The Suborbital Refueling Dance

Jon Goff has an interesting variation on a concept that’s been around for a long time, but never implemented: refueling a suborbital vehicle in space to allow it to get to orbit. It’s in between Black Horse, which did an aerial fueling (or rather, oxidizing, since the propellant transferred was the oxidizer rather than the fuel), and standard orbital refueling. There’s an up side and a down side to it, relative to aerial refueling.

The down side is that unless the suborbital trajectory is fairly high, at least in velocity, you don’t have a lot of time for the operations before entering the atmosphere. You’d only have a few minutes, but that might be enough to transfer several thousand pounds of propellants. You’d have a trade as to whether to transfer just fuel, or just oxidizer or both. The latter would increase the likelihood of failure, since you’d have to mate two transfer booms, and simultaneously transfer two fluids.

The up side is that, out of the atmosphere, it’s easier and safer to fly in formation, because there are no wind gusts to worry about, and the physics is much more predictable (gravity doesn’t tend to vary much over non-astronomical times).

In a sane world, NASA would have long ago built an X-vehicle to prove out the concept, but that’s not the world in which we live. What I’d like to see is a prize for the first demonstration of such a propellant transfer operation, which all of the suborbital folks – Scaled (or VG), XCOR, Masten or Armadillo or others — could go after. You could have tiers of total propellant transferred, or total propellant transferred in a given time.

The other appealing thing about it is that, as Jon notes, it has benign abort characteristics. Which brings to mind another prize that NASA could offer (again, in a sane world). If reliability is really valued (the focus on heavy lift in general, and Ares in particular, would indicate that it’s not particularly, despite the advertisements), like low cost, it will only be achieved through high flight rates, and no one will really believe it until it’s been demonstrated. Fortunately, reusable suborbital vehicles are capable of lots of flights for low marginal cost per flight. So all they need is funding to do lots of flights. I would propose a prize for a consecutive number of successful deliveries to orbit (you could even start off with suborbital missions). Or, rather, consecutive number of non-failures, where failure is defined as losing the payload. In other words, you wouldn’t be penalized for an intact abort. The prize would be won when the requisite number of missions were flown with no losses. Abort rate could be a tie breaker for multiple winners.

If you wanted to have a demonstrated reliability (defined as non-payload loss, not mission success) of 0.999, you’d have to fly a thousand flights. If the marginal cost of a suborbital flight is, say, $10K, this would cost ten million, about the same as the X-Prize. So offer a fifty-million dollar prize, and see who goes for it. Once that’s won, offer half a billion for orbital.

9 thoughts on “The Suborbital Refueling Dance”

  1. Pumping only one propellant works best when you have a really large O/F ratio, like what Blackhorse would’ve had with its peroxide/kero combination. For more balanced O/F ratios (like what you see with most LOX/alcohol or LOX/hydrocarbon combos), being able to pump both definitely makes a difference. I’d have them both be on the same boom, just separate hoses though, because having to make two boom connections might be more hassle than it’s worth.

  2. This concept was actually explored before Pioneer Rocketplane was even founded. I don’t have a copy of the AIAA papers from back then handy but there is a mention of this in this article from Analog in 1995.

    We were focused on a “near SSTO” design using suborbital refueling as an enhancer for payload. You could think of it as a biamese idea except for the fact that you do the stage integration with a hose, and after launch. Mathematically, they’re not so different. Scroll down; it’s below figure ten.

    It’s also worth pointing out that the receiver-side propellant transfer hardware arguably could be common for both the atmospheric propellant transfer, needed to load the rocket propellants in the first place, and the exoatmospheric propellant transfer. We imagined a “transfer kit” in the payload bay of the tanker-configured vehicle, with the necessary pump or pressure feed systems to push the propellant out into the receiver.

    The precision flying needed to effect the transfer is probably easier outside the atmosphere, given the lack of disturbances. We pictured a back-to-back concept, but didn’t think it through completely, so perhaps another approach would be preferred.

    An aircraft designed for the suborbital point-to-point mission would be a good fit for this application. The extra delta-V obtained from a second propellant transfer could deliver net payload to orbit. A first look at the thermal protection system requirements suggests that the more rapid heat rates from the suborbital case is probably the designing requirement.

    Finally, if you can refuel exoatmospherically in a suborbital trajectory, you can do so in an orbital trajectory. And that makes a great many things possible, clearly, as advocates of orbital propellant depots have argued for some time.

  3. Reminds me about the high school math puzzle…. you want to cross the desert, its 250 miles, you can carry enough water to go 50 miles.
    How many people does it take to get one man across if everyone survives, if only the guy making it across survives…..

    Who said High school algebra puzzles were useless…. 😉

    With H2O2 the OF ratio is so skewed you only need to transfer one propellant.

    Also if the vehicles are VT is there any downside to having them take off already coupled…..

  4. With H2O2 the OF ratio is so skewed you only need to transfer one propellant.

    Which is also very good news for ISRU. Kerosene/H2O2 would be nocryogenic like MMH/NTO, which solves boil-off issues and allows propellant transfer with proven technologies. In addition it would not be nearly as nasty. Performance would be slightly less, but not critically so and ISRU potential would be a lot better. All in all, a fine near term propellant combination for landers and other transfer craft based at Lagrange points.

  5. Which is also very good news for ISRU. Kerosene/H2O2 would be nocryogenic like MMH/NTO, which solves boil-off issues and allows propellant transfer with proven technologies

    Cryogenic boil off is not such an issue if launching from above most of the atmosphere.

  6. Speaking of Transterrestrial prize suggestions: Peter Homer wins $250k 1st prize and Ted Southern wins $100k 2nd prize in Astro Glove Challenge. Congrats to both for a great competition! Re-Pete! My astronaut glove wins first prize at NASA Centennial Challenge!

    Worthy opponent Ted Southern teamed with Nik Moisiev, both past competitors, also beat NASA glove to win 2nd prize.

  7. “Also if the vehicles are VT is there any downside to having them take off already coupled…..”

    Dunno, the adverse aerodynamics of the two vehicles in “biamese” configuration at max-Q? That is, if the vehicles are mated together, they may be less aerodynamic or less controllable than if they flow by themselves. The Shuttle “stack” is not a true biamese, but the shock waves from the SRB’s and the Orbiter and the ET mixing it all up must have been a serious aerodynamic design challenge.

    The other problem could be the “stuff falling of the tank and hitting the tiles.” Imagine if the Shuttle Orbiter could be lofted into suborbit, the External Tank lofted into suborbit, and the tank fueling (or oxidizing) up the Orbiter outside the influence of aerodynamic forces and in free fall. You would not have the problem of, ahem, “stuff” falling from the Tank and damaging the Orbiter.

    On the other hand, the suborbital in-flight refueling mission profile — it would probably come about like “synchronized diving” (do they do that in the Olympics?) Two spacecraft on two pads, far enough away that they don’t risk blast damage to each other, but with synchronized engine start and synchronized liftoff. A computer gives the “GO” for both launches if both sets of rocket engines successfully reach operating thrust at the same time for the hold-down arms on each pad to be released at the same time.

    We would watch two rocket plumes streak skyward and then lose sight of them. We would “watch” the suborbital refueling represented by the “artists’ animation” on TV.

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