Spacecraft configuration

[Aero]

While searching the Internet for data on the MythBusters Water Heater Rocket, I found a number of articles stating that water is now considered to be widely available throughout the solar system. We now know that water exists on the moon, Mars, and the moons of Jupiter and Saturn, and is expected to exist widely in the asteroid belt. The general theme of the articles was that our future spacecraft design should take advantage this availability to simplify design and reduce the cost of spacecraft. It should come as no surprise that steam rockets were proposed for propulsion. The other component was a nuclear power plant. One concept that I found most interesting carried water in the form of ice, almost eliminating the mass of tankage. By using water prolifically, the spacecraft could go pretty fast perhaps even putting the Kuiper belt within reach. Using this concept, what would be the ideal configuration?

If we assume that Polywell direct conversion works, then a relatively small tank of composite material, heated by microwave radiation and coupled to a thrust nozzle should be all the tankage and plumbing needed for propulsion. If direct conversion doesn't work, then the design would need heat exchanger plumbing. With enough heating power the thrust efficiency of a steam rocket approaches that of LH2-LOX. A spacecraft configured with fuel storage as ice and a high power reactor or two could be very large but also very powerful. Use the fuel reserves in the form of ice to shield the crew compartments from reactor radiation and shut down the reactor for a day or two if it requires maintenance. Just make sure that you carry enough ice reserves. But with the fuel storage area between the reactor and the living quarters, the separation distance between the reactor and the crew quarters would reduce radiation risk. Assuming an isolated launch location, could such a spacecraft be designed as an SSTO? Maybe the design could be a one and a half stage to orbit. That is, maybe the spacecraft could be cradled in a wing set that lifts to altitude and speed, then is ejected. Like a carrier aircraft but with the power from the spacecraft instead of the carrier airplane.

I note now that one significant advantage of a steam rocket is that it can operate using other liquids that may be found in space as reaction mass. All that is needed is material that can be vaporized and handled with the on board reaction mass feed system. Frozen CO2 should work. So should liquid hydrocarbons if they exist in space. It may be necessary to melt the ice and filter the water before it is used in the rocket but that is simple compared to electrolyzing it into its constituent hydrogen and oxygen, then liquefying it for use as fuel.

Comments? Thoughts? Improvements?

Edit - By storing fuel as water ice, the design could allow for strapping on as much extra ice as you could find, as long as the spacecraft could move it.

[kunkmiester]

You still need to heat it, which takes you back to all the threads talking about rocket designs. Also, your approach would really only be reasonable in orbit, where getting to ice on a regular basis isn't that big of a deal.

For in space, VASIMR and other designs are more practical mass-to-power, though the actual propellant might indeed be water. For surface-to-orbit, you have a whole 'nother set of issues, and REB powered steam rockets might work.

[Aero]

The point of the idea is that water is available on the moon, in the asteroids, Mars and other locations that are as easy or easier to access from space than Earth surface water. Yes, the water will need to be heated to high pressures for thrust, hence a nuclear reactor. That makes the spacecraft quite large and massive. The benefit of water as propellant is that the rocket engine is simplicity in itself. The combustion chamber is nothing more complex than a pressurized microwave oven or a cooling loop from the nuclear reactor. The number of parts in the engine would be minimum so the engine reliability should be very good. No hot turbines, no complicated electrical gear, no need to break water down into hydrogen and oxygen or cool them or to store them, no related plumbing.

Does anyone know what the maximum achievable combustion chamber pressure might be? Some articles gave extremely high values for chamber pressure but I wonder what would be available with real, obtainable materials for pressure vessels. Engine thrust efficiency goes as the square root of pressure so the steam rocket might be quite efficient.

[MSimon]

The point of the idea is that water is available on the moon, in the asteroids, Mars and other locations that are as easy or easier to access from space than Earth surface water. Yes, the water will need to be heated to high pressures for thrust, hence a nuclear reactor. That makes the spacecraft quite large and massive. The benefit of water as propellant is that the rocket engine is simplicity in itself. The combustion chamber is nothing more complex than a pressurized microwave oven or a cooling loop from the nuclear reactor. The number of parts in the engine would be minimum so the engine reliability should be very good. No hot turbines, no complicated electrical gear, no need to break water down into hydrogen and oxygen or cool them or to store them, no related plumbing.

