By Dr. Gary L. Deel, Ph.D., J.D.
Faculty Member, Wallace E. Boston School of Business
Close your eyes and for a moment and imagine the kind of vehicle you would climb into if you wanted to go to space. What does it look like in your mind?
I’d bet you’re probably imagining a rocket of some kind – a tall, slender cylinder or maybe a few cylinders strapped together – standing erect on a launch pad. There would be big engines at the bottom and a crew compartment way up top. Am I close?
If so, this intuition is probably because at this point in our development of space logistics, we can safely say we have a “typical” space launch design, the traditional chemical rocket. However, the modern era is bringing with it new innovations, and among them are very creatively engineered spaceplanes.
One such spaceplane is Virgin Galactic’s SpaceShipTwo. SpaceshipTwo is a rocket-powered plane that is built for suborbital parabolic flights up to altitudes around the Kármán Line, the traditionally recognized border between earth’s atmosphere and space.
Technically, SpaceShipTwo only manages half of the total launch effort. The other half comes from a carrier plane called WhiteKnightTwo, to which SpaceShipTwo is attached.
On the ground, SpaceShipTwo is suspended beneath the twin tethered fuselages of WhiteKnightTwo. WhiteKnightTwo then takes off on a normal airplane runway with SpaceShipTwo in tow.
When WhiteKnightTwo reaches approximately 46,000 feet in altitude, it releases SpaceShipTwo. Upon its release, SpaceShipTwo then immediately ignites its rocket engines and continues its powered ascent into space. So these two planes actually work together for a choreographed space launch.
But the most interesting thing about SpaceShipTwo is not the partnership with WhiteKnightTwo for launch. Rather, it’s the interesting way in which the spacecraft utilizes a “feathering” fuselage design to assist with stability and orientation on re-entry.
I wrote at length about SpaceShipTwo’s feathering mechanism in a previous article. To summarize, SpaceShipTwo essentially has a hinged rear wing assembly. This assembly has the ability to fold itself on re-entry to maintain a consistent orientation and create the desired amount of drag in freefall to keep the plane headed on its proper bearings and at the proper velocities.
Now, in that same article, I also offered a view that SpaceShipTwo’s feathering design would not be at all useful for orbital spacecraft because they travel much faster and their collision with atmospheric resistance is much more violent. But in retrospect, I think this idea deserves more thought.
Related link: Spin Launchers for Space Missions Are Now Coming Around
The Argument for a Feathering Heat Shield Design on Spacecraft
To be clear, I still think that the intense forces involved with atmospheric reentry from orbital speeds would make a feathering spacecraft unworkable or at least impractical. However, I do think there might be an argument for a feathering heat shield design that allows for dynamic adjustment of drag and resistance as a spacecraft descends back to Earth for landing.
Imagine, if you will, a heat shield of the kind commonly used by space capsules upon re-entry today. They typically have a convex shape like the outer surface of a contact lens. The purpose of such a shield is to absorb and disperse the kinetic and heat friction energy as a spacecraft reenters Earth’s atmosphere and to protect the spacecraft while it makes its way to Earth.
These heat shields are typically static – in other words, they don’t pivot or move independently of the spacecraft to which they are attached. In essence, they’re just a fixed armor plate that takes the brunt of the atmospheric impact. But what if they could move?
Imagine a heat shield that could angle itself slightly in different directions. By analogy, imagine that you stick your hand out of a car window while you’re driving down the road; this dynamic angling effect is similar to the way the wind affects your hand outside the car window.
If you hold your hand perpendicular to the direction of the wind – with your flat palm facing forward – you will feel a significant amount of force from the drag of the air trying to push your hand backward. However, the moment you turn your hand 90 degrees to be parallel with the direction of travel, suddenly the resistance is dramatically reduced.
How the Angling Ability of a Heat Shield Would Be Useful for Spacecraft
For spacecraft, this heat shield angling ability could provide several beneficial functions. First, it could help to reduce drag on reentry if a spacecraft were to come in at a less-than-desirable angle and experience dangerous resistance forces. This kind of adjustment for gradual re-entry could have potentially averted a disaster such as the one that befell Space Shuttle Columbia’s final mission.
The angling ability of a heat shield could also be used to alter spacecraft direction to align the re-entry with a desired trajectory for eventual landing. Referring back to our “hand out the car window” analogy, you’ll probably recall from the last time you did this that as you shift the orientation of your hand in the wind, the drag forces had the effect of pushing your hand all over the place. Your hand moved up, down, and from side to side as the drag pushed against the different sides of your hand that are exposed due to the different angles of incidence.
This same phenomenon would happen to a spacecraft if its heat shield were not more or less perpendicular with the body of the craft behind it. Any change in that angle would create a steering effect – and this effect could be useful to the extent it could be safely controlled.
So the feathering design may still not be right for orbital spacecraft in the same way that it has been deployed for Virgin Galactic’s SpaceShipTwo spaceplane. But the concept might yet have use in some interesting heat shield applications for the atmospheric re-entry of much faster-traveling spacecraft.