Can you have a Hybrid Turboprop-Jet engine?

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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.