Suborbital refueling: a path not taken
by Francis Castanos
|Make no mistake: hypersonic refueling was insane and was rightly dismissed.|
At the dawn of the Space Age, the Air Force performed what are known as the Aerospaceplane broad studies. It was to be the next step beyond DynaSoar: eliminating the Titan III and LC-40 and flying into orbit from a very ordinary runway. From the Aerospaceplane studies come the three broad families of SSTOs, corresponding, respectively, to the VentureStar, the X-30 Orient Express, and Skylon; all-rocket, air liquefaction, and scramjet.
Yet buried deep inside the Aerospaceplane studies was a fourth path that was not taken. That is, applying aerial refueling to a SSTO. Called HIRES (Hypersonic In Flight REfueling System), it was to be tested by two X-15 flying in formation at Mach 6, and trying to hook up, wingtip to wingtip. Make no mistake: hypersonic refueling was insane and was rightly dismissed. Yet things might have been different. Because, you see, the X-15 not only flew fast, to Mach 6.7. It also flew very high, up to nearly 108 kilometers, providing its pilots with three minutes of microgravity. Now imagine, if HIRES flight testing had been repurposed to those high altitudes rather than Mach 6. The refueling would now happen in suborbital flight. Outside the atmosphere, riding a segment of orbit, there is no turbulence, kinetic heating and hypersonic shock waves.
Three decades later, Mitchell Burnside Clapp re-introduced aerial refueling on the way to orbit but subsonically, behind a modified KC-135. That was the Black Horse study, followed by the Black Colt and Pathfinder iterations. Yet at various times between 1993 and 2003 he flirted with suborbital refueling as far more efficient. According to Gary Hudson. it was briefly examined again for RASCAL circa 2002.
Two years later in 2004 come Alan Goff of Novatia Labs, a brilliant thinker who went much farther with FLOC. He replaced suborbital refueling with suborbital docking and went from 2 to 64 machines.
Having read their papers avidly and contacted these two people, I intend to strike right between the two concepts and at a crucial juncture: two, three, and four rocketplanes, and eventually eight. No more, because the suborbital ballet then become overly complex.
Three variants of the rocketplane exist. All weigh 120 metric tons with full tanks and 18 metric tons with empty tanks, a propellant mass fraction of exactly 0.85 (although hydrolox can go lower at 0.78 thanks to its superior specific impulse).
The Mk.1 burns kerosene with hydrogen peroxide at a specific impulse 327 seconds (Reaction Motors LR-40.) In order to make orbit with a decent payload, it needs military turbofans augmented with mass injection pre-compressor cooling (MIPCC) to push to Mach 4 before ignition of the rocket. MIPCC drops water ahead of the compressor and either hydrogen peroxide or liquid oxygen in the exhaust, augmenting a military turbofan from a F-15 max speed of Mach 2.5 to Mach 4. At this point, the delta-v gain to orbit is two kilometers per second, lowering orbital speed from nine to seven kilometers per second. Note that the oxidizer/fuel ratio of hydrogen peroxide/RP-1 is vastly unbalanced, at seven. This mean that the fraction of kerosene is minuscule compared to peroxide (15 versus 85 metric tons), hence only the later needs to be transferred.
|The US Navy has used buddy-buddy refueling for at least 60 years, all the way from Skyhawks to Super Hornets. Suborbital refueling would be very similar.|
The Mk.2 switches to kerosene/liquid oxygen and uses that outstanding Soyuz third stage engine, the RD-0124, with a specific impulse of 359 seconds. That way it doesn't needs MIPCC to get into orbit, but the price to pay is a much different oxidizer to fuel ratio of only three that forces a kerosene transfer along with liquid oxygen. The ratio would be 25 to 75 metric tons.
The Mk.3 uses liquid hydrogen and liquid oxygen (hydrolox) and the specific impulse reaches 455 seconds. No need for MIPCC either, only to modify Pratt & Whitney F-110s to run on hydrogen. At least the oxidizer to fuel ratio of hydrolox matches that of kerosene and liquid oxygen at six, so once again, only liquid oxygen needs to be transferred.
