Exo-Atmospheric Suborbital Refueling

[jongoff]

I've been sick at home for the past week and a half, and decided to write-up a short blog post about the concept of exo-atmospheric suborbital refueling.  It's a sort-of crazy idea, but probably physically possible, and has been historically investigated as well (by Pioneer Rocketplane and HMX as part of their RASCAL project work).  I mostly go into some of the benefits of the approach, how one might go about it, and a few observations about the technique:

http://selenianboondocks.com/2009/11/random-thoughtsorbital-access-methodologies-vii-air-launched-glideforward-tsto-with-exo-atmospheric-suborbital-refueling/

[mlorrey]

hi Jon,
Nice article, interesting thinking. Would you recommend that the two vehicles utilize some anti-collision formation flying AI software, esp given the high accelerations these vehicles are undergoing on the way up, or do you think human pilots could manage this sort of flying/maneuvering under boost?

[MP99]

Jon,

how weird - this morning I mused about DIRECT CaLV & EDS docking or PT-ing sub-orbitally (would have to happen in that 30mins before the circ burn). About 5 secs later I decided it was way too risky for NASA!

You mention military refuelling, but are there any existing applications where they must approach, dock & begin PT anything like that quickly? (In danger of coming under fire, maybe?)

cheers, Martin

[khallow]

While obviously, the big market is Earth to orbit, this approach would work well for travel in the Solar System in cases where you'd have days or longer to make fuel transfers.

[Danny Dot]

Clever idea.  It might just work.  The formation flying would be challenging, but no worse than tanking a fighter aircraft at night in the weather.

Danny Deger

[jongoff]

Clever idea.  It might just work.  The formation flying would be challenging, but no worse than tanking a fighter aircraft at night in the weather.

In many ways it might be easier.  Most of the formation flying is taking place at really high altitudes where there isn't wind to buffet things around, and where dynamic pressures are really low/negligible.

The trick is finding a way to balance all the competing demands while still leaving margin.  You want to do the staging, hookup, and transfer at as high of an altitude as possible, but you don't want too lofted of trajectories in case of aborts, and you want to set things up so that all three of the non-orbital stages can make it back to the landing site without having to spend lots of fuel on a boostforward/boostback or lots of weight on wings or a lifting body.  I think it can be made to work, but I just don't have the 3 or 6DOF and the chops to run the simulation.  It'd make for a fun thesis or dissertation level project though...

~Jon

[jongoff]

While obviously, the big market is Earth to orbit, this approach would work well for travel in the Solar System in cases where you'd have days or longer to make fuel transfers.

Yeah.  In some ways this is similar to one of the early propellant depot ideas (from ~80 years ago).  I'm not sure how much it buys you compared to a fixed depot though.  The only reason for doing a vehicle-to-vehicle transfer was because there's no convenient stopping point between the surface and LEO.

~Jon

[jongoff]

hi Jon,
Nice article, interesting thinking. Would you recommend that the two vehicles utilize some anti-collision formation flying AI software, esp given the high accelerations these vehicles are undergoing on the way up, or do you think human pilots could manage this sort of flying/maneuvering under boost?

I think it could be automated or manually flown.  More likely to be done with software though, because the tanker would likely be unmanned, and it's likely going to be the half doing the formation flying maneuvers--the orbiter would be flying a preplanned trajectory, and the tanker trying to match up with it.  Or at least that's probably the way that makes the most sense.  And for automated flying you don't necessarily need "AI".  You just need a good guidance algorithm.

~Jon

[aftercolumbia]

I'm going to bump the "Cheap Reusables" thread in a moment, since there is a bit of a common theme in the high orbit rendezvous strategy of the Mars plan that I attached to that booster.  (This idea was given to me by Grant Bonin, that instead of his previous plan to rendezvous kick stages on low energy orbit, as in his paper "Mars For Less", use a slightly larger booster/smaller payloads to launch the stages to a higher, elliptical orbit, where it is easier to store them during the assembly phase.)

