Are rocket engines practical for commercial aircraft?

Question

What are the prospects of seeing rocket engines become the standard for commercial aircraft? It might be a dumb question; I really don't know all that much about the topic. The reason I ask is that rocket powered aircraft are much faster—and who doesn't like speed?

My concerns are:

1. I don't know whether there any designs that would be economical. I understand rockets burn a tremendous amount of fuel and that is obviously a limiting factor.
2. There could be safety concerns. Just because we can do something doesn't necessarily make it a good idea. We have seen the struggles Musk has had with his rockets.

We keep hearing about all these super trains that can get us from Chicago to NY in an hour, even though the whole idea of traveling on a track at those speeds seems quite dangerous. However, up in the air there is nothing to hit, which is a different story.

Answer 1

At a fundamental level, it should be clear that throwing stuff out the back is going to be less economical than sucking in stuff in the front and throwing it out the back faster. For one thing, the former has to carry around a lot of stuff at takeoff.

The only possible way to save you from the fundamental issue above is to throw the stuff out the back so fast that you don't need much of it. In fact, rocket engines strive to do this. If you were able to throw stuff out the back at near the speed of light, then a rocket engine would look pretty good. With our current technology, especially what we can reasonably carry on a plane, we can't get the speed anywhere near fast enough.

Another fundamental issue favoring jets over rockets is that jets only have to carry half their chemical reactants. The other half is available in the atmosphere they fly thru.

For flying around earth's atmosphere, rockets are unlikely to be competitive with jets and propellers any time soon.

That said, rockets are sometimes used in special cases where the extra cost is worth it. This is done routinely by the military to allow large aircraft to take off from small runways, for example. In this case, the rockets are only to assist takeoff, and only last seconds. That's enough to get the plane going fast enough so that the wings make more lift than the plane's weight. The regular plane engines are also used at full power. The rockets add just enough thrust to allow the necessary short takeoff distance.

Answer 2

Economically a rocket engine will always lose out to a jet engine.

We'll ignore solid fuel rockets, they are impractical for commercial air travel due to their fixed thrust and inability to turn on and off.

A both liquid rockets and and jets need fuel but a rocket engine also needs oxygen, the jet engine pulls this from the air. So for the same amount of fuel burnt your cost of fueling a rocket is far higher, you need to supply two fuel tanks instead of one. You also need to lift twice as much mass into the air with you which means you need to use more fuel for the same trip since your aircraft is now heavier at takeoff.

People are working on the idea of a rocket engine that can use oxygen from the air until it gets above the atmosphere (e.g. http://www.reactionengines.co.uk/sabre.html) in order to minimize the amount of oxygen you need to carry. But that is intended more for space planes than conventional aircraft. If you were to stay in the atmosphere it would make the fuel costs a lot closer but it adds a lot of extra complexity which will have an impact on operating costs.

The main reason you are asking is for faster travel times. You can go a lot faster with a jet engine than any airliner in operation today. Concorde could give most fighters from it's time a run for their money when it came to cruising speed. There is no reason why you couldn't build a jet airliner that went even faster.

In the end it doesn't matter how you power your super fast airliner, jet, rocket, big elastic bands, you always hit the same problem. Pushing something through the air at those speeds creates a LOT of drag. You end up burning a lot more fuel to got the same distance if you go fast which means the operating costs are a lot higher. While some people are willing to pay the extra price it's not a big enough market to make it pay.

Answer 3

To answer first let's look at the different types of "rocket" engines.

1 - Solid Fuel: The issue here is that one a solid fuel rocket is ignited it can't be shut off or throttled very effectively. Imagine the pilot ignites the engine and the aircraft WILL (and I mean WILL) start moving, continue moving and get into the air regardless if he wants to stop.

2 - Liquid Fuel: In general, for best efficiency you'd want cryogenic fuels like Liquid Hydrogen but Kerosene also works which add a huge issue in terms of weight (for the tanks) as well as volatility of the fuels and the cost.

3 - Hybrid solid/liquid: Again, solid fuel is difficult/impossible to stop the chemical reaction and difficult to throttle.

Answer 4

A rocket engine is impractical for 2 reasons

As other answers have stated, you must carry the oxidizer on board.

More importantly though, a rocket accelerates much faster, burns much more fuel, generates much more thrust because the target velocity is orbital velocity, 9.4 km/sec or mach 27.5 for LEO, or escape velocity 11.2 km/sec or mach 33. A supersonic airliner would have great difficulty going faster than mach 4 through the atmosphere for a number of reasons, primarily friction or drag and the thinness of the air for power requirements of the jet.

At mach 4 you can use a scram-jet instead of conventional jet. The scram-jet does not need the compressor turbine but rather the speed of the airflow at mach4 and higher would be sufficient enough for the compression, thus less moving parts. This would allow the supersonic aircraft to fly high in the stratosphere and avoid the weather. A hybrid system that may work well is rocket propulsion for a quick 2 minute or less boost to reach the stratosphere at mach4 and then the scram-jets taking over. You could theoretically go up to mach 15 with a scram-jet, which while not orbital velocity, would certainly please any traveler.