The rise, fall and rise again of refueling – in space

Range anxiety was invented by NASA. Well, perhaps not (or Velcro), but space exploration gives new meaning to an obsessive awareness of how much further you can go when there is not a charger on every corner. Now imagine that feeling in outer space, or back on the ground watching your spacecraft, not just for power but for fuel. In 2011 NASA looked seriously at refilling a rocket in orbit for the journey still to go. The team was asked to add meat to the bones of an idea that another NASA team had mentioned in passing the previous year as just an option versus go-to massive rockets.

Refilling stages in space makes too much sense. If low Earth orbit is halfway to anywhere in the solar system, and it was all hands-on deck to figure out how to go beyond Earth orbit, then filling up before the other half of the trip had to be on the table. Yet for an assortment of reasons as events unfolded ten years ago the task did not end well. Not for the team, the technology, or its business approach. At least not just then.

Logo of the Pur and Kleen Water Company, stylized, pure without the “e” at the end, and “clean” with a “k”, from the science fiction TV series The Expanse.
In “The Expanse” it appears ships will fill up on everything in space, including water.

If figuring out how to refill a spaceship in Earth orbit were a movie, after the opening where the characters are gathered up from their day jobs, the leader’s pep talk ended with “…the secretary will disavow any knowledge of our actions”. Even so, at its start refueling in orbit was more than just promising. Technologically difficult yet manageable, with some vague benefits passes for promising. Here though the benefits were well defined and significant. At the time NASA was even funding real hardware with plans for a demonstration in space. As far as setting the stage, the NASA Constellation lunar return program had recently been canceled, unable to add up against likely budgets. Now with refueling in space there was a chance to fit those lunar exploration plans back into foreseeable budgets. Refueling in space added up to exploration sooner, compared to other options that just didn’t.

It’s completely impossible

Arthur C. Clarke is a favorite author of mine since I was gifted a book of his short stories by my 6th grade teacher Mr. Brown. Coincidentally, a story in the collection involves refueling in space (stealing the fuel). Clarke is quoted as saying revolutionary ideas go through three stages of reaction.

1. It’s completely impossible.

2. It’s possible but it’s not worth doing.

3. I said it was a good idea all along.

The story for the NASA gas station in space, begun with such promise, fell in 2011 somewhere between Clarke’s stages 1 and 2, between declarations of impossible or at best and reluctantly as possible but not worth doing. This was so even as the work did the math and answered the frequently asked questions, showing that transferring large amounts of cryogenic propellant in space was doable and well worth doing. As innovative ideas often show convergence where more than one champion comes along at the same time, a team at United Launch Alliance lead by Bernard Kutter reached the same conclusions. With flight proven technology and those already in development refueling in space was a game changer.

Technically, filling the tanks of a large rocket stage in space is nothing like filling them on Earth. Loading a Space Shuttle’s external tank or any rocket tanks sitting on a launch pad has something a stage in orbit lacks – namely gravity. It’s ironic how handy gravity comes in to load a rocket we use to overcome gravity. Yet the task of moving fluid between two sealed containers without gravity (not an easy trick) is well understood. It was never in doubt that it’s possible to transfer super-cold cryogenic fluid from one sealed bottle to another sealed bottle (“ventless” or “no vent fill”). Think sealed heat pump, thermodynamics, and slowing down compared to the time it takes to load on Earth and you are good to load and go.

Chart from NASA study on the "Tank Chill and Fill Technology Approach".
On the ground the basic principle to fill a cryogenic tank is to have the receiving tank open through most of the process, it’s “vent valve” open at the top. Liquid flows from the high pressure storage facility to the low pressure rocket tank. The loss of liquid out the top at the vent valve (boiled off as gas) is acceptable, as there is much more where that came from. In space, a sealed and “ventless” approach assures most all of the propellant pushed to the receiver tank remains inside the receiving tank. From a copy of the 2011 NASA Propellant Depot Study at NASAWatch.

Yet the demise of refueling in space after 2011 had nothing to do with technology, as much as another game changer. New technology and its perceived readiness for prime time (or not) is often not a real issue against adoption, even if it’s the first excuse offered. Everyone loves new toys. It’s when technology is hooked to changes in ways of doing business that the barrier arises, now as if asking to change the laws of physics. An idea part and parcel to propellant transfer in space was going commercial for NASA’s space transportation needs, not just propellant tankers.

Then along came another poor reason offered against refueling. While rockets are supposed to get cheaper as they get bigger, this is only true for apples to apples, say comparing two “cost-plus” programs (the old business model, NASA owned, no ongoing competition, NASA as the only customer). This notion a larger truck will haul cargo more cheaply than a smaller one – cost as measured by the pound – does not apply when comparing a large commercial rocket, as the propellant depot team was doing with Falcon Heavy, to an only marginally larger cost-plus rocket. Not even close. The relationship is then reversed – the only slightly smaller commercial and very competitive systems do better on all measures. Nonetheless, this ill-conceived notion any larger rocket must be better by the pound, among other seemingly technical criticisms, spelled the demise of refueling at the time.

It’s possible but it’s not worth doing.

Ideas are bulletproof, especially good ones. The work a decade ago heralded as “gas stations in space” came and went, the studies and the hardware scrapped from further consideration. Another team would come along in 2015 to go for a second bite at the apple (this time paid by NASA, but independent). Some members of the 2011 team (myself included) were at it again. Here the gas station in orbit was ditched and the space tankers would just refill the stage in orbit directly. Imagine a couple of gasoline tanker trucks filling up an even larger truck. Rendezvous, mate, and fill-er-up. (In my DOD circles as well, it was common for me to hear a sigh of relief years after that the depot, the “first big target”, was gone from this equation). As well, the refueling was no longer central in the 2015 work, necessary to the task, but with no need to overly defend it as feasible technology. Instead, the report went to the heart of the matter, new ways of doing business. This was after all the hard part for many in the target audience. It was the new ways of doing business that could create a true propellant economy in space, starting in Earth orbit. Yet, as with the earlier effort, this work came and went. Its value remains in the persistence of memory, spreading like so many thoughts that leave an impression.

I said it was a good idea all along

It’s now 2021 and refueling is at the stage now where it’s common to hear “I said it was a good idea all along”. NASA is back in the refueling business. It may have had something to do with physics, that thing about Earth orbit being that first most difficult climb out of a gravity well, and asking “if only we had a full tank”. More so it may be how new (commercial) ways of doing business have taken ground across all the new NASA programs – from low Earth orbit to the Moon. It may also be ideas like refueling stages in Earth orbit just have to pass through Clarke’s stages of revolutionary ideas first, fueling up along the way.

Thoughts? Comment below.

Also see:

  • Cryogenic Fluid Transfer for Exploration, Dr. David J. Chato / NASA Glenn Research Center, 2008 – “A study was conducted to compare flow rates demonstrated in experimental rigs [14 -17] to the requirements for flight systems. Currently there are no constraints on EDS fill time but most studies tend to recommend a single shift (8 hours or less). For propellant quantities required we used the 22 Metric Tons of propellant reported as burned during orbit insertion by the ESAS EDS. The fastest nonvented transfer in Taylor and Chato [15] is 534 kg/hr using a 6.35 cm diameter pipe. If this is assumed to be the maximum flow rate achievable, transferring the propellant required to top-off the EDS will require 42 Hours.”
  • For NASA, Longest Countdown Awaits, Kenneth Chang, January 24, 2011 – “”All our models say ‘no,’ ” said Elizabeth Robinson, NASA’s chief financial officer, “even models that have generous affordability considerations.”” [Well, except for some models.]

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