By Dr. Gary L. Deel, Ph.D., J.D.
Faculty Director, School of Business, American Military University
This is the first article in a four-part series on the concept of spaceplanes and modern iterations of spaceplane technology.
When does an aircraft become a spacecraft? This was once a subject of significant debate until a clear line of demarcation was established at an altitude of approximately 100 km (60 mi) above sea level; this came to be called the Kármán line, named after an early 20th-century Hungarian American aerospace engineer. This definition is accepted by the governing bodies responsible for regulation and recordkeeping in aeronautics.
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Once an aircraft crosses the Kármán line and becomes a spacecraft, the next question is whether the craft will follow a suborbital or orbital trajectory. Suborbital trajectories are those that will inevitably result in the spacecraft’s return to Earth; orbital trajectories are those that allow for sustained flight in orbit around the planet without ever returning to the surface.
Suborbital flights require far more energy than conventional aircraft flights because as an aircraft climbs higher in altitude, there is increasingly less lift force through flaps and wings due to the thinning of the atmosphere. Thus, airplanes that might be used as space planes must sport additional hardware like rocket engines that allow for vertical or near-vertical ascent through the altitudes where lift is negligible.
Suborbital Flights Require Significantly Less Energy than Orbital Flights
However, suborbital flights require significantly less energy than orbital flights. This is because the speed necessary for maintaining a given altitude is far greater than the speed necessary to simply reach that same altitude and then descend. For example, to reach the Kármán line in a suborbital trajectory requires a minimum speed of about 1,000 m/s. However, in order to reach and maintain altitude at the Kármán line, the speed required would be approximately eight times greater!
Consider by analogy the amount of energy a pole vaulter requires to successfully clear the high bar and land on the other side versus the amount of energy that clears the same bar and then travels a lateral distance before landing. How much faster would the pole vaulter have to be going? How much more force would he or she have to exert?
For this reason, traditional rocket-style craft are usually best suited for orbital flights. But for suborbital flights carrying passengers, a different category of craft — space planes — are better choices. Because of the lower acceleration rates and speeds, suborbital flights don’t experience nearly the G forces and aerodynamic pressures that orbital flights must endure, neither on launch nor on re-entry.
Space Planes Are Able to Take Off Horizontally Like a Conventional Aircraft
Accordingly, space planes are able to take off horizontally like a conventional aircraft and then accelerate into a parabolic ascent up to and over the Kármán line. Depending on mass, speed, acceleration, and other factors, these flights might allow for anywhere from a few seconds to a few minutes of sustained spaceflight before falling back into the atmosphere.
Space plane technology began with the X-15 hypersonic rocket-powered aircraft developed by the U.S. Air Force and flown in the 1950s and 1960s. And although the challenges of spaceflight are no less daunting since the days of the X-15, modern aerospace engineering nevertheless has come a long way. Today’s space plane engineers have learned many lessons about design infrastructure, material application, and adaptability.
In the next part of this article series, we will discuss two conceptual space planes that have been developed to take humans on suborbital flights, and one concept for a space plane capable of orbital flight. These concepts are the XCOR Aerospace Lynx, the Virgin Galactic SpaceshipTwo, and the Sierra Nevada Space Systems Dream Chaser. We’ll compare their unique characteristics and attributes, as well as their practicality for regular commercial use.
About the Author
Dr. Gary Deel is a Faculty Director with the School of Business at American Military University. He holds a J.D. in Law and a Ph.D. in Hospitality/Business Management. Gary teaches human resources and employment law classes for American Military University, the University of Central Florida, Colorado State University and others.