The Tyranny (and Power) of Rocket Travel

Being in “outer space” has the connotation of being far from Earth. But here on Space Station, we are only 400 kilometers (240 miles) above the planet—not that far away considering the distances we deal with on a daily basis. Space Station is only about as far from Earth as Houston is from Dallas.

So why is being in low Earth orbit such a big deal? It’s not the distance, but the energy.

Rockets are momentum machines. They spew hot gasses out their nozzles at high velocity, and the rocket moves in the opposite direction. If the thrust is pointed correctly, and the rocket burns long enough, the rocket will go into orbit.

To achieve the required momentum takes energy, and this energy comes from rocket fuel. A rocket sitting on the launch pad is mostly propellant; only 15 percent of the mass is the actual rocket and payload. Turning that 15 percent into something that can withstand the dynamics of spaceflight, carry people and payload, and perform a meaningful mission—and bring everyone safely home—is right on the edge of our engineering ability. By comparison, airplanes, at about 70 percent fuel and 30 percent structure, are easy to design and build.

Once the type of rocket fuel has been chosen (and we only have a few choices), the required rocket propellant fraction to achieve orbit is dictated by the magnitude of Earth’s gravity. And there is very little we can do about Earth’s gravity. As long as rockets are the mode of transportation for traveling into space, we are stuck with a vehicle that is 85+ percent propellant.

Reaching orbit also requires a precision rarely seen in any form of travel. If the rocket engines miss their target by as little as 0.2 percent, achieving your desired orbit will not be possible and the Earth will repossess your spacecraft in a not-so-gentle way. This is like being two pennies short of a 10-dollar purchase. For the space shuttle, the difference between a trans-Atlantic abort to Spain and reaching orbit happened in the last eight seconds of powered flight. For both bull riders and astronauts, eight seconds is a long time.

The momentum balance that governs rocket dynamics is aptly called “the rocket equation,” and it holds a tyrannical grip on anyone who desires to leave this planet. During our ascent to orbit, our rocket transfers momentum to the vehicle and the crew. My body has stored about 32 mega-joules (7.6 million calories) per kilogram of energy that originated from the rocket fuel in our Soyuz booster launched last December. This is five times more than the energy stored in an equivalent mass of nitroglycerin, and seven times more than the energy stored in TNT. Thus, the energy now stored in my body is seven times greater than what would be in an 80- kilogram pile of TNT.

For my return to Earth, this energy must be removed slowly, in a controlled manner, or we risk explosive results with an equivalent bang. No wonder spacecraft accidents are so spectacular. Any perturbation to the carefully designed and engineered atmospheric entry, and you will be torn limb-from-limb, with the torched remains sprinkled as cosmic croutons on the garden salad of Earth.

This is why being in space is being “out there.” The remoteness of the space frontier comes not from the distance of separation, but from the enormous quantities of energy expended and dissipated. Thus, the tyranny of the rocket equation creates a separation that makes low Earth orbit a frontier with no parallels.

For more of my thoughts on this subject, see my longer essay on the NASA website.