Descent and Ascent

By Eric M. Jones

ISAAC NEWTON WAS A “momentum and velocity” kind of guy, and although he presumably believed in energy, he didn’t find the concept particularly useful.

But since the mid-19th century, physics has been all about energy. This “energy” approach to physics was the greatest single change in physics since “The Principia,” particularly the notion that all types of energy, and even mass itself, were convertible, one form into the other.

To understand why this approach is so much more revealing, consider the two main forms of energy:

• Potential energy is stored energy, such as mechanical tension, and compression, but also chemical, nuclear, gravitational, magnetic, voltage, chemical, etc.

• Kinetic energy is the energy of motion, but also thermal, sound, vibration, current, radiant energy, etc.

All of industry and engineering today comes from understanding and using the conversion of this potential energy to kinetic energy, and back again.

So this small physics lesson is presented so it will be easier to understand how a spaceship gets into orbit, and back down again.

Launching Into Orbit

A big space-bound rocket on a launching pad is a fearsome beast. Even before launch it is usually breathing out vapors and creaking and groaning. It is 85 to 98 percent fuel. Chained down and serviced to by a hundred captains, it is the creation of the best aerospace engineering Earth has mastered.

But the rocket on the launching pad already has substantial kinetic energy, as a consequence of being bolted to a rotating planet. This is useful. At the equator, the best place to launch most rockets, the Earth’s surface spins 40,000 kilometers in 24 hours, so the surface speed is about 1,670 kilometers/hour. This speed decreases by the cosine of the latitude so that at Cape Canaveral’s latitude of 28.5°, cosine (28.5°)=0.879, the speed is 0.879×1670 k/hr=+1468 kilometers/hr. As a convention, the plus sign says we’re launching eastward. This speed-kick launching is why almost everything put into orbit travels to the east. Traveling west subtracts velocity.

The rocket has an enormous store of potential energy in the form of chemical fuel. The kinetic energy that can be obtained from various propellants was figured out in the 1920s. Using liquid hydrogen (LH2) and oxygen (LOX) is technically quite difficult but has been the preferred liquid fuel for big orbital missions.

Many rockets are solid fueled, such as the boosters on the Space Shuttle. The advantage of solid fueled rockets is simplicity. But once they are ignited, turning them off is virtually impossible. They are always burned until they’re exhausted and then jettisoned.

The advantage of liquid fueled rockets is controllability. They can be restarted, throttled, or used in short bursts.

Of course there are hybrids too. The Rutan Spaceship One and Two use rubber which burns furiously in an oxidizer of Nitrous Oxide. What these lack in propulsive capability is more than made up for in safety and reliability. Both of these systems have the simplicity of solid fuels with the advantage of controllability.

It is possible to use non-chemical rockets, such as nuclear propulsion, but this is a subject for another time.

When a rocket launches, usually but not always vertically, it quickly becomes a hypersonic vehicle ... yet it hardly looks like a sleek spaceship from the early 20th century. The German V2 used steerable carbon vanes in the throat of its engines, but fins were dispensed with when rocket motors were put on gimbals. Computers, sensors, and gyros balanced the vehicle. Pointy noses, fins, and sleek bodies were just cost-adders. A straight cylinder with a streamlined nose cap makes a fine rocket. The Space Shuttle won’t be the last orbital craft with wings, but wings make sense for short orbital and return missions. They are not worth carrying around the solar system.

The shape of the rockets’ pointy end is important, but as the rocket increases its velocity and altitude, the best shape keeps changing. So an average shape is chosen. The specific angle of the engines’ bell cone is also critical to the efficiency of the engine. Engines for low pressure and vacuum are very wide-mouthed. Lift-off engines have narrower bells. Again, average configurations are often chosen.

Expedition 30/31 Flight Engineer Don Pettit wrote “The Tyranny of the Rocket Equation,” an excellent article on the physics of getting into orbit. In it, he makes some eye-popping points:

• Rockets are usually 85 to 95 percent fuel. An aluminum soda can (an astonishing piece of modern engineering, by the way) has a lower ratio of structural-mass to useful-contents than the Space Shuttle’s external liquid fuel tank, which is cryogenic, withstands 3.3Gs and goes almost into orbit.

• Partly due to the fact of its reusability, the Space Shuttle only delivered one percent of its mass as payload to orbit.

• The tyranny of the rocket equation numbers dictate that if the Earth were only a little bigger, escaping from its surface into space by rocket would be physically impossible.

This last point is remarkable; chemical rockets are doomed. They barely allow humans to get into space at all. To go much further, humans need nuclear propulsion or something we have not yet invented.