Because of their lower specific impulse, the Mk.1 and Mk.2 needs at least J58 turboramjets or MIPCC to make it to orbit with a meaningful payload. The Mk.1 and Mk.2 have subvariants according to the many different airbreathing engines that can be interchanged with MIPCC. It starts with Rolls Royce BR.715 turbofans to Mach 0.95; stock F-110s to Mach 2.5; J58 turboramjets to Mach 3.5; MIPCC F-110 to Mach 4 and SERJ to Mach 4.5. Even a SABRE vehicle could be done (think of a "baby Skylon"), pushing to Mach 5.5.
Most of these engines could switch to hydrogen fuel and thus apply to the Mk.3 although it doesn’t really need them thanks to its vastly superior specific impulse. This doesn’t mean the lower performance engines are not useful: they fly close enough to orbit to be used for point-to-point suborbital passenger transportation, and the BR.715 turbofans can take off and land at ordinary airports.
For each one of these engines I calculated the delta-v gain they bring on the way to orbit. Combined with optimized ascent trajectories and high density propellants smaller gravity losses, it is possible to lower the delta-v from a classic ground-launched, all-rocket SSTO 9.2 km/s to 6.2 km/s for a SABRE vehicle. Together with suborbital refueling, the gain in payload is already immense. Then add a third and a fourth rocketplane into the FLOC, and very large payloads can be lifted into orbit.
The concept of operations is as follows. Two rocketplanes take off on jet engine power from a standard airstrip, military base, or airport, accelerate to the upper limit of their airbreathing engines (Mach 0.95 to Mach 5.5), then fire their rocket engines up to five kilometers per second and 130 kilometers. Once there, they settle into a suborbital parabola, get close together by firing thrusters, and proceed with an oxidizer transfer using the revolutionary Boom Rendezvous system: essentially a handshake with flexible booms (Bonometti, Sorensen, Jon Goff). Overall, it is essentially buddy-buddy refueling as used by the US Navy and French aéronavale, applied to rocketplanes. The refueling pack or pod can be placed into the payload bay as a removeable kit provided of course it can be connected to the oxidizer tank. This mean that the tanker is not a specialized rocketplane, just like F-18E/F Super Hornets replaced the KA-6 and KS-3.
Vehicles ascend into space with a trajectory to bring them together with a separation or one kilometer or less at engine cutoff. They then use active sensors, RCS, maneuvering thrusters and a cooperative rendezvous system to approach to 15 meters or less of each other. They then use cold gas thrusters to maneuver to less than 10 meters and activate the automatic refueling booms. These are equipped with a beacon and sensor tuned to the same frequency, which are used along with built-in boom articulation to connect and seal the transfer pipe. A test “slug” of propellant is sent through the system at high pressure and bleed off into vacuum through the purge valve to ensure connection, and check for leaks and contamination in the lines as well as to clear the for propellant transfer. Propellant is then transferred till completion.
The probes’ booms deploy at five to ten meters and use active connection (they are articulated to a degree and can extend or retract) systems to find and connect with each other. Once the booms are connected, the receiver and feed nozzles connect, verify, and propellant flow begins. As the booms can adjust for slight variations and with active RCS on the vehicles, that should keep them stabilized within a meter during the process.
There are several risks with this approach. From one kilometer to 50 meters, there’s the risk of an RCS or maneuver engine failure on one or both vehicles. From 50 to 15 meters, the RCS system on one vehicle could fail, aborting the mission.
In the final stages of the rendezvous and transfer, there are several additional issues. The booms could fail to connect for some reason. Also, propellant or other contamination cause detonation in propellant booms, nozzles or connections. As the systems are cleaned and inspected on Earth this should be a low probability but it has a high chance of significant danger so it is addressed.
|Suborbital refueling is nothing more than a different way of staging, as imagined by Tsiolkovsky 120 years ago.|
A failure at the entry point to the transfer vehicle boom arm would damage the boom and likely also the joint assembly. Kevlar lining in the boom bay should protect the vehicle from any damage and the boom will be able to be disconnected and jettisoned if needed. Alternatively, there could be a failure along the transfer vehicle boom length, in which case there may be some shrapnel damage or external damage to the upper side of the vehicle. Since the explosion would be of low order any shrapnel generated should have low velocity and thus extensive damage is unlikely.
