Achieving VTOL with jet engine instead of rocket engine

[Mr. Barracuda]

Jetpack International's T-73 achieves vertical takeoff with an actual jet engine as opposed to the rocket engines used by the majority of jetpack models. This allows it to have longer flight time (9 minutes as opposed to 30 seconds). Some fighter planes also manage VTOL with a jet engine, the most well-known of which is probably the F-35.

Most jets aren't powerful enough to achieve vertical takeoff and rockets are usually required. What is it about jetpacks and fighter plane engines that gives them the extra boost in power? I'm guessing a very powerful compressor to intake enough air even when starting from rest?

 

[Borek]

Some fighter planes also manage VTOL with a jet engine, the most well-known of which is probably the F-35.

And not Harrier?

 

[etudiant]

As always it is a tradeoff.
An engine able to lift the aircraft vertically needs a large intake, which then creates big drag at higher speeds.
The F35B instead achieves VSTOL with a Rube Goldberg engine gear box driven fan built into the middle of the fuselage, with folding doors to cover it in forward flight. It is hugely costly in terms of payload and performance, but probably the only acceptable way to combine supersonic capability with VSTOL.

 

[AlephZero]

The F35B instead achieves VSTOL with a Rube Goldberg engine gear box driven fan built into the middle of the fuselage, with folding doors to cover it in forward flight.

In the beginning ...
https://www.youtube.com/watch?v=f8W2SI4c93s

 

[etudiant]

Great footage, AlephZero!

The US Army was very active in the 1950s in the pursuit of VSTOL, with a whole series of experimental airplanes and devices. None was able to match the helicopter for overall effectiveness and none went into production.
The USAF did have an effort with West Germany in the 60s to develop a supersonic VSTOL fighter bomber with dedicated lift engines placed in wing tip pods.
The dead weight of the VSTOL propulsion and the cost of qualifying the lift engines killed that project.
But the siren song of VSTOL has clearly caught the USMC leadership in its spell, even though it is of questionable cost/effectiveness.

 

[russ_watters]

The goals of speed and lift are opposites of each other for engines, so that makes it difficult for jet engines. Lifting works best with a high volume at low velocity (which is why helicopter rotors are so large) while speed works best with a high velocity at low volume.