Spaceship Designs

The simple but effective spaceships of Buck Rogers and Flash Gordon have been superseded by flashy new designs with interesting shapes, sizes, and abilities. The advent of the space age in the 1950s brought about a heightened awareness of the practical physics characterizing a successful rocket or spaceship. Yet today, more than 50 years later, the ingenuity of the movie industry continues to defy the laws of physics. We see the latest nuclear-powered spaceships operating in space coming in for a landing on Earth (or other comparable planet) at a spaceport and then taking off for space a little while later in the same ship from the same spaceport. Why can’t we do this feat with present-day space vehicles?

Answer

Landing a spaceship on Earth and then taking off for space involve the same forces, but the gravitational force always acts toward the center of Earth, sometimes being a help and sometimes being a hindrance. The major problem is the enormous energy requirement in getting from the surface of Earth to a reasonable distance away. Once the spaceship is more than a few Earth diameters away, its nuclear engine operation can be reasonably efficient in accelerating the vehicle. However, to get off the surface requires a tremendous amount of energy, and its rocket engines must throw out a lot of momentum in the exhaust gases at high speeds to achieve “escape velocity.” Newton’s third law dictates this momentum requirement. The particles ejected backward act on the rocket in a force pair, the rocket pushing particles backward while the particles are pushing the rocket forward.

To reach outer space from Earth, the vehicle must provide a large supply of energy and be able to eject a large amount of momentum, usually by having a large supply of mass to eject. The energy needs can be accommodated by a variety of engineering designs. However, the physics is quite demanding on the amount of mass ejected per second. The fuel mass used for this propulsion is not consumed instantly, so this fuel mass adds to the mass of the vehicle at launch time. Consequently, even more propulsion fuel mass and energy are required for a launch than simply accounting for the payload itself. The mass of the fuel supply soon becomes many times larger than the actual payload launched into space.

So when the spaceship leaves its Earth spaceport and doesn’t eject a lot of stuff backward out of its rocket engines, the film is expressing a mode of operation that is not achievable with present technology. But perhaps the propulsion will be different in the future, say the disbelievers. So let’s now go to the extreme propulsion limit. The most efficient process would be particle-antiparticle annihilation, conversion of fuel and anti-fuel completely to energy in the form of high energy photons according to Einstein’s famous E₀ = mc². If we ignore many problems such as a source for antiparticles, radiation exposure, and so on, and also assume that all the photons are eventually directed rearward, each kilogram of fuel could provide 3 × 10¹⁶ joules of energy and 3 × 10⁸ kg m/ sec of linear momentum. To accelerate upward at about 10 m/s², a kilogram of this fuel can provide a million- kilogram spaceship with 30 seconds of thrust. If one requires 3,000 seconds of thrust, simply use 100 kg of matter-antimatter fuel. We look forward to the future of space travel with antimatter engines, but for the present we can enjoy the entertainment provided by space travelers in films.