So, a way of getting around the "why not just use a bigger booster?" problem, I'm going to assume Lilmax 5 module boosters in this case, and a once around suborbit for the refuelling activities.  I'm going to mirror the phases in your article:

1. The two Lilmax pads would be near each other, but not particularly close.  The performance critical vehicle is the tanker, and so the upper-stage equipped booster will maneuver to the tanker's trajectory, not the other way.

2. The tanker vehicle launches first, I don't know by how much, since I haven't analyzed the concept at all, but it will definitely be taking off first.

3. The lower stage composite burns of the two boosters will be very similar (described in the "Cheap Reusables" thread).  If the vehicles are close together, there might be an issue with the upper stage booster taking the plume of the tanker, especially in the later stages when they are closer together, the plumes are huge, and the fairings are off.

3b. The upper stage equipped booster has a Lilmax second stage equipped with a main engine and pressure-fed verniers, while the tanker is equipped with with a Lilmax upper stage tank section with verniers only.  The Lilmax tanker without the main engine might have either an empty or short interstage; either way, it will be visibly shorter on the pad since its only payload is the refueling kit.  These tanks operate at about 200psia, so a respectable 150psia vernier chamber pressure can be achieved.  The full blown upper stage will be operating on its main engine and expending a lot of its propellants catching up to the tanker, whose upper stage composite is quite a bit lighter.  The core module of the tanker module will be landing near the Morocco coast in a CCAFS/easterly scenario.

4. With the two vehicles now abreast about 50m apart and with speeds matched by the upper stage engine throttling (the upper stage being the active vehicle), the upper stage cuts off its main engine and continues to operate on verniers.  Both vehicles will cut off verniers once the once-around recovery trajectory for the tanker has been reached.  They will have about half an hour to transfer propellants.

The tanker, with the refueling kit, will probably release a small RCS equipped active vehicle which rides a set of laser beams (probably three) to the upper stage and docks.  Trailing it, and reeling out from the tanker, are the two propellant hoses.

5, 6. Not applicable

7.  Most likely the transfer would be enabled by having the upper stage vehicle vent pressurant from the tank's ullage space, and this would thus maintain settling accelleration.  The tanker stage would be in the lead with settling accelleration maintained by a centerline thruster.  The tank differential pressure would be maintained by adding pressurant to the tanker tanks and venting it from the recipient upper stage.  The settling accelleration would add some head, but since the settling accelleration is probably in mm/s2, it wouldn't be significant.  The main point of the settling accelleration is to ensure the tanker is drawing liquid, and the upper stage is venting gas.

8. The tanker has to be empty for recovery purposes, so this system would be designed so that the upper stage has plenty of room to accept propellants and will likely be about 90% full at the end of the transfer, rather than 100%.  Another issue is that it is harder to ensure you are getting gas at your vent outlet as the fill factor increases, and there is a limit to how reliably the liquid will stay down in a tank at low settling accelleration, especially when your are filling it.  I know who to call, though, Dan Jaekle of ATK's Space System's Division (a.k.a Pressure Systems Incorporated.)

9. After the boom tug separates from the upper stage, the tanker will have another half hour to reel it back in.

10. At the apogee of the suborbit, the upper stage will ignite its main engine to circularize its orbit, while the propellant given to it by the tanker will fuel a big kick to a high energy elliptical orbit (for example, GTO, or the aforementioned high energy orbit for the assembly of a mission to Mars.)  Two five module Lilmax boosters used in this way could probably put 40,000kg or more on GTO without needing an Ares-class booster.

[Archibald]

bumping this old thread because this idea has a possible huge potential. A SSTO easier to build than either Skylon, the DC-X, or the Orient Express.

With two refuelings - the first at subsonic speed, a second during suborbital flight - then a SSTO with a meaningful payload to orbit become a reality.

What would be needed ?

A large aircraft tanker - a 747-8 could do the job.
Two identical suborbital space planes. Rocket-powered to Mach 15 and a height of 130 km. Propellant: H2O2 / kerosene.

