Knockoff Lunar Landing Training vehicle for ~500k (a jet engine powered chair!)

In My Humble Opinion
1

So I’m just spitballing here, after seeing this video. Wow, that is not very safe. So I was thinking about what you’d need to make a remotely safe VTOL jet powered thingy. You’d want the engines mounted to a frame, away from the pilot. You’d want a chair - seated position with safety straps make for better survival in rough landings. You’d want the flight control to be mostly automated - some type of high end software solution, trained on a simulated model of the vehicle and tested against all regions of the flight envelope.

As I understand it, jet engines have huge control lag when you adjust throttle. So to hover, you’d need to mount them in pairs. Normally, the pair would be angled slightly away from each other, so the thrust vectors are purely downward, but they are at less than maximum vertical thrust. If you want more thrust, a big servo motor would have to push the engines towards each other, and if you want less, they get pushed away from each other.

So for stability, you’d need 8 engines. Each engine pair is the equivalent of a rotor on a quadcopter. Doing napkin math here, assuming the engines are running at 80% of max thrust, and are mounted such that in neutral control position they are only developing 70% of total thrust in the downward direction (the rest of the thrust is wasted sideways), and using TJ40-G2s, you get 221 newtons of thrust per engine. Or 22 kilograms of downward thrust, or 48.5 pounds.

So if pilot is 200 lbs, and the welded tubing frame + electronics + chair is 100 lbs, and the engines are each 12 lbs, including a beefy servo motor and motor driver and mount, with 16 engines you have 776 lbs of thrust and 492 pounds dry weight. 158 liters of jet fuel gives you a flight time of 10 minutes, assuming 1 liter per minute per engine. (they are not running at max thrust per the data sheet for redundancy)

I have not been able to find a per engine price tag, but am estimating it’s at least $10,000.

It does appear feasible. I wonder why moderately rich hobbyists haven’t built something like this. Obviously, one would extensively test a vehicle like this under remote and automated control using a weighted dummy before anyone even considers riding it.

Waiting for an smackdown of my napkin design from Stranger on a Train and LSLguy in 3, 2, 1…

2

This will blow your mind

Frank Zapata, Flyboard Air

Flying ‘hover’ board for those that can’t open the link. Sort of like what the Gremlin would use. The guy does it over water for obvious reasons, and races jet skis (he’s faster). I suspect the jet skis are really just life guards in case.

I could drill a lot of holes in the ground with something like that. Well, probably just one.

3

eh (quoted myself), I’m doubting that that is real. But it is well done if it’s fake. Can’t find anything on Snopes.

If it was real, we would be hearing a lot more about it.

4

April 1st is not a good day to read about real-life superheroes. Let’s discuss all this next week.

5

If the Flyboard Guy was wearing this:

Then we’d have something!
BTW, has anyone wore those microjets while wearing a wingsuit?

Don’t tell me about Yves Rossi, I already know about Jetman.

6

Several thought-bites not making a coherent essay.

I think you’re on the right general track. The obvious problem as you’ve already identified is the fuel efficiency of direct jet-powered lift sucketh greatly.

So as a toy with a 5-10 minute flight time it works. As anything useful, e.g. aerial motorcycle, it doesn’t.

Existing quadcopter control software could be modified readily enough for the gimbled engine idea.

I suspect you may have a weight budget problem for the gimballing mechanism and it’s motive power.

FYI the power vs. time response curve for jets is very non-linear. At the low end coming up from idle it’s real slow. Up near the top end the lag is no different from an ICE. Part of why we fly a very high-drag arrival is to keep the power setting high enough that we’re in the fairly fast response area of the curve.

7

Yes, but that’s relative to ICE. Piston engine helicopters feather the rotor blades and tilt it for immediate response instead of throttle control. As I understand it, quadcopter hovering control - with simple fixed propellers - is only possible because the electric motors can change torque from 0% to 100% within a few update cycles (which are 15,000 times a second or more), and because the actual rotor and shaft into the motor are light and thus have low angular momentum.

I was thinking it would be only moderately difficult to install a big, high torque servo to tilt on only 1 axis, keeping costs and complexity low, for each engine.(maybe direct drive, maybe with a gearbox - whichever offers more torque per weight)

And yes, severe weight budget problem with the servo motor/engine mount/motor controller. If they are 12 pounds each, it means this thing can’t fly. The motor controller is the circuit board that drives the brushless DC motors you need to use if you want rapid and precise servo control and high torque. It has to have big, relatively heavy capacitors on it…

It sounds like you’d be better off using turboprop engines. The same Czech Republic aerospace company offers relatively cheap ones. You’d want a similar mounting system.