Does anyone know what the maximum achievable combustion chamber pressure might be? Some articles gave extremely high values for chamber pressure but I wonder what would be available with real, obtainable materials for pressure vessels. Engine thrust efficiency goes as the square root of pressure so the steam rocket might be quite efficient.

The SSMEs provide a guide. You then have to derate for water cooling and 1 million hours MTBF (to get the failure rate low enough).

I did some spread sheets on water rockets. SSTO. SSME parameters. The numbers were horrible. The mass fraction was lousy.

LH2 was much better.

Once you are in space water does become a lot more attractive. OTOH. Think of H2 refueling stations all over the system. And eliminating about 2/3s of the mass. ie sqrt(18/2). And if you can atomize the H even better.

[D Tibbets]

I believe that water (or amonia, or methane ices, etc.) can serve as excellent and easily processed sources of hydrogen. Even hydrates may be a good source of hydrogen. But water (H2O) as a propellent is less attractive, except as augmented high power thrust when you need to get off a planet or heavy moon surface. Seperating the hydrogen and then accelerating it seperatly will give much higher efficiency. The oxygen would be left behind on the moon. A reason for not leaving the oxygen behind would be storage issues. The tank size and leakage rate for storing pure hydrogen may allow water storage to be attractive, despite the excess weight that would result. In this regard amonia or methane, or even longer chain hydrocarbens may be better choices. ie Titan may make a better refueling stop than the water ice moons of Saturn.

Dan Tibbets

[taniwha]

Assuming such a beast can be built, I worked out that if you can split O2 into two O- ions, you can get about 153ks ISP (1.5e6m/s) using 186.6kV. Using 603kA allows for 0.1kg/s giving 150kN thrust. This gets a 1000ton(*) ship with 1000ton(**) LOX out to Neptune in about 8 months. Of course, the drive power is 112.5GW (ignoring efficiency issues). Also, the ship never lands and it's probably best to use tugs in the vicinity of a space station.

I imagine electrode/grid erosion will be nasty.

* I now know my polywell design is probably unrealistic (38T, 4.5m radius: 130GW) and probably overly optimistic in the first place, but I got a basic design, including radiation shielding but not conversion equipment, to be about 1100tons. Unfortunately, the magrid can't support itself

** Nasty stuff, especially with titanium. Much better off using H20 and having a separate set of ion drives for the H2. Not sure I'd want to just dump 11% of my reaction mass.

[Aero]

Assuming such a beast can be built, I worked out that if you can split O2 into two O- ions, you can get about 153ks ISP (1.5e6m/s) using 186.6kV. Using 603kA allows for 0.1kg/s giving 150kN thrust. This gets a 1000ton(*) ship with 1000ton(**) LOX out to Neptune in about 8 months. Of course, the drive power is 112.5GW (ignoring efficiency issues). Also, the ship never lands and it's probably best to use tugs in the vicinity of a space station.

I imagine electrode/grid erosion will be nasty.

* I now know my polywell design is probably unrealistic (38T, 4.5m radius: 130GW) and probably overly optimistic in the first place, but I got a basic design, including radiation shielding but not conversion equipment, to be about 1100tons. Unfortunately, the magrid can't support itself

** Nasty stuff, especially with titanium. Much better off using H20 and having a separate set of ion drives for the H2. Not sure I'd want to just dump 11% of my reaction mass.

Yes, it will be a big power plant, but if we consider what we already know about human operated ships on extended voyages, (sea ships, yes, but more particularly space craft carrying a crew) then we know that these spacecraft will be big. Just take a look at the ISS. It has a crew of 6 or 7, and how big is it? And it doesn't have a propulsion system or power plant, just station keeping thrusters (plus the shuttle) and solar wings for power.
Science fiction has been a predictor of a lot of science breakthroughs, so look to Sci-fi for the size of human controlled spacecraft. I guess the X-wing fighters are about the smallest out there in the Sci-fi universe.

[MirariNefas]

What, you mean no space motorcycles?

[taniwha]

Star Wars isn't SciFi (the most generous description I've heard is Science Fantasy. More accurate is either Space opera or space fantasy).