The two space planes take-off from a very ordinary airport on turbofan power. They meet the tanker and takes H2O2 oxidizer - lots of it, hence the need for a 747.
Together, side by side they climb into suborbital flight.
Once out of the atmosphere and on a parabolic arc, the first machine sprouts a refueling boom and refuel its twin.
The twin reach orbit.

Bonus: according to Mitchell Burnside Clapp, by refueling so high and fast some massive payload boost happens.

the weakness of the concept: you need three aircrafts, one tanker and two suborbital machines. Fair enough.

the nice side: here's a SSTO without a) Skylon precooler b) DC-X impossible mass fraction c) the X-30 impossible scramjet.

https://www.risacher.org/bh/analog.html

A Speculative Idea

Beyond their baseline mode of operation, certain speculative operabonal modes exist that could signiticantly enhance the capability of APT vehicles in the future.

Consider the case where we have two Black Horse type vehicles, each using JP-5/H2O2 with an Isp of 335 s. The vehicles have a dry weight of 15,000 lb and a propellant load of 180,000 lb, which assuming a required Delta-v to orbit of 27 kft/s, allows them to deliver 1,000 lb to LEO. Now, let's say that we fly the two of them off together, accelerating them jointly not to orbit, but rather to a suborbital trajectory with a velocity of 18.5 kft/ s. The two space planes are now outside the atmosphere, in free fall (i.e. zero gravity) in the immediate vicinity of each other. Let's say we now bring the two together and extend a refueling boom, allowing the 20,000 lb of residual propellant from one to be transferred to the other. The two then separate, the empty vehicle to return to Earth, the enriched vehicle to ascend to orbit with a payload of 12,000 lbs. Without any new hardware, the orbital delivery capability of the system can be increased by a factor of 12.

Such a non-material enhancement by teamwork would allow even an APT spaceplane that was designed for suborbital flight to achieve orbit. Or put another way, let's say that it turned out after the construction was done that the actual Black Horse dry weight came in not at 15,000 lb, but at 24,000 lb, a 60% mass growth over the estimate. The vehicle would now only be capable of suborbital flight to 23 kft/s. However, if two such vehicles were flown, performed a suborbital propellant transfer at 15.5 kft/s, the enriched vehicle would be able to make orbit with a 1000 lb payload. Since the propellants being transferred are non cryogenic, such a suborbital zero-g propellant transfer could be done using bladders. If the APT in question used LOX for its oxidizer, the transfer would require a weak gravity field, which could be created by both vehicles firing their RCS systems continually during the transfer.

The plan certainly sounds incredible, and to be frank, we don't expect such maneuvers to be done anytime soon, but it's not impossible. On a suborbital trajectory with a velocity of 16 kft/s and a 120 nautical mile apogee, the vehicles in question will be out of Earth's atmosphere for about six minutes. The actual propellant transfer can be done in less than two minutes. With sufficient training, good pilots could eventually do the job.

Just saying... this is amazing.

[clongton]

Clever idea.  It might just work.  The formation flying would be challenging...

Not necessarily. Instead of 2 vehicles being flown independently in formation during rendezvous and propellant transfer, slave the tanker autopilot to the orbital vehicle's autopilot and have the orbiter vehicle's autopilot control both vehicles, running a subroutine designed specifically for the task. At that altitude the formation would likely be rock solid.

[aero]

The two space planes take-off from a very ordinary airport on turbofan power. They meet the tanker and takes H2O2 oxidizer - lots of it, hence the need for a 747.
Together, side by side they climb into suborbital flight.
Once out of the atmosphere and on a parabolic arc, the first machine sprouts a refueling boom and refuel its twin.
The twin reach orbit.

Sort of like a rocket takes off with a large liquid booster strapped to its side, then before the rockets run out of fuel, the first rocket refuels from the tanks of the second rocket which runs dry and separates, and the first rocket now with full tanks, continues on to orbit.

sort of (exactly) like crossfeed.

[Rugoz]

Well you wouldn't have to attach two launchers to each other, like in usual TSTO proposals, making it easier to design them as planes.

However refueling at mach 10, full thrust, with little time to approach each other, sounds pretty crazy.

