If you want a propellant depot and space tug architecture get small ones operating and allow NASA to come to you. The DoD may be willing to pay for some of this.

Shuttle extension and building new pads for Atlas and Delta (preferably at LC-39 to make return of the shuttle or an SDLV more difficult) would be a good way to direct pork to Florida in the short term. The same goes for increasing Orion’s budget, accelerating work on Altair and work on new hardware, say – my favourite – a hypergolics depot. COTS-D would work in the slightly longer term.

However, they are sufficient to show that an L2 rendezvous mission can be accomplished with two launches of a vehicle capable of sending 25 tonnes to TLI. This is less than half the capacity of the baseline Ares V, and opens up a range of more cost-effective launch vehicle options.

Well written letter.

I am really looking forward to reading your article about how to use dual-launched EELVs for manned missions, when you are able get around to it.

You could also use higher silanes instead of hydrocarbons, which are supposed to be less toxic than MMH and have slightly better Isp and combustion properties than the corresponding hydrocarbons. They also have better ISRU potential.

In the short run, I would still prefer MMH/NTO since that’s what’s being planned for Orion and Altair and I would like both to be refuelable. Propellant gels and metal loading also decrease handling risks and improve Isp and ISRU potential. Toxicity and low Isp with MMH/NTO is clearly an issue, but the more I read about it, the more it appears there are good ways to mitigate that.

And there’s also the issue of reusability. I think that for the near term LOX/LH2 is going to be much harder to make reusable than MMH/NTO which has been used and reused for ages with OMS. I have no idea how difficult it would be to make kerolox engines that are reusable in space.

On UDMH/MMH/hydrazine and NTO/MON I’m told by knowledgeable sources that the required modifications are minimal.

Of course there’s also HAN and other green propellants. If you can find a propellant that is dense, hypergolic, completely non-cryogenic and non-toxic that would be great. Higher silanes + H2O2 might be that combinations, especially with metal loading. For certain applications high up in the Earth’s gravity well, or other circumstances where delta-v’s are small you may not need very high Isp.

There is a cost trade, between low complexity hypergols and existing engines, and moderate complexity cryo storage and a billion dollar engine development program that needs to be made. If we are launching hundreds of rockets with fuel for a lunar program, and particularly if we are pulling that fuel using high isp electric tug to lunar orbit, at some point going with more complex mild cryogens like lox/methane or lox/propane may make sense.

If we are going with hypergolics we most certainly should be going with what the russians use, UDMH and N2O4, because we can take fuel transfer hardware, fuel lines, valves, engines, etc used with zvezda, soyuz and progress straight off the shelf with zero development.

So apparently there is this ~4kW/kg thin film solar suggested for solar power satellites. This could also make some pretty effective solar powered high ISP stages using far less of a perhaps easier to store propellant. This does not have the power density to get to and from the lunar surface – though a hybrid system might (perhaps one reusable stage all the way there and back to LEO).

Will higher ISP solar powered systems be possible by the time propellant depots arrive? Or by the time anyone gets back to the moon? Should they be developed and planned on now?

Pete.