Another risk is at the boom-to-boom interface. That is similar to the above but the more robust structure would absorb more of the blast energy. This would damage both booms and require they be jettisoned and the mission aborted. Failure of the propellant transfer system during operation would mean aborting the mission unless sufficient propellant had been transferred before system loss. Finally, the booms could fail to disconnect after propellant transfer is completed. In this case the “orbital” vehicle with payload detaches its boom and moves away to fire its engines and proceed to orbit.
“Are gravity losses manageable?” is a recurring question, and a very good one. I carefully examined the issue and the results are positive. Optimized ascent trajectories, dense propellants, and airbreathing engines drastically lower delta-v and largely compensate any losses.
But there is more. To make a long story short, centrifugal forces are the key. For “low” velocities, the effective gravity of Earth does not change much, and it is reasonable to assume the velocity change with the engines off is 10 m/s2. As the refueling happens closer to orbital speed, this can drop to the point that it is reasonable to moderately extend a suborbital trajectory.
This is confirmed by Wikipedia article on gravity drag , and also by more reliable sources: “As orbital speeds are approached, vertical thrust can be reduced as centrifugal force (in the rotating frame of reference around the center of the Earth) counteracts a large proportion of the gravitation force on the rocket, and more of the thrust can be used to accelerate.” Astronautics: the physics of spaceflight (Ulrich Walter, 2018, p.155) makes clear that, during acceleration in suborbital flight the centrifugal force is increasing, until at 7.8 kilometers per second it evenly balances gravity and prevent the vehicle from falling back—orbit. The trick is to refuel at a speed close enough from orbit that centrifugal force partly balance gravity losses, or above 4 kilometers per second.
Suborbital refueling is nothing more than a different way of staging, as imagined by Tsiolkovsky 120 years ago. In turn, staging has a massive effect on payload because it hits the rocket equation’s one and only parameter we have a solid grasp on: the mass ratio. All others parameters are hopeless. Staging turns the logarithm on its head and it becomes a bonanza rather than a liability. Suborbital refueling basically launches identical rocket stages separately and unites them only briefly for an oxidizer transfer in suborbital spaceflight. Not only are the rocket stages identical and thus of lower cost through mass production (see FLOC) but they can be turned into rocketplanes even with all the dead weight that entails. More than two or three stages, er, rocketplanes can be launched, up to four or eight, or even FLOC craziness if you are bold enough to attempt it: 32, 64 or 128.
A mind-boggling aspect of the whole thing is that even without suborbital refueling there are a whole bunch of missions that can be done even by the low performance Mk.1. Among them: satellite launches with an expendable upper stage; suborbital point-to-point transportation with Preston H. Carter “Hypersoar” ricochet trajectories; suborbital tourism at six kilometers per second rather than Virgin or Blue Origin’s one kilometers per second, with longer zero-g time for passengers.
Suborbital refueling offers a clear path toward making Clarke and Kubrick’s Orion III spaceliner come true. Remarkably, the concept could have been applied to the X-33, and could apply today to Skylon once in rocket mode, not to mention ISINGLASS and many others past concepts. More generally, an oxidizer transfer between two rocketplanes brings large gains in payload to orbit. The system is extremely flexible and can achieve a wide range of missions from suborbital point-to-point passenger transportation to ISS crew and cargo missions to cislunar flights. An entire family of rocketplanes can be created according to commercial, military or space agencies’ requirements.
The most exciting prospects however remain with three far-reaching, profound breakthroughs.
|The same rocketplane, if entirely refueled in Earth orbit, could lift a very large payload into cislunar space and return via a mix of propulsive braking and aerobraking.|
First, the introduction of suborbital refueling lead to a complete paradigm change that crushes the rocket equation tyranny once and for all. Whatever the propellants and specific impulse, suborbital propellant transfer acts as a colossal “force multiplier” that rise payload to orbit tremendously.
Second, with the mass fraction stuck into the rocket equation no longer an issue, it is possible to “burden” an RLV with turbofans and wings, which give it access to very ordinary airstrips. The final objective being to make a rocketplane as easy to handle as a Boeing 737, from runway to orbit.
The third, and most profound, implication is that the same rocketplane, if entirely refueled in Earth orbit, could lift a very large payload into cislunar space and return via a mix of propulsive braking and aerobraking. In the end it would be possible to fly all the way from LAX airport to lunar orbit.
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