NASA hasn't completely given up on propellant depots. At least, not all of it.

A space depot for liquid hydrogen might be asking for too much. Less extreme cryogenic propellants such as propane or methane might make more sense.

My favorite scheme is an ammonia + xenon orbital depot for fuelling a dry-launched Mars rocket. Instead of conventional hydrogen NTR propulsion (with 1000 ISP), the Mars rocket would use a hybrid combination of ammonia NTR (500 ISP) and xenon NEP (3,000 ISP). Using ammonia and xenon avoids the low density, heavy tank insulation, and active refrigeration requirements of liquid hydrogen. Plus existing methods of orbital propellant transfer and storage, such as the progress cargo missions to the ISS, would probably work.

For missions to Mars, spacecraft would make pit stops at fuelling stations orbiting the Earth or Moon to fill their tanks with liquid hydrogen and liquid oxygen generated in space.

If we can harvest lunar materials to produce propellants and other consumables (water, O2) at a price lower than launching the propellants from Earth, then it would make sense to put a refueling station in Earth orbit. Not only could such a refueling station fill up spacecraft bound for the moon or Mars, they could refuel orbital transfer tugs to carry payloads from a low Earth parking orbit to their mission orbits (semi-synch or geosynch). Instead of having to launch a large upper stage to carry the payload into a transfer orbit and to include an apogee kick motor, you'd only have to use a booster big enough to put the payload into the parking orbit. This could save many million dollars per deep space launch. It might even be possible to use such a tug to put payloads into an interplanetary trajectory, then have the tug do another burn to return to Earth orbit. Aerocapture and aerobraking could greatly reduce the amount of propellant that the tugs need to return to their original parking orbit.

For missions to Mars, it makes absolutely no sense to put the refueling station in lunar orbit. The delta-v required for trans lunar injection isn't much less than that required to go directly to Mars. The difference is definitely less than the delta-v required to enter lunar orbit, the accelerate to Mars.

What if you parked it at EML-1 instead of LLO?

While I commend NASA for realizing that depots are essential to opening up the fronteir, a $5M prize is hardly worth it. Heck, you cannot even launch anything for that prize.

Now if NASA would give the prize AND a free launch, then it may start to get worthwhile. Though I'd be very surprised if people could still get it that cheap. Even the DoD responsive satellites, TacSat, which is a very simple satellite, cost around $10M to build. I'd personally be surprised and very impressed if someone could get the cryo system alone downto under $10M.

Now if NASA is serious, the prize needs to be boosted to $25M and a paid for launch. Then we'd start seeing companies getting excited about this.

Ryan, if NASA is serious, that is exactly what it will not do. The purpose of a prize is to provide an incentive for private entities to compete, not to pay for a competition. Believe me, a company that developes space based fuel depots will make a lot of money in the long run.

I'm not in the industry so forgive me if the question is ignorant but... Robert Zubrin suggested kerosine as fuel for spaceplans as it was far easier to handle (for midair refueling or standard airports) and had a 'good enough' impulse. Why couldn't an Earth orbit gas station use Kerosine?

Yes its better if we can mine the stuff up there and that would change the equation but I don't believe Kerosine has the same storage or cryo problems that most other rocket fuels have.

Actually, I thought he was a big proponent of methane, as many people are, because *that* can be made in a Sabatier reactor.

The big innovation in Mars Direct was making the fuel for the return voyage on Mars before the mission team ever launched.

Again I'm not an expert but why don't we just fire pre-packaged containers of solid rocket fuel up into orbit. Its easier to handle and store and has a higher impulse rate.

There are a few safety issues but space is life is nt safe and it should be easier to dump a solid rocket fuel that starts to burn early than deal with liquid hydrogen issues. Send up a dozen solid rocket cannisters, and the first ship to Mars takes six. Two for the trip out, Two for the trip back and a Two extras to leave on Phobos in case someone needs them in the future.

The Robert Zubrin reference was specifically for space plans. Earth orbit. Nothing to do with Mars. I do believe he was a big fan of methane for Mars missions. No reason we need a one-size fits all fuel.

Again I'm not an expert but why don't we just fire pre-packaged containers of solid rocket fuel up into orbit. Its easier to handle and store and has a higher impulse rate.

I think you've got it backwards. Solid propellants have a much lower specific impulse than just about any liquid propellant. For example, the Shuttle SRBs have a reported Isp of 267 seconds while the liquid (H2/O2) fueled SSME has an Isp of 467 seconds, almost twice as great. Plug those numbers into the rocket equation and you'll find that it'd take an awful lot of solid propellant to send a sizeable spacecraft to Mars. You'd end up having to either launch smaller rockets and cluster them somehow to your spacecraft or find a way to pack the propellant into a big booster while in orbit. Either way doesn't sound like a good idea to me.

If you have the means to cache solid propellants in Earth orbit, it'd be much easier and more cost effective to use that launch mass to send liquid propellants instead.

Is there any obvious problem with propellant stations that slowly spin, to make liquids settle at the far end of the tanks? Doesn't have to be 1-g, just enough for pumping purposes...

My solution is bags and straps. Fill them with fuel, collapse them by tightening the straps. There's no question of settling because the contents are being forced into the plumbing. Think of them as Murphy Bags.

I don't understand the fixation on lightering(moving fluid from one tank to another). Why not design a system where you attach the full tank to the vehicle. Kind of like plugging a battery into a flashlight?

Why not design a system where you attach the full tank to the vehicle.

You have to fill the tank somewhere (remember, a large point of this is to create an infrastructure for space-manufactured propellants). All you're doing is moving the problem. I can't think of any other transportation mode that operates this way. It would be much less flexible than allowing tanking in variable amounts. It's also much less efficient from a structural overhead standpoint.

One drawback to a LEO depot is the question of orbital inclination. Where do you put the thing? Not a show stopper but certainly a limitation on the usefulness of having one depot.

Multiple depots (including EML-1) is one obvious solution.

Perhaps grab the tank with a robotic arm and then wave it around to generate the desired acceleration.

Whoops, wrong blog...

One drawback to a LEO depot is the question of orbital inclination. Where do you put the thing?

If we are at all serious about launching into space economically, equatorial. This is ofcourse right above where you'll be doing the majority of your launches from.

The only US launch facility that can launch directly into an equatorial orbit is SeaLaunch. The minimum inclination you can reach from the Cape is 28 degrees (the same as the site's latitude).

Picking an orbital inclination is important but not enough by itself. You need to pick an orbital plane (inclination, RAAN, etc.) that's easily reachable by your ground launch site. An orbital plane change consumes a LOT of energy and is required even if you're in the same inclination but not the correct right ascension of the ascending node.

One of the attractive features of air launching is that it solves all those problems. Of course, it doesn't work for heavy lift (which is another feature, in my opinion).

What we need to do is get Equador to build a launch facility near Quita. High up, on the Equator, its the capital city so it has to have air networks available, there is a port not too far away, and downrange is the amazon forest (no major population centers).

If someone could talk Equador into building facilities and tax incentives for the space industry they could become the leading space nation of the coming century.

Either that or work with China on building an Orion launch program. Tell them to use the Neutron bomb tech they stole during the 90s, hint that Tibet would make a nice launch platform, and give them a list of stuff we'll pay them to put into orbit.

Hey they're dictators, whose gonna complain about environmental and safety problems to them. Yeah there might be security issues but imagine what we could put up there with a single Orion launch).