I am a YA science fiction writer - not a scientist. This will be apparent by my question. Could...
a rocket be equipped with two scramjets? One to take an unmanned rocket out of Earth orbit and the second to propel the rocket to, say, Mars, and so cut down on the seven months' travel time? How many scramjets would be needed to lift a payload of, say, one ton, beyond Earth orbit?
Thank you for the question, but unfortunately Scramjets are not known for their high propulsion in terms of final acceleration into orbit. There is still generally a requirement for a rocket stage to break this barrier beyond the earth's atmosphere into orbit. Scramjets are air-breathing engines still, so the benefits of using them in space are still not a viable alternative to the current space-based propulsion systems. The final breakthrough into orbit and the associated acceleration once the vehicle breaks free are generally the driving forces behind ongoing propulsion through space.
As a stage for launch into orbit it still is a viable option to replace current rocket-based propulsion, or at least most of the heavy lifting required to get the final stage into the upper-atmosphere. And the main theoretical benefit is as a reusable stage, with a reduced weight to allow larger payloads.
So as a direct summary to your question, to lift a payload of one tonne, you would need a rocket-based stage to first lift and break the minimum efficient operating speed of a Scramjet and help increase acceleration over time. Then the jet would need to have around 29810 Newtons of thrust for 560 seconds to allow one tonne to break into orbit. Currently this is only just about theoretically possible depending on which side of the fence you fall.
One Scramjet engine would be available as a single stage, but not available in space or as a single stage take off vehicle.
Thank you very much, Simon (may I call you Simon?) for your easily understandable explanation. It looks as if it's back to the drawing board for sending my rocket to Mars.
Forget about explaining anything technical. It will be obsolete in a year. Instead explain what happens to the crew in the seven month you thinks it gets to Mars. Your first answer will be wrong.
Look up my Hubs, I am also writing about Mars.
Will your readers want to read about the boring seven months trip?
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