In the harder scifi I've read, the smallest (interplanetary or interstellar) ships were all big enough to have families, or nearly so. And in general, forget landing. My ship seems to be a little on the small side.

The funny thing about some of the discussions going on here is it looks like anything using fusion that can get into orbit can probably cross a solar system in reasonable time for little extra cost (in size: ion engines, reaction mass). However, carting wings and air-breathing "engines" across the solar system seems a little wasteful. Out to L4 or L5 might be reasonable, then a transfer to a stripped down interplanetary model.

BTW, at 0.1m/s/s, it takes 2-3 days to get from LEO to L4. That's boosting parallel to current velocity from LEO to about half way, then boosting towards Earth (slowing down) until the apogee is right, then circularizing. It really makes me wonder how much a one way ticket to L4 will cost when your expendables are hydrogen and boron (in kg) and water (tonnes: with my above design, it works out to about 17tonnes of water)).

[Aero]

The point of the idea is that water is available on the moon, in the asteroids, Mars and other locations that are as easy or easier to access from space than Earth surface water. Yes, the water will need to be heated to high pressures for thrust, hence a nuclear reactor. That makes the spacecraft quite large and massive. The benefit of water as propellant is that the rocket engine is simplicity in itself. The combustion chamber is nothing more complex than a pressurized microwave oven or a cooling loop from the nuclear reactor. The number of parts in the engine would be minimum so the engine reliability should be very good. No hot turbines, no complicated electrical gear, no need to break water down into hydrogen and oxygen or cool them or to store them, no related plumbing.
...

The SSMEs provide a guide. You then have to derate for water cooling and 1 million hours MTBF (to get the failure rate low enough).

I did some spread sheets on water rockets. SSTO. SSME parameters. The numbers were horrible. The mass fraction was lousy.

LH2 was much better.

Once you are in space water does become a lot more attractive. OTOH. Think of H2 refueling stations all over the system. And eliminating about 2/3s of the mass. ie sqrt(18/2). And if you can atomize the H even better.

It took me some time to get back to this. I needed to work out a Hot Water Steam Rocket, (HWSR), concept and performance for comparison. This concept is for a space operation, so vacuum performance of the SSME is the appropriate comparison. The conceived HWSR thrust equals that of the SSME and the engine nozzle design point is 60,000 ft altitude, same as for the SSME.

Yes, the fuel mass flow for the HWSR exceeds the mass flow of the SSME. The mass flow ratio is 1.25, HWSR to SSME. However, the fuel volume is much lower. Fuel volume ratio is 0.22 HWSR to SSME. That would result is significant savings in fuel tank structural and plumbing mass. I think the issue might be the choice between accelerating fuel mass or structural mass. Then there is the major question of fuel availability in space and support infrastructure not to mention the energy cost to power the HWSR. The HWSR requires electric power to melt ice, filter the water, pump it through and heat it in the “Combustion” chamber. For the SSME, energy needed to do comparable functions is extracted from the fuel, put there by the support infrastructure when cracking the water into Oxygen and Hydrogen. I should note here that the SSME uses a fuel rich mix of 6:1 meaning that only one third of the oxygen made in this cracking process is used by the rocket engine.

If anyone wants to check my calculations, feel free to tell me where I have slipped up. The conceptual HWSR operates at critical conditions of water/steam. That’s about 3200 psia and 705 degrees F. A constant pressure variable flow pump (a wobble plate pump, for example) raises the pressure of the engine heating chamber to critical (equivalent to a combustion chamber). The water is heated to critical temperature and exhausted through the thrust nozzle. This must be a continuous flow process to achieve reasonable burn times. Due to the nature of steam expansion the nozzle of the HWSR will be larger than that of the SSME, but due to the low operating temperature of the HWSR the nozzle can be simply made of common material. The HWSR engine is conceptually simple compared to the SSME. Read about the details of the SSME here:

http://en.wikipedia.org/wiki/Space_Shuttle_main_engine

As for myself, I prefer a spacecraft that gives me the freedom to go where I want to go without depending on infrastructure to exist at all the points along the way.

