sevenperforce

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?

Right, that's why I had it at the edge. The long, skinny axially-rotating ship suffers from two issues: first, it requires a high RPM to produce even relatively low gravity, and second, you introduce asymmetric bending moments in the axial linkage that can rip your ship apart.

I'm afraid I'm not sure I follow. Obviously, the fuel will be liquid. Are you suggesting that the energy flux from radiation will cause the fuel to heat up and boil? Radiation doesn't really carry a lot of energy, at least not this type. Plus, denser fuels will typically have better boil-off performance than less dense fuels.

There is neither oxygen nor water in hydrazine. Perhaps you were thinking of hydrogen peroxide? And it wouldn't be getting close to a reactor core anyway. I asked what would happen if hydrazine (which, BTW, decomposes into diatomic hydrogen and diatomic nitrogen) was injected into the exhaust of an ion thruster that happened to be powered by a (separate) nuclear reactor.

How hard is it to crack vegetable oil into ethanol and mix that with sugar and lots of iron oxide?

Well, it does; most of the volume of any SSTO is tankage.

What would happen if you injected something like hydrazine or ammonia into the propellant stream of a nuclear-reactor-powered ion engine? For that matter, what if you injected highly-compressed air into the propellant stream? Are there any ion thrusters which can even operate in anything other than vacuum?

Multimodal nuclear engines are a pretty good deal, actually. The coolant loops you already need in order to run safely as a nuclear thermal rocket can be repurposed to provide electricity without much additional weight.

Nuclear engines are mostly a problem for takeoff from Earth. Expelling radioactive propellant into the atmosphere, even if it is nearly harmless, isn't something people are happy about. For interplanetary transfers, or for landing on other worlds, people won't care as much about the use of nuclear engines. The average layperson wouldn't have the first clue about the difference between an NTR and an RTG. Also, launches from Earth wouldn't be very common. This is, after all, a long-term, extended-persistence interplanetary transfer vehicle capable of supporting a large crew for a long time. The initial launch would be followed by a series of missions to other worlds, returning to low Earth orbit to refuel and take on more supplies, but "dry dock" on earth wouldn't happen very often. Much easier to treat the ship like an orbiting space station and simply ferry up and down using capsules. It would definitely be more economical to refuel, even using inefficient chemical rockets, because this contains its own engines, tankage, and hab. This ship would be able to support a large crew for a long time, drop them on another planet, and then come back. Hard to beat that. Another option (not pictured) would be to attach a cluster of ion engines with their own fuel tanks, either passing through the open center of the ship or in a toroidal ring mated underneath. The ion engines would run off the nuclear reactors, which would continuously expel small amounts of coolant in order to keep from overheating. The internal fuel tanks would still end up partially depleted by the end of the burn, since the reactors would be using propellant as their coolant, but you could develop an extremely high delta V. This eliminates some of the major problems with using ion engines for interplanetary transfer (the high power requirements of ion engines typically result in either extremely low thrust, extremely large solar panels, or extremely large radiators and coolant loops with nuclear-thermal designs). Plus, you arrive with enough propellant to still use your engines the normal way. In theory, this ship could mate to any number of additional cargo containers, pods, or fuel reserves. Big, overpowered engines are a large initial investment, but they pay great dividends. Especially because a moon shot ends up being as simple as flying to the moon, landing, taking off, and coming back.

Obviously the radshield is also the aerobrake heatshield.... =P

Not superluminal, but damn if it ain't pretty. Of course I'm always also a sucker for the fastest ship in the 'verse.

You can equip the ancient civilization with whatever industries are necessary in order to build the rocket, but you only have your own lifetime to do it in. Which means you'll need to limit yourself to stuff which can be done rapidly, without a lot of precursors and development and construction time. Basically stuff where manpower is your limiting variable...because that's where ancient civilizations have the edge.

Errr...yes. The fuel tanks serve as the shielding; when the fuel tanks are depleted, shielding isn't as great, which is why you definitely want to refuel quickly. But it wouldn't be too bad; you could always enter a portion of the ship with additional shielding temporarily. See below -- I linked a mockup. Well, speak of the devil....

