Astronauten Don Pettit melder litt om hvorfor Science Fiction Space Operaene i film og fjernsyn har noen utfordringer. Han tar for seg "rakettligningen" og hva den betyr for nyttelast. Kapselen med astronauter som returnerer til jorden, og det vil jo gjerne astronautene, utgjør 0,2% av rakettens vekt på oppskytingsrampen.
Fint lesestoff med mange bra ting å ha med seg i hodet neste gang verdensrommet er fylt med romskip i en film:
Another less recognized side effect of the rocket equation is the sensitivity of completing the rocket burn to obtaining your goal. To illustrate this, I will use some numbers from my Shuttle flight, STS 126 in November 2008. Our target velocity at main engine cut off was 7824 m/s (25819 ft/s). If our engines shut down at 7806 m/s (25760 ft/s), only 18 m/s (59 ft/s) shy of the target value, we would make an orbit but not our designated target orbit. We would not be able to rendezvous with space station and would lose our mission objective. Like being two pennies short of a ten dollar purchase, this is only 0.2% less than the price of admission into space. In this case, we do have some options. We could burn our orbital maneuvering propellant and make up this difference. If we were 3% shy of our target, 7596 (25067 ft/s) we would not have sufficient orbital maneuvering propellant and we would not make any orbit. We would be forced into a trans-Atlantic abort, falling back to Earth and landing in Spain. This final 3% of our required velocity comes during the last 8 seconds of our burn. For astronauts and bull riders, 8 seconds is a long time.
If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.
NASA - The Tyranny of the Rocket Equation