# How small can turbojet engines be made whilst remaining fuel efficient, light, cheap, and producing enough thrust to carry a human, gas, and casing? [closed]

I've seen many videos of small turbojet engines (small enough to hold in your hands) that produce 30-40 pounds of force. If you mounted a couple of these on a small craft, that wouldn't even support human + gas, and especially wouldn't support a fuselage or any other casing.

What size is efficient with gas but still produces a lot more than 40 pounds of thrust?

An extra question you don't need to answer: what is an effective way of tuning fin direction on the exhaust end of one of these engines? Basically, how can I twist a knob and make the fins tighten or twist in a direction I want the thrust to move in?

• A small, fuel efficient turbojet. Not a combination of words I've seen very often. Aug 1 '17 at 22:37

## 1 Answer

The figure of merit here is thrust to weight ratio (https://en.m.wikipedia.org/wiki/Thrust-to-weight_ratio) For a good turbojet, you get about 5:1. So a 100 lb engine could produce about 500 lbf of thrust. Modern High bypass turbofans are significantly more fuel efficient than turbojets, but the thrust to weight ratio is still about 5:1. So calculate how big your human is, how much gas you want, and divide by 5

• So say the person is around 150lbs, I wan't 10 gallons of kerosene (68.1 lbs), and add 50 lbs for extra casing and stuff. Divide this by 5 and I get 53.62. What does this number signify?
– user24531
Aug 1 '17 at 17:18
• @LucasSkarpness Are you building a jetpack or a airplane? Aug 1 '17 at 18:57
• I am building a jetpack. The JB-9 JetPack uses a Me 262 engine that produces 220 newtons of thrust (about 52 pounds) and they use 2 of those (as I can see visually). That only produces 102 lbs of thrust though, yet it propels human and the "fuselage" (casing).
– user24531
Aug 1 '17 at 22:10
• @LucasSkarpness, you have 268 lb of stuff that you want to lift. You'll need 268 lbf of thrust to do that. Well probably more than that actually. 268 is enough to hover. To actually go up you'll need more. To generate that much thrust with an average turbojet, you'll need approximately a 53 lb turbojet. Except don't forget that the engine has to lift itself too, so really you need maybe 350-400 lbf of thrust, so the engine probably weights 70-80 lb. With top of the line military hardware, you can probably get something lighter, but for a basic turbojet engine, 5:1 is a reasonable ratio. Aug 1 '17 at 22:28