If you're going to build an actual spaceship -- a craft you can take to another planet as easily as a pirate could sail across the Mediterranean -- there are a few things to keep in mind. Gravity. You're going to need artificial gravity if you want to be able to manage long trips, so you'll need to either have a spinning hab, or you'll need to spin the whole ship. Power. You need high-thrust engines to get on and off of planets, high-impulse engines to make your transfer burns propellant-efficient, and energy to run your ship in transit. Trimodal nuclear thermal engines are your only real choice (a trimodal NTR has three modes: high-impulse, where low-density propellant is heated and ejected by the nuclear core, high-thrust, where LOX is injected into the propellant stream to increase thrust at the expense of impulse, and thermal-electric, where the circulation of coolant generates electricity). Shielding. Your hab needs to be shielded from both solar radiation and the nuclear radiation of your engine(s). Volume. You need a large internal volume to carry an enormous amount of fuel if you're going to be able to make a transfer, land, take off again, and head back. Granted, you'd use in-orbit refueling wherever possible, but you need the flexibility to make a round-trip to unvisited worlds. Surface area. Although you need a large volume, you also need a form factor with a blunt-body surface area, allowing re-entry heating to be as minimal as possible. Further, large surface area will help with radiating heat away in space. Finally, a lifting-body shape will make launch and re-entry a bit nicer. What, then, is the optimal shape and configuration? Here you go. Yep, it's a flying saucer. The hab is located in the center column and is a single floor, preventing any unpleasant gravity gradients. Because the center is open, it allows windows to be embedded in the ceiling, which will feel more natural. The hab is far more "open" overall than most designs, while still being well-protected from radiation and micrometeoroid strikes due to its location in the center of the ship. It will feel very natural to have the sky "up" and the ground "down". The hab is shielded by wrapping the ship's tankage completely around it. The triangular cross-section maximizes internal volume while also having the optimal shielding profile. Obviously, the entire ship rotates. The ship is powered by six small nuclear reactors, feeding three linear exhaust nozzles: Each of the three exhaust nozzles is capable of running on a single reactor, so you still have maneuverability even if you need to scram one or two of your reactors. The coolant cyclers and generators are also in this area. Only minimal shielding is necessary, due to the placement of the large internal tanks. For on-orbit burns, the three nozzles all fire together, providing moderate thrust even at the highest operating impulse: During any such burn, there will be a slight misalignment of the apparent gravitational field, but it will likely be no more disorienting than standing on a train while it starts to move. It is also likely that in most cases, the ship will only "spin up" after its transfer injection burns, relieving this issue entirely. Takeoff and landing use the same orientation as on-orbit burns, but with the injection of liquid hydrogen or another oxidizer to dramatically increase thrust at the expense of specific impulse: During takeoff, however, maintaining this thrust orientation would make drag losses altogether unmanageable. For this reason, the nozzles are able to change orientation in order to thrust backward during in-atmosphere climbs (for reaching orbit from Terra, Mars, etc.): Because the "flying saucer" shape is one of the only shapes which is capable of achieving reasonable lift in subsonic, supersonic, and hypersonic flight, this allows for the higher-specific-impulse burn to be used (when applicable) to achieve orbital velocities. Obviously, on worlds without atmospheres, liftoff would be purely vertical without any intermediate horizontally-oriented burn stage. Re-entry uses the large blunt surface area underneath to dissipate heat passively (although active cooling could be used, in principle): Passengers are seated in the upper ring during takeoff and landing, both for gravitational orientation and for safety reasons. Launch abort escape would be achieved either through individual ejection seats or through the ejection of the entire upper ring using built-in thrusters. It is possible that the upper ring could also be configured to serve as an ejectable lifeboat in the case of an on-orbit accident. \ Attitude control could be achieved either by vectoring the thrusters, or by venting coolant. Placing the heavy engines at the outside isn't ideal, but given that this is going to be designed with enough structural integrity for powered landing, it shouldn't prove too problematic. The hab would have nearly 8500 square feet of floor space under artificial gravity, with more than 30 times the pressurized volume of the Space Shuttle crew cabin. I'm estimating a nominal dry weight of 1200 tonnes. The body encloses enough space for 9800 cubic meters of tankage; using a dense propellant like hydrazine, this corresponds to 9,900 metric tonnes of fuel. With pebble-bed reactors giving a specific impulse of around 520 seconds, you get a lovely 11 km/s of dV. Enough to reach orbit as SSTO (if you use LOX-injection augmentation). Also enough to fly to the moon, land, take off, and come back. Now to build the damn thing...

Well, the heat from the hab can be eliminated by depressurizing the tankage; no heat sink without a heat transfer medium. And you'd probably want to use something with better boil-off properties, like hydrazine or ammonia or liquid methane (if you can get the latter two to disassociate). Trimodal engines inject LOX just after the chokepoint to augment thrust at the expense of specific impulse; you'd use these for takeoff and then refuel with something lower-density for orbital transfer.

What kind of ISP could a dry ice cannon develop, theoretically? Would it be able to use a converging/diverging nozzle and achieve supersonic exhaust velocities? A high-thrust design would be set up so that the hot water was also the propellant. I was originally thinking specifically of chemical rockets based on combustion rather than something like phase change, but this is quite interesting.

Blasphemy! 1 + 1 = 10 is far purer!