Chris Lakomy, of Waikanae, asks :-
Why do rockets need to go so fast to get into outer space? Jumbo jets lift off with an almost agonising slowness, so why can’t rockets do the same thing and just keep going up at a similar gentle pace until they reach orbit?
Andy Edgar, a physicist at Victoria University of Wellington, responded.
In rocketry, the payload (a satellite or space capsule) has to reach a certain minimum speed to enter earth orbit (around 25,000 km/hr) or to escape completely from the earth’s gravitational pull, (around 40,000 km/hr), as for a deep space mission.
This is much faster than the cruising speed of an airliner, typically 900 km/hr.
To achieve the very high speed for the payload requires a very large rocket full of fuel and oxidiser which react chemically and provide the necessary forward thrust via the rapid ejection of the exhaust gases. For a fixed payload, the smallest and therefore cheapest rocket which can reach the necessary speed is achieved if it is designed so that the fuel is burnt at the maximum possible rate, resulting in a rapid acceleration, since the mass of the rocket rapidly decreases as fuel is burnt.
To prove this statement requires some analysis, but one can see from a simple case that it is not unreasonable - if the thrust were deliberately reduced, at some stage the rocket would just hover, and use up all the fuel without actually getting anywhere. A technical point is that the thrust from a solid fuel rocket cannot be controlled anyway as it is just a chemical reaction between solid components, and that from a liquid fuel rocket is difficult to adjust – it is set by pumps which are designed to run at a fixed and high speed.
When a rocket such as the Saturn V lifts off, the initial acceleration is quite small, and similar to that of an aircraft on take off, but it rapidly increases as the rocket uses fuel so that after 1 minute of burn time the rocket is travelling at around 3,000 km/hr.
In contrast, an aircraft maintains the same, constant, and low rate of acceleration down the runway until after about one minute it achieves enough aerodynamic lift to take off at a speed of only around 300 km/hr; shortly after that the pilots throttle back and gradually take the plane up to its final cruising altitude and speed.