[Elmar Moelzer]

Sound pretty much like parallel staging with cross fueling to me (what SpaceX is doing for the FH), but with two parallel stages that launch separately. I am not sure that this would really bring that big of an advantage. There was a concept in the late 90ies that did in flight refueling. Not sure anymore which one it was, might have been the Pioneer rocket plane. I always thought the idea was rather strange.
Kelly Space wanted to town their Astroliner to altitude behind a 747, which might be preferable to in flight retanking (at least for the proposed lower altitude first retanking step)?

[Robotbeat]

Yeah, no better than cross-feeding. Much more complicated, actually.

[RanulfC]

Well you wouldn't have to attach two launchers to each other, like in usual TSTO proposals, making it easier to design them as planes.

However refueling at mach 10, full thrust, with little time to approach each other, sounds pretty crazy.

Mach-10 or less actually and NO "thrust" you'd be in free-fall till after link-up. Then the two vehicle throttle up together.

Sound pretty much like parallel staging with cross fueling to me (what SpaceX is doing for the FH), but with two parallel stages that launch separately. I am not sure that this would really bring that big of an advantage. There was a concept in the late 90ies that did in flight refueling. Not sure anymore which one it was, might have been the Pioneer rocket plane. I always thought the idea was rather strange.
Kelly Space wanted to town their Astroliner to altitude behind a 747, which might be preferable to in flight retanking (at least for the proposed lower altitude first retanking step)?

"Black Horse" Aerial Propellant Transfer Concept, later Pioneer, etc
The idea was to use an "off-the-shelf" aircraft for the subsonic propellant transfer aspect to lower operations costs. In the case of the BH it was USAF KC-135 tanker aircraft, and later C-141s with "parked" propellant tankers and a "boom" kit added. The big advantage was the LVs were not "all-up" orbital vehicles and there were more than one of them which helped spread the costs. (The aforementinoed article talks about FLOC which had DOZENS of vehicles through out the whole trajectory and is quite interesting if a bit unweildly

The reason it's "better" than cross-feeding are several actually. Number one of course is that "cross-feed" hasn't been done yet while refueling has You're also not building a single all-up orbital vehicle but smaller, less costly (supposedly at any rate) vehicles and adding multiple abort options all along the flight path. Of course BECAUSE you're vehicle is going to be heavier and more complex than a "simple" cross-feed booster your total overall advantage is going to be slight.

Of course the "other" method is to simply put recovery gear on your "booster" in the first place (thereby gaining your "cost" savings from mass production) and stacking them together from the start ala-MUSTARD, and avoid the whole rendevous and docking process. But then you're getting into multiple vehicles that all pretty much have to be full-up orbiters no matter what....

Well you can see where this goes

Randy

[Elmar Moelzer]

"Black Horse" Aerial Propellant Transfer Concept, later Pioneer, etc

Guess that is why I was not sure anymore what it was called, it had multiple names

[RanulfC]

"Black Horse" Aerial Propellant Transfer Concept, later Pioneer, etc

Guess that is why I was not sure anymore what it was called, it had multiple names

It's that whole "edgy" name for the military then a more "mundane" name 'cause the civy's is wussies thing... I know it gets confusing but it's a military thing

Not

Randy

[Archibald]

Good points about cross-feeding but... it's a red herring here. The main advantage is elsewhere.
Unlike a crossfeed Falcon 9 it doesn't takeoff vertically from a huge pad.
Unlike Skylon it doesn't burns hydrogen, weights 400 tons and takes 5 km to takeoff at 500 km per hour.
A sizable number of RLV studies and concepts (from the 1968 space shuttle and StarClipper to Reagan Orient Express of 1986) insist about flying out of ordinary airports. Easier said than done ! That concept can do it, and I think its important.
Same thing goes for noise levels: subsonic refueling doesn't bother anybody ears.

[Rugoz]

In principle that would be a very nice concept. Two space planes, one with payload bay and better TPS for orbital reentry. Both start at the same time, refuel at subsonic speed (standard tanker), then launch to space and the suborbital plane refuels the other one before returning.

A bit complex though, especially that refueling in space.