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To start things off I am writing this to gain information for vehicles in a story, so I am more so looking for theoretical possibilities, unless there are real world examples.

The background and my current understanding: Turboprops have great low end thrust and are more efficient for VTOL and low airspeed flight with the downside of low altitude flight. Jet engines, even going so far as Scramjets, are for faster and higher altitude flights.

The goal is to create a typical science fiction dropship that can land on unprepared surfaces and fly into space where another thruster onboard will propel the craft in space (unless there is a really diverse hybrid engine that could do that job).

So the key takeaways are the landing part and the high altitude. I know it is possible to take something like the V-22 Osprey and swap the Turboprops (Proprotors) for another higher thrust engine, but with the downside that you will burn and destroy any unprepared surface you land on.

The simple question: is it possible to have a Turboprop engine that essentially shifts to another form of jet engine for higher altitude and faster flights?

This is probably much more of a theoretical question than this particular site is used to so I am good with any helpful information.

Thank you

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  • $\begingroup$ Do you want hard sci-fi (everything is actually physically possible with the laws of physics as we know it)? Is VTOL a hard requirement or a nice to have? How much actually horizontal flying do you need to do? Is it just drop down from space to one site and then straight back up to space, or do you want to take off and fly around and land a few times? Also, if you don't get answers here, Worldbuilding.SE may be a good resource for story questions. $\endgroup$
    – Daniel K
    Sep 28, 2021 at 14:44
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    $\begingroup$ Since it's sci-fi, have the props of the V-22 fold forward (away from the engine), then retract into the center of the spool to get them out of the way for "other" propulsion methods. $\endgroup$
    – FreeMan
    Sep 28, 2021 at 14:53
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    $\begingroup$ One idea I had was to fold the propellers back, then I wondered if the internal mechanics of the Turboprop could be "switched" to become a jet engine midflight. $\endgroup$
    – Markitect
    Sep 28, 2021 at 14:57
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    $\begingroup$ @mins yes, I am assuming Earth atmosphere. As for the mother ship, at least at orbital speed. So up there between 8 to 10km/s+. $\endgroup$
    – Markitect
    Sep 28, 2021 at 16:42
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    $\begingroup$ Have you heard about the Rotary Rocket? It's a crazy hybrid of a helicopter and rocket that people made a fairly serious effort at building. Explanation and some footage of manned test flights: youtube.com/watch?v=OIuGfXp-Ok8 $\endgroup$ Sep 28, 2021 at 23:33

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TLDR: Pretty much impossible with current technology as I understand the question.

I think you are focusing on the wrong part of the problem. Turboprop vs turbofan is pocket change compared to the amount of rocket fuel required to get to orbit. Let's put some numbers to it.

I'm assuming the following mission 1) Deorbit from low earth orbit, 2) landing (potentially parachute assisted), 3) a 10,000 km range for either one very long flight or a number of short ones, utilizing vertical takeoff 4) return to orbit. Further assuming that all fuel for the entire mission is onboard at step 1.

We are going to start at the end, and work backwards.

First we need to know the mass of the vehicle. I'm assuming based on "typical science fiction dropship" that the actual vehicle weight might be similar to an F-35 fighter. Let's take the empty weight of 13,290 kg (also similar to the v-22 osprey that you mentioned). Some sci-fi movies depict a "drop ship" as having a crew of dozens, so this might actually be on the low end, but let's start here. How much rocket fuel do we need to get that kind of a vehicle back into orbit?

Assuming the delta-v required is around 11,200 m/s, assuming a sporty specific impulse of 470s (the absolute best rocket listed in this table), and utilizing the rocket equation gives the fuel required as 150,000 kg. For comparison, that's about the empty operating weight of a Boeing 777. Now, that's assuming that you start from the ground and use the rocket the whole way. Of course, if you use the jet engines to get started and launch from in-flight, you'd need a little bit less fuel, but it's not as much as you might think. Elon Musk thinks its only about a 5% savings. Let's just keep the 150,000 kg figure.

Now, we need to fly something with the weight of a B777 around a bit. For B777, a fuel load of 100,000 kg is enough to get around 10,000 km range,

So now our vehicle plus fuel is 250,000 kg. To do a vertical takeoff, you'd need a whopping 2.5 MN thrust, which in jet engine terms is around five GE9xs at max takeoff thrust (which by the way are about 10,000 kg each, so now our vehicle is way over our initially assumed weight, which means we need even more fuel!)