[CaptainBeowulf]

Hm, smallest quasi-hard science spaceship I've heard of is Chris Van Den Broeck's modification to Alcubierre's warp drive, where most of the mass of the spaceship is contained in a "pocket universe". I forget how to explain it properly, so I'll just plagiarize wikipedia here (not a reliable source IMO but acceptable in the same way as BOE calculations are acceptable):

"By contracting the 3+1 dimensional surface area of the 'bubble' being transported by the drive, while at the same time expanding the 3 dimensional volume contained inside, Van Den Broeck was able to reduce the total energy needed to transport small atoms to less than 3 solar masses." http://en.wikipedia.org/wiki/Alcubierre_drive

But, more realistically, yeah, spaceships will be fairly large - they will either have to carry shielding against cosmic rays/solar flares in the form of: (1) thick metallic hulls, (2) water between inner and outer hulls, (3) the apparatus to deploy some sort of magnetic shield. Bonus of thick metallic hull is it might provide some minimal micrometeroid resistance. Bonus of magnetic shield is you could possibly use it as a solar sail for outbound journeys, although you might have to switch it off in order to make inbound journeys.

I'd probably go for isolating the hydrogen from water and use that for either fusion, or isolate the hydrogen and oxygen and burn them in the standard high-performance conventional rocket approach. But, a steam rocket could work as a fairly simple alternative.

Basically, there's lots of alternatives when you're in interplanetary space. Ion drives and VASIMR look good too, although the possibility of a Polywell powered fusion rocket does really excite me (I'm not convinced yet polywell will work, but like others here I'm hoping that the current round of experiments won't find any show-stoppers). Earth to orbit is the problem, and like MSimon says, steam rockets don't look promising there. As they say: Get to orbit and you're halfway to anywhere.

[MSimon]

Time is money. A REAL space faring civilization is going to want to get from point A to point B in the shortest time consonant with economics.

H2 reaction mass is it. With a small water fed impulse engine for emergencies. Or exploration where H2 sources are not common. And of course custom designed craft for those who desire them.

[Aero]

Time is money. A REAL space faring civilization is going to want to get from point A to point B in the shortest time consonant with economics.

H2 reaction mass is it. With a small water fed impulse engine for emergencies. Or exploration where H2 sources are not common. And of course custom designed craft for those who desire them.

I see that as farther down the road. Sure, a space liner running a scheduled trip from point A to point B will use the best economical propulsion available. That is sort of like the far end of the spectrum, time wise for the developing space faring civilization. And someone wrote, maybe it was you, something along the lines of, "The first SSTO system developed will be the propulsion system that does the exploring and mapping of the solar system." That may also be right. But I'm thinking that perhaps in between these two extremes of time, it may be the HWSR that opens up space for civilization.

Consider that if you want to haul a load of manufactured product out to a new colony, and a load of ore back, maybe you would want to use the least expensive fuel available. Water gives you the freedom to carry that load whenever and where ever you want to go. Hydrogen, not so much.

Of course there are other propulsion systems being researched, hopefully one of them will pan out.

Edit - Of course there are already perfectly workable SSTO conceptual designs out there, its just that the need doesn't exist. The first one I saw was in the late 70's. Designed as an unmanned, heavy lift, expendable SSTO, it consisted of one spherical aluminum liquid natural gas tank Iike they used to put in LNG tankers. It was to be used for LH2, and the design had 8 SSMEs around the lower periphery of the tank. Each SSME had its own LOX tank and the cargo was stowed on the nearly flat top of the LH2 tank. The numbers gave 25 tons to LEO plus the launch vehicle. If anyone needs 25 tons of cargo, 8 SSME's and a 70 foot diameter, 6 inch thick walled aluminum sphere in LEO, that design is probably still rotting in some file cabinet somewhere.

[kunkmiester]

Use the tank for a pod on a space station. Stow the stuff that goes inside it and other accessories on top, and up it goes. Design the SSMEs to reenter for reuse, or use a more easily refurbished engine with similar thrust.

Strap the engines to a new tank, and repeat a few times. Instant space station.

[MSimon]

That is sort of like the far end of the spectrum, time wise for the developing space faring civilization.

When would that be? 100 years from now or two hundred? It won't take long and might even be part of initial asteroid belt mining operations. Raise the payload mass fraction.