron oxide/sugar rocket get?