That's already a tremendous vehicle. And we haven't even talked about how to get this thing to survive re-entry without burning up. You'd need a massive heat shield, which would just add even more weight. I would call this impossible with current technology.

So to modify it to make it work, you'd need to do one or more of the following

  • Massively cut back on the final mass. i.e. make the final thing look more like an tiny Apollo command capsule than a "typical sci-fi drop ship".
  • Re-fuel on the ground before each flight and before returning to orbit.
  • Give up on VTOL
  • cut down on the flight range
  • give up on hard sci-fi, and invent a magic rocket engine with a tremendously better specific impulse than current technology.
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  • $\begingroup$ Awesome answer, thank you for that. I figured it would be virtually impossible. And for the sake of the story I have contemplated something like a Sky Hook from the orbiting ship. That seems a little slow for the future setting. To give some more perspective since I think you settled the impossibility: the setting is about 600 years in the future where interplanetary travel is a few days only with torch drives similar to the Expanses Epstein drive. The idea I am toying around with is to get the craft to a high enough altitude to turn on the fusion thrusters and get into space that way. $\endgroup$
    – Markitect
    Sep 29, 2021 at 0:54
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    $\begingroup$ Well if you're going to go the route of "fusion thrusters" and "epstein drives", I don't think I'd worry too much about whether a turbofan is a bit less efficient than a turboprop at low speeds. That's like saying you've got a Ferrari that can go 150 mph, but only if you tow it out to the highway with some draft horses first, and you are wondering what kind of horse would be the most efficient for towing. Just say that you have something like a jet engine but it runs off energy from your "fusion reactor" instead of jet fuel. $\endgroup$
    – Daniel K
    Sep 29, 2021 at 11:26
  • $\begingroup$ or to put it a different way, the reason we care about the efficiency difference between a turboprop and a turbojet is because jet fuel is expensive and heavy. The specific energy of jet fuel is about 40 MJ/kg. The specific energy of hydrogen fusion is 640,000,000 MJ/kg, so about 16 million times more energy per kg than jet fuel. You could fly a B777 from New York to Tokyo on about 6 grams of hydrogen if you have fusion available. So once you have fusion, turboprop vs turbojet is asking about whether you need 6 grams of hydrogen or 8 grams. Not a big deal. $\endgroup$
    – Daniel K
    Sep 29, 2021 at 14:39
  • $\begingroup$ That's fair. I guess my question was more of a mechanical aspect on whether an engine that runs a propeller could also run a jet engine and compress the air and use that as the propellant in atmospheric flight. The next phase I need to figure out is at what elevation a fusion engine could be used without disastrous affects. $\endgroup$
    – Markitect
    Sep 29, 2021 at 20:06
  • $\begingroup$ I guess I'm not sure what you mean by fusion engine. If you have a fusion reactor, use it to generate electricity, and then use that electricity to turn an electric motor connected to a big fan or propeller. A fusion powered quadcopter would work nicely at sealevel up to where the air gets pretty thin. $\endgroup$
    – Daniel K
    Sep 29, 2021 at 20:33
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Any VTOL arrangement will put a lot of blast on the landing surface, so a practical, off the shelf solution would be the Pratt & Whitney J58 hybrid turbojet/ram jet to get you off the ground and into near space at around Mach 3, before switching to rockets.

Although it is possible to mount a prop on this type of engine and fold it, drag and turbulence (on the intake air) may render it impractical. Better to blow a few things over with the turbojet (great movie special effects). The turboprop will exhaust hot gasses as well, at a slightly lower velocity.

Another approach may be a VTOL version of the British Skylon ground to space aircraft. Perhaps in your book it actually makes it into production.

But for vertical landings, it may work best to supplement the turboramjets with rockets. Then you wind up with something more akin to what Elon Musk is working on with jets and a little more wing.

Although these "hybrids" are a popular sci-fi concept, in reality only a small portion of even orbital flight is beyond the atmosphere. Once past the point of lifting or oxygen consumption for thrust, jets, wings, and propellers become so much dead weight.

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The F-35 Lightning has an approach to VTOL that is similar to what you are requesting. There is a propeller (strictly speaking, a ducted fan) pointed straight up in the middle of the fuselage and has doors that open above and below it. It is powered by the jet engine and used during vertical takeoff and landing.

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