guest blogger john hare
The ability to refuel in space is one of the critical technologies for opening up the high frontier. Without the ability to refuel in microgravity, destinations will be restricted to either low DeltaV missions or planets with in situ refueling capabilities. Any high DeltaV missions will have to be extremely valuable with massive launchers for tiny robotic explorers. These points have been made so many times by knowledgeable people that they should be beyond dispute. Unfortunately that is not the case. People are still designing programs and missions based on the assumption that propellant transfer in orbit is just too hard and dangerous to be in the critical path.
While I haven’t studied this problem to the extent Jon has, his posts got me started thinking about the problem. I think most of the approaches use just too much finesse. This seems to be caused by the attitude that mass in orbit is so expensive that minimum mass solutions must be sought even at the expense of time. Gravity gradient settling with a small tether, slow spin settling, magnetic attraction settling, and some type of tank vortex generator. Sometimes a sledgehammer is the answer. Put some equipment in orbit that yanks liquid, gas and all out of the delivery tank, replace enough helium to keep it from collapsing on reentry, and send it back for another load.
A bad case delivery scenario (not worst case, it can always get worse) would be an SSTO tanker has to rendezvous, deliver propellant, and reenter in one orbit. It carries the propellant delivered in the same tank as used to supply the vehicle main engines so that you have 10 tons of LOX drifting in random blobs around a 100 cubic meter tank mixed with helium and the GOX that vaporized on the way up. It takes 180 degrees of longitude to launch and rendezvous, and needs 60 more degrees to reenter, leaving one third of an orbit to empty the tank. We have 30 minutes to empty the tank of useful propellant and excess helium, and replace enough helium to prevent tank collapse of the delivery vehicle during reentry.
At 1,800 seconds to harvest 10,000 kg of LOX, a 6 kg per second (~90 gallons per minute) transfer pump would have margin to spare in a well defined pumping situation. Or the receiving tank could be at a lower pressure for a vacuum transfer with no pump at all. If settling of whatever flavor could be guaranteed, then that is all that is needed and there wouldn’t even be a question of orbital refueling feasibility.
This concept assumes that settling cannot be guaranteed in the allowable time frame, and that the delivery vehicle does nothing to cooperate. The cooperation assumed not offered include GOX vapor pressurizaton to eliminate helium. Dedicated delivery tanks so that only a relatively small tank would need to be emptied. Bladders to separate liquid from gas or sliding cylinder bulkheads. Tanks shaped to facilitate vortex generation for automatic separation during transfer. And so on through dozens of things that could be done to make the orbital storage facilities’ life easier.
If a very large pipe is used off center in the delivery vehicle, rapid emptying of the tank could create a vortex that would mostly yank the denser liquid first. This requires having equipment that will rapidly separate gas from liquid and do something with the gas so that it doesn’t interfere with the rest of the process. If two thirds of the gas must be removed in the process of removing the LOX, it has to be separated into oxygen and helium and sent to the proper tank without interfering with the main liquid transfer. After the liquid is out of the delivery tank, the remaining gas is helium and the GOX that boiled out during the rapid pressure drop of initial liquid removal.
There is a lot of gas to be removed during this process. During the initial LOX transfer phase, two thirds of the helium and GOX might be removed. By having a compressor remove the gasses from the vortex separator in the receiving vehicle, a liquid transfer pump could move the LOX to the holding tank without problems. The compressor sends the GOX and helium mixture through a heat exchanger to cool the gas well below the boiling point of LOX. A second vortex separator after the heat exchanger sends the LOX to the orbital tank and helium to another compressor that forces it into the orbital helium reservoir. After the initial bulk transfer, the compressor keeps sucking the delivery tank to nearly vacuum and sending the gas through the heat exchanger and secondary separator until pressure is so low that there is no useful quantity of GOX remaining. Then the delivery tank gets some helium back for reentry.
A 1,000 gallon per minute pump could empty the liquid in two and a half minutes with a clean intake. Assume 5 minutes and it should work. The compressor (or vacuum depending on which side of the machine you stand) needs to suck enough gas through the separator out of the delivery tank fast enough to to keep some vortex action working. I arbitrarily chose 30 seconds as the amount of time the compressor should empty the delivery tank to keep a strong enough swirl going to get the liquid out early. It works out to a 7 inch diameter compressor inlet in the initial vortex separation chamber. During the 5 minutes of initial transfer, the compressor sucks in a volume of gas equivalent of 10 times that of the delivery tank. Most of that will be GOX that boiled out during the pressure drop. If the right equipment is employed, there is a very small penalty in liquefying the GOX and sending it to the station tank. The pressure will be fairly low by this time, so there will need to be more compressor stages online to compress the oxygen to a temperature that allows it to condense in the heat exchanger.
If the assumption is made that no LOX is pulled through the pipe and all the material is sucked through the compressor, then this size would pull about 7 pounds of air per second at STP (standard temperature and pressure). Since the temperature is much much lower than STP, and the initial pressure double or more, The compressor would initially be capable of removing ~70 pounds of GOX per second. The problem with just planning to take it all that way is that some of the LOX will freeze into possibly inaccessible solid chunks during the pressure drop. There may not be time to thaw it and recover the material before the tanker leaves.
As always, I make a lot of assumptions that would need investigation. My first cut estimate is that the extraction equipment and heat exchanger would mass on the order of a ton. Better case scenarios could cut this to a fraction. Cooperative tankers, longer available time on orbit, aerospace class minimum weight engineering, and such. If a ton of equipment could do the job with low development risk, it would seem a cheap way to enhance capabilities. The more elegant ways would be much better if they can be brought to use economically and reliably.
Testing this might be cheaper and easier than some of the better methods. As fast as the reaction takes place, testing could be done on parabolic flights. Each phase could be tested in turn during one roller coaster series. Since the early applications would probably be smaller tanker vehicles, full scale extraction equipment could do a serious shakedown before being sent to orbit. Some fairly basic simulation would come before that to make sure the physics is valid.
If this means for emptying the delivery tank are valid, then an FBC method could be an inflatable tank in orbit of large relative size. If a 1,000 cubic meter inflatable tank were in vacuum, it could suck the delivery tank nearly dry in a minute or so without the complex methods mentioned here. Then the delivery tank with no liquid and 10% of the mixed gas remaining could disconnect and reenter after a fractional orbit. The inflatable could be then emptied into permanent tanks at leisure.