Jan 2, 2017

6 min read

The Potential of Hypersonic Aerofoils for Interplanetary Transit

In this post, I lay out a possibility for the use of hypersonic spaceplanes that are constructed in space, use planetary atmospheres as a source of momentum, never make a terrestrial landing, and fulfill the role of transporting people and materials from one planetary orbit to another. To do this, I am outlining some gaps from the existing knowledge-base on the subject, and speculating on several very radical aspects of my proposed design that we may not be able to either substantiate or rule out at this time.

My intention is to write several posts on the subject, as well as develop some simple mathematical models in-code. This post is to break open the subject.

Applications of Hypersonic Aerofoils

My interest in space revolves around human habitation and space industries. As such, I’m looking at technologies through an interpretation of how they can be useful to those endeavors. However, in the current climate, technologies are mostly interpreted through applications that could be used in science missions currently on the table — which means robotic space probes.

Because of that status quo, the engineering and scientific treatment of the applications of hypersonic lifting bodies are only analyzed in-depth for use with near-future expendable space probes. For hypersonic lift, that limits us to a marginal gain in gravity assist maneuvers for deep space probes. In that context, it is a niche proposal that is unlikely to be practical when weighed against the risks of a first-of-a-kind demonstration and the myriad of other systems on-board which all impose their own design constraints. This is exactly how the Wikipedia article on the subject reads.

I am interested in how this technology can be used as a reusable component of a larger space infrastructure which transports people and cargo between a variety of destinations from Jupiter to Venus on a regular schedule.

Once you dig into the references of that Wikipedia article, you find one paper (Sims et. al.) that is more interested in the orbital mechanics implications of aerogravity assist, rather than the aerodynamics of the flight through atmosphere.

The Tremendous Benefits of Lift

Lift is critical to both reusability and for sending humans into space. You can not expect space tourism to be a popular attraction if the g-forces the riders must tolerate exceeds 4g-s. Lift allows a spacecraft to slow down at lower accelerations, and at lower heating loads as well. For reentry, instead of a fast plop-down by a capsule, you can cruise a great distance downrange and, in doing so, keep the acceleration relatively very moderate, perhaps as low as 2 g-s.

Upside-Down Planes

Reentry is a different problem from capture. Except for a small fraction of time at the start of the reentry, almost all of reentry slowdown time is spent at speeds lower than orbital velocity.

On the other hand, if you are looking at aerocapture of a spacecraft that came from outside the planet’s orbit (probably a heliocentric transfer orbit), it flies at speeds greater than what orbital velocity at those altitudes would be. Because of that, the wings would need to generate downward lift like a car’s spoiler in order to keep it from flying away from the planet again. If you sustain balanced flight, then you can greatly extend the time spent slowing down — generating the same kinds of benefits that lift produces for reentry trajectories.

Volume Tradeoffs

Nonetheless, lift is not a panacea because of the tradeoffs that you must make in order to obtain it. Most notable out of these is the volume constraint. After all, it is basically the cargo bay that caused the mediocre lift-to-drag coefficient of the Shuttle.

Interplanetary space shuttles must traverse a substantial Delta V relative to the propellant velocities we have available (in the inner solar system, for instance). This would require an enormous volume, which almost always runs counter to the aerodynamic design objectives.

However, the lifting body helps to solve its own problem. By avoid the need for propellant for stopping, it reduces the total Delta V the engines must provide. In that way, we are trading some of the mission’s propellant for its bulky lifting surfaces.

The other half of the mission Delta V (the initial Hohmann burn of the mission) could be off-loaded onto another system. For example, an Earth-departure tug could be used to push the spacecraft into its transfer orbit, then separate, and return to LEO for rapid reuse. This isn’t my idea, Zubrin has proposed the same type of reusability for Mars missions.

Insufficient Mach Number Research at High Speeds

Capture from speeds higher than escape velocities involves flight through atmosphere at extraordinarily high speeds. Because of this, the “Newtonian model” might actually be the most accurate available model, where the gas is treated as a bunch of molecules that behave independently and ballistically like billiard balls.

A spacecraft coming into Earth for capture could exceed Mach 50. I have not found good categorization of the flow and lifting methods for these incredible speeds, but it is abundantly clear that it would involve a very slender waverider design. The subject of waveriders are a tremendously interesting topic to read about on their own. The first source is a fantastic read and has somewhat more detail available in a corresponding patent.

Here is another chapter on the subject.

You can notice that the highest mach number that is even illustrated in graphs is Mach 30. The first literature reference above, however, is optimistic about the asymptotic behavior at high speeds for optimized designs. They claim up to L/D ratios of 8, which seems overly-optimistic to me, but if mission design allows optimization (which I argue here that it can), then it might not be completely out of the question.

Ionization Problems in the Upper Atmosphere

Extreme speeds are comprehensible because the altitude can be adjusted by the mission design to compensate, flying in lower-density regions of the atmosphere. Since drag scales with the general relationship of ρ v², and since we are talking about radically high velocities, the spacecraft would need to fly in extraordinary low density regions of the atmosphere. This will impact the assumptions about fluid mechanics firstly (lending more toward the Newtonian approximation, again), and also cause a different type of molecular composition. At higher altitudes, Hydrogen becomes more prevalent, and at high enough altitude, almost all molecules are ionized. This might wreck havoc on the assumption that the scattering angle follows from geometry as you might expect, and also might cause degradation of the lifting surfaces, even if it has tolerable temperatures.

This is a complex problem, and it will be specific to the particular numbers that come from a particular planet-to-planet scenario.

An odd option is that the charges mean that magnetic deflection might actually be usable as a means to generate lift. Unlike similar ideas (for instance, station-keeping via accelerating charged particles over a voltage difference), this would work completely passively. As such, it could be much more effective than our intuition might predict. It’s still not clear if such a solution is necessary in the first place, but the effect of charged particles is a subject that needs to be taken seriously.

The Application’s Context

None of this would be relevant unless several other complementary infrastructure components also exist.

As argued above, a distinct departure first-stage might be necessary to make this idea viable. If you do that, the payoff could be enormous because the design only has the payload and its own aerodynamics to worry about, without additional bulky tanks and engines. Some engines would still be necessary, but not requiring a large mass-fraction.

The equipment could also be largely foldable for manned interplanetary missions to help people cope with the months of transit time. As a reusable vehicle, relatively sophisticated facilities might be possible, including shielded quarters for solar storm events and rotating artificial gravity. The rotation of gravity wheels would need to be stopped and folded into the wing before the reaching the destination planet.

Some timing benefits might also be obtainable, since the control of altitude or several elliptical flybys could be used to add small amounts of additional time while the spacecraft circles the planet. The end of its trajectory would result in a departure to an apogee at the altitude of an orbital space station that it has a rendezvous with. Subsequently, this station would need to receive a regular planetary shuttle to and from the surface, stock a propellant depot, and service the departure stage specific to that planet’s gravity well.

Obligatory analytical writing, online participation account for Medium. Engineering, software, books, space, constant daydreaming.

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