Before getting into my thoughts on potential options for the carrier plane itself, I wanted to mention a few nice-to-have options for the carrier plane itself. I don’t know that any of these is strictly required, but all potentially help:
Top-off Tanks
For many reasons cryogenic propellants would be the best option for truly competitive air-launch. But both for boiloff reasons, and for providing cross-fed propellants during the gamma-maneuver, having some smaller propellant tanks on the aircraft itself could be useful. These tanks could be insulated more thoroughly than flight tanks, since the carrier plane is the least weight-sensitive part of the system.
One clever option that Doug Jones mentioned to me at Space Access if you have such tanks is to fly up to 30kft, vent the launch vehicle propellant tanks (one at a time)1, let the tank vent until it is at the now much lower ambient pressure, and then refill the tank till nearly full. Cryo propellants will boil at a colder temperature at altitude, and the heat absorbed by boiling off some of the propellants will chill the remaining propellant to this lower temperature, densifying it2, and significantly reducing the pressure needed in the tanks to suppress pump cavitation. Both of these can result in a non-trivial reduction in system mass, especially if your system is large enough that you’re not at minimum gage levels for your tank wall material.
Crossfeed Pumps and T-Zero Disconnects
A lot of ground launch vehicles have propellant, pressurant, and electrical umbilicals/quick disconnects that only separate right as the vehicle is taking off the pad. You might not want to cut it quite as close here (T-1 disconnects would be fine too), and you’ll definitely want features for retracting the hoses out of the air-stream after they’ve disconnected, but making these types of hoses and disconnects shouldn’t be that hard. In the case of crossfeed pumps, you only need a pump that can keep up with the flow-rate of the engines operating at whatever throttle level they’re running at during flight operations, and push against the backpressure from the main propellant tank pressurant, since you’re feeding in through the same fill ports the vehicle uses for ground filling. The power levels required would be low enough that an electric powered pump would make a lot of sense–easy to control the pump flow-rate/pressure to make sure you don’t over or underfill the tank. You’re probably talking about needing a pressure of 15psi or less, which means that compared to an electropump for a main propulsion system (like RocketLabs and Ventions are using), you’re probably only looking at 1-5% of the power needed for the cross-feed pumps. I see how cross-feed for dual strap-ons on a tri-core rocket stage like Delta-IV or Falcon Heavy might be hard, but this seems relatively straightforward by comparison.
Propellant Umbilical Reconnect Mechanisms
A slightly harder task would be designing the disconnects in a way that they could be in-flight reconnected. This might involve some level of robotic or mechanism hardware to make the reconnection, but could be handy in case of a last-second abort. Also, this same sort of hardware would likely be exactly what you’d want for refueling or detanking the upper stage at an orbital propellant depot.
Emergency Detanking Hardware
In addition to cross-feed/tank-up pumps, it might be good to have a way of detanking the propellants from the rocket in case of an aborted mission. This could possibly use some of the same hardware, but thought should be taken on how and where you route the dumping propellant, and how you sequence them, so you avoid building up hazardous concentrations of flammable materials near spark sources during an emergency propellant dumping operation.
In-air Propellant Transfer Hardware
I wasn’t thinking about this in my baseline Boomerang system, but having the ability to transfer propellants to the carrier aircraft in-flight might enable launch vehicle performance enhancements without requiring a bigger carrier aircraft. While kerosene transfer is routinely done by the military, LOX and cryogenic propellant transfer should also be technically feasible, but would require some demonstration3. I’d probably have prop transfer go into the holding tanks on the aircraft, and from there into the launch vehicle (that way you minimize the odds of damage to the vehicle, and reduce the dry mass impact on the launch vehicle itself).
Most aircraft mass limits are due to take off thrust and abort considerations. If you could launch with the rocket empty or mostly empty of at least one propellant type4, you could carry a much bigger rocket and payload at takeoff with the same carrier plane. This would allow growing to a larger system over time if desired without requiring a new carrier vehicle design. Depending on the range of the tanker aircraft, this might also give the carrier airplane more flexibility on how far it flew prior to launch operations.
Tow Cable Reattachment Hardware
At least some carrier plane options would use a glider-based carrier plane towed by a larger, more traditional aircraft. It might be challenging, but in the case of an abort with a glider based carrier aircraft having a mechanism similar to the in-air-fuel transfer mechanism that allows reconnecting a tow cable to the nose of the carrier aircraft might be valuable. It might even be useful for normal operations, where the towing aircraft could maneuver out of the way for launch, and then catch up with the glider and reconnect with it to tow it home to the launch site. If you’re particularly crazy/clever, you might even be able to find a way to combine this with in-air propellant transfer hardware to enable retanking the rocket in case of an abort, though I’m not sure what scenarios that capability would make sense.
Anyhow, as I said at the start of this blog post, most of these capabilities are nice-to-have, not have-to-have. Prioritywise, the top-up tanks and cross-feed pumps are the ones I think would be most worth looking into for a first generation Boomerang system.
Next up in Part IV: my thoughts on carrier plane options.
Jonathan Goff
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- And I’d urge extra caution in how you vent any cryogenic fuel like methane or hydrogen, since they can form explosive mixtures with the ambient air if you’re not careful
- Doug was saying you could increase LOX density by something like 7%, and LH2 by about 5%
- The Rocketplane concept over the years relied on transferring some form of non-standard propellant (either Hydrogen Peroxide or LOX) from a tanker aircraft to the launch vehicle, and while they never got to flight demonstrating the concept, there didn’t appear to be any real show-stoppers.
- If you transferred only one propellant, most likely LOX would be what you’d want to transfer since it would be used on both stages, and tends to make up the majority of the mass of any typical propellant combination.
An alternate to in-air propellant transfer is LOx harvesting, which would place an Air Collection and Enrichment System (ACES) within the aircraft. However, as ACES would require LH2 to liquefy the incoming air before separation, it best suits RLVs that use LH2 as the fuel but still shows useful payload performance gains (~30-40%) with 1st stages using RP-1.
Of course, there are many draw-backs compared to in-air propellant transfer (e.g. the LOX is not pure, with >5% N2) and the obvious mass penalty of the ACES equipment (~4t), but it does reduce the number of vehicle involved in each launch and so may enable a more streamlined operation.
Re: Towing
The tow-plane doesn’t have to be larger than the carrier-glider, most aren’t. It does need a large engine, but because it isn’t lifting the weight of the rest of the system, it can be surprisingly small.
Re: Towing, refuelling
Essentially the glider becomes a fly-back tank. In that case, once you release the glider from the tow-plane, the rocket-stage doesn’t need to throttle down. Indeed, you want to go as hard as a conventional launch (within the structural limits of the glider), until you’ve drained the glider-tank. Then jettison the glider as you would a big-dumb-tank, leaving the booster fully fuelled, at the highest possible speed/altitude, and at the optimal trajectory to continue.
And because you can release the glider before you ignite the rocket, you greatly reduce the risk to any pilots on the tow-plane. (And assuming the glider is unmanned.) If the rocket doesn’t ignite, you have the option to try to reattach the tow-plane and return-to-airport, or failing that, to dump fuel and glide to a controlled (soft-crash) landing on a convenient bit of dirt road/large-paddock/beach/etc. Ruin the landing gear, but salvage everything else.
Speaking of…
Re: Throttling down the engines to merely counter the weight of the rocket-stage.
Just realised: Wouldn’t be really wasteful? Like using low acceleration in a conventional vertical launch?
Sure you can vent both cryo propellants at the same time, as long as the vents are a few feet apart. You have a 500 knot wind blowing them away….