Wednesday, January 6, 2016

Back (from outer space)

I'm back from holiday laziness and failure to post. Also have a certain song stuck in my head now...

The big news lately is the successful return of a Falcon rocket first stage to Earth. Congratulations to the team at SpaceX and all those who were involved.

The question is, who wants to be the first to risk their precious payload on that rocket?

I have a suggestion: SpaceX should absorb the cost up front and launch their own payload. The launch itself should in theory only cost them a few million dollars. As for what to launch, I'd say a big tank of water. It would have been nice if they had a payload like this waiting in the wings so they could turn the vehicle around and launch again quickly, but that is still an option for a future flight.

There are details to consider, like power, communications, stationkeeping, designing a fluid transfer interface and a hundred other things I haven't thought about yet. The reason for shipping up water is that it's inert and in demand, both for life support on ISS and for making propellant.

There are a handful of interesting cubesat propulsion systems that electrolyze water then recombine it for thrust. These are limited both by mass and by volume, so if a cubesat carrier were to rendezvous with the tank then its payload satellites could fuel up in orbit and be able to pack more hardware or perform more ambitious missions. This would not be a trivial operation, but it would provide useful experience with microgravity fluid transfer. This would open the door to a larger number of customers and would provide the flight experience necessary to move to the next step.

Once this is a proven technology, equipment for water transfer can be shipped to the ISS. Being able to pump water directly into the station instead of transferring it with contingency bags would save quite a bit of time. (Astronaut time is quite expensive.) At first this would benefit only the big cargo missions, but it would open the door to using extra payload margin on other vehicles for shipping up water. With enough water mass in orbit, it would become viable for someone to launch a tug or satellite tender to transfer it from point to point.

Once there is a functioning interorbital cargo transfer service we can expand the types of cargo to include other propellants, spares and perhaps larger component assemblies or modules.
Several companies are working on refueling existing satellites, but if there was a standard interface for fluids then new satellites going forward would be very easy to refuel. They might use hydrazine, storable bipropellant, electrolyzed water, argon or other materials. The ability to launch partially fueled would allow satellite owners to use less capable launch vehicles or (for example) use a reusable F9 rather than an expendable F9 for significant savings.
Refueling in orbit will be expensive, but reducing their up-front launch costs can reduce their financing costs and allow them to generate some revenue for a few years before they have to pay for more fuel. It also provides contingency in case a launch vehicle failure places their payload in a useless orbit: enough fuel can be transferred to the satellite to put itself into the correct orbit. Simply having that option available would reduce the cost of insurance.

If refueling becomes commonplace there is no reason we couldn't include remote manipulators similar to Dextre. For best results this would require designing new satellites with orbital maintenance in mind, basically using a similar system of orbital replacement units as used on the ISS. Being able to replace or upgrade transponders, solar panel wings, power units, guidance units, even propulsion units would be a transformative capability for two reasons. First, the owner can choose to go with less costly components at launch and upgrade them 5 or 10 years into the craft's lifespan. Second, component failures would be repairable at a reasonable price; a failed gyro or blown power unit would no longer mean end of mission.

Establishing experience with cargo transfer in low-Earth orbit is a necessary step to larger-scale resource transfers like asteroid mining. Developing remote maintenance and manipulation capabilities will be an important step as well. Reusable launch vehicles will make this process affordable enough for many smaller players to get involved instead of waiting for Boeing to set the pace. A little chaos and competition in the market would be a good thing in this case, and SpaceX is in a unique position to push things over the hill.


  1. Sadly Musk hasn't shown much enthusiasm for cislunar infrastructure. Mars seems to be his focus to the exclusion of everything else. But re-using just the booster is not sufficient to realize his goals. To have affordable MCTs he also needs to re-use the upper stage. It's my hope he'll come around to recognizing benefits of orbital infra-structure in our earth moon neighborhood.

    1. Reusing the upper stage is a much rougher problem to solve without orbital infrastructure.
      If you can't rely on anything but what you take with you then the second stage needs a reentry heatshield and quite a bit of leftover dV, both of which come with a heavy mass penalty. You also have to use your vacuum-optimized nozzle deep down in the muck, overexpanded and perhaps unstable.

      Several options would be available given appropriate services / facilities in orbit:
      The engine could be scavenged and reused for building chemical tugs. (Not terribly useful, but better than nothing. The fuel tanks could be repurposed for storage as part of a depot.)
      Heatshields could be built using lunar or asteroidal materials and delivered in orbit. (Good example of a potential early market for lunar products.)
      Momentum exchange tethers could be used to capture much of the orbital energy of the stage, allowing it to deorbit with minimal propellant. (That energy would be used to boost other payloads from LEO to higher orbits.)