The Karman Line

Simply stated, as an aircraft goes higher, its forward speed must increase to develop enough lift to keep it flying. The higher the airplane goes the faster it needs to go to stay flying. Long wings can somewhat compensate for this, but present their own problems. At about 100 km, called the Karman Line, an aircraft reaches Earth’s escape velocity. (The line is named after Theodore von Karman, a renowned Hungarian-American aerodynamicist from the 1920s to the 1960s. He was the first to calculate that around 100 km, the atmosphere becomes too thin to support aeronautical flight.) Brilliant! That’s why the limit of the atmosphere is officially considered to be 100 km. This is not necessarily the “edge of space” or the edge of anything. There is no abrupt change in the atmosphere. There is still some “air” practically as high as you want to measure, but to “fly” there, you would be at escape velocity, so it would be impossible.

Descending From Orbit

Temperature is defined as the measure of the average kinetic energy of particles, so a thermometer can be used as a kind of speedometer. The spacecraft in orbit has an enormous amount of energy. In fact, the spacecraft can be viewed as the agglomeration of atoms each travelling at orbital velocity. Lower orbit contains gas molecules which are suddenly knocked forward by elastic collision with the vehicle. Molecules that move fast due to the collision are hot molecules and if they get hot enough the molecules come apart and turn to turn into ionized plasma which is incredibly corrosive, extremely hot, and highly electrostatic; but due to the hypersonic flow, they never quite touch the spacecraft. Remember Daniel Bernoulli? The faster the gas flow goes, the lower the pressure.

[Right, huge fins like the ones on this 1930
“Popular Mechanics” cover were once thought to be essential for rockets to launch or land.]

This energy of motion has to be dissipated to get it back to Earth. When it is time for a spacecraft to land, it has to change orbit from the relatively empty and drag-free vacuum in which it is traveling to a lower orbit which has some residual atmosphere. Perhaps the spaceship could just fire its retro rockets to reduce its orbital velocity to essentially zero (or to the speed of the Earth’s rotation) and park over its intended landing spot, then fall like a stone. But this would require an enormous expenditure of (unavailable) energy. But that is why the SpaceshipTwo (SS2), manufactured by Scaled Composites for Virgin Galactic, requires very little heat shielding or fancy technology to get into “space” or get back to the ground. It never gets close to orbital velocity even if it is in “space.” It goes up and comes down where it started. SS2 can hardly be compared to orbital vehicles.

It was not at all obvious to early designers what shape a spacecraft needed to be to return to Earth from orbit, but it soon became apparent that the shape of the craft that got into orbit would simply not do to get it back down. Many assume that the heating in re­entry is from friction, but more complicated physics is at work here.

The hard work on this problem had already been done in 1951 in relationship to ICBM warheads by H. Julian Allen and A.J. Eggers Jr. of NACA (predecessor to NASA). They determined mathematically that a blunt shape generally made the most effective reentry shape. (The Space Shuttle uses its big flat belly.) More importantly perhaps, they showed that the heating of an entry vehicle was inversely proportional to its drag coefficient—not an obvious result at all as frequently meteorites were found with dished blunt front ends that showed obvious signs of having been melted. Then again, the ones that were found had made it through the atmosphere. Making a reentry vehicle with a blunt front end produced an air cushion that would push the hypersonic shock wave and its incandescent shock layer forward. The hot plasma would simply—or not so simply—spill around the vehicle, taking energy with it.

Most heatshields are “ablators,” meaning that they continuously shed some of their substance during reentry. Heatshield ablator materials must turn to gas or tiny particles when heated. A popular heatshield ablator used in the Mars Lander and many earlier missions was composed of a mixture of cork, silica spheres, and wood glue. A mixture one might have cobbled together in any basement workshop, and still a reliable workhorse for non-reusable vehicles. The SpaceX Dragon Capsule uses a Phenolic Impregnated Carbon Ablator (PICA) designed to enable complete reuse of the vehicle.

Finally, the spacecraft either lands with wings, like the Space Shuttle and the secret X-35B space plane, or deploys parachutes which are light, reasonably reliable, but not very steerable. There are variation on all these. The SpaceX reusable Falcon 9 rocket has recently made a historic landing upright on its tail, just like the fantasy spacecraft of the 1930s.

Spacecraft reentry heatshield development is still ongoing. There are other methods of reentry being studied and even prototyped. Some think wings will return to reusable spacecraft, some think ram-air parachutes will be a preferred method of touchdown.

Future spacecraft will surprise us all. 

Eric M. Jones is the Associate Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur, working in the experimental aircraft community, NASA, space transportation companies, and the International Space Station.