Once man is finished with exploring the neighborhood, then it's probably the time he would be thinking to go places wherefrom - he today suspects - the cigar shaped visitor "Oumuamua" just rendezvoused our world. I am sure rockets propelled by chemical fuels won't be of much use compared with more advanced options, say ion propulsion, that are yet experimental at best.
The method of propulsion always depends on the types of missions and thus the types of constraints. For most practical purposes, power density of your propulsion system, as is thrust specific fuel consumption, control what you must provide to your customer. Chemical Propulsion is not dead. A rifle bullet goes 1,700 miles per hour. The Parker Solar Probe will go 430,000 miles an hour, or about 125 miles a second. Fired from a Delta IV Heavy chemical rocket, the probe achieves it's record-breaking speed via clever trajectory planning and gravity assists.
For high thrust SFC (or 'thrust per weight flowrate of fuel') you have a few superior options. Instead of liberating the energy of chemical bonds, you could reach in to the superior nuclear binding energy. Nuclear thermal rockets or nuclear electric rockets (with a generator as an extra step) offer far better energy density. But nuclear propulsion is environmentally very dangerous, and the research has already claimed lives.
Without onboard reaction mass, you need a different option. Either push against the radiation pressure of your nearest star (or LASER) using a solar sail, or capture particles and accelerate them out the back. Nature just released footage of the ion aircraft at MIT, and earlier this year, ESA (European Space Agency) proved in a lab that they could build a rarified air-breathing electric thruster which could operate in low-Earth orbit. It is possible to build a satellite, powered by solar, that would never run out of fuel.
Electromagnetic propulsion is a great way to go, and is not in its infancy. In some respects it is not a more advanced option and is not in an experimental phase. Because unlike nuclear rockets for example, it's already mature technology. Almost all of our satellites and probes use some method of using electrical energy to accelerate charged particles; the method which has allowed us to build networks of communication satellites around Earth, and allowed us to explore planets, dwarfs, moons, asteroids and comets. Ion propulsion is not just a future concept. It was, it is, and it will be a viable option for interplanetary exploration. BepiColumbo was launched October this year, on a seven-year interplanetary cruise to Mercury using solar-electric propulsion (ion thrusters) and gravity assists from Earth, Venus and eventual gravity capture at Mercury.
Thanks, J. Smith, for updating me, among others, on ion propulsion. Probably, bit by bit, mankind will one day surmount all the obstacles to achieving velocities that will help confront in time, at least, whatever heavenly ever approach our home dangerously.
https://futurism.com/the-space-travel-of-the-future-ion-propulsion-2
Recent progress is in research of supersonic combustion that supports the feasibility of very high speed flight vehicles by supersonic combustors.
Hello S. Dey
Yes, SCRAM jet has promise for aircraft applications. However, as for rocket missions to expanses of space, there is still huge scope for innovative solutions. Increasing awareness about how insecure we are on this, as yet, the lone world is one reason why mankind is getting too restive over our inability to camp away in case some apocalypse lurks out of the blue.
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