# How powerful would an engine have to be to lift 150lbs?

1. Aug 17, 2008

### BigFly

just out of curiosity if i wanted to lift something about 150lbs out of the air with a jet engine or rocket, to hold it against the earths gravitational pull 9.8m/s how much energy/power would i need? would it also depend on how heavy the engine is? and what other factors are there in this problem?

2. Aug 17, 2008

### chayced

Really just comes down to how many lbs of thrust the engine can put out. You need slightly more thrust than the weight of the engine and payload in order to go up. All just basic physics.

Want to build a jetpack? Just assuming since you said 150 lbs.

3. Aug 18, 2008

### jaap de vries

Might be handy to bring fuel as well.

You only need more thrust than the weight of your contraption if you need to go straight up. You could also use wings and you would only need 1/5 or so.

4. Aug 21, 2008

### minger

Yea, I wouldn't use power. Power is really just work per unit time. So, an infinitely small engine could be geared to lift the weight infinitely "fast". But rockets and jet engines are typically measured in thrust force, so you would really just need slightly more than the weight of the vehicle + fuel + weight. Excess force can be used to calculate a possible acceleration.

It all really depends on how high you want to go and how long burn time you have.

5. Aug 21, 2008

### Swiftuh

If you want to lift a total load of a 150lbs, you would need to engine to produce slightly above 150lbs of thrust in a focused direction.

6. Aug 21, 2008

### Staff: Mentor

One would have to consider all masses involved, i.e. not only the 150 lb payload (or person), but also the mass of the engine and propellent (fuel - and oxidizer if the engine is not airbreathing). If one want's to accelerate vertically, the thrust must exceed the total weight, or thrust/weight ratio must > 1.

7. Aug 22, 2008

### FredGarvin

Don't forget drag forces too.

8. Aug 22, 2008

### Staff: Mentor

9. Aug 25, 2008

### kmarinas86

Like other people said, the thrust has to be greater than 150 lbs.

To generate 150 lbs of thrust, the output power required depends on how fast you have to move the propellant to generate the force needed. A propellant that is moved faster to generate the same force will result in a higher specific impulse, which means it willl reduce amount of that propellant mass need to deliver the same momentum (e.g. ion propulsion rocket). However, to take advantage of that lower propellant mass, it will require more output power to generate the same force.

In this sense, for your purposes, a jet engine would be far more efficient than a rocket. A fan of larger diameter would be even better in terms of efficiency, since it distributes the power over a larger swath of air, reducing the power needed to generate the same force. That's why helicopters don't use jet engines and why airliners don't use rocket engines.

10. Aug 25, 2008

### minger

But helicopters do use jet engines (gas turbines). Well said either way ;)