How to talk to a 9yo girl about

Emily

doesn’t seem to like

engineering

much, but she’s certainly smart.

Today, I discussed with her about the basics of rocket engine designs, and she correctly figured out the followings on her own:

1. Two tanks, one for fuel one for liquid oxygen, two straws underneath, so two liquids can flow through to a place, where they go kaboom. why this design doesn’t work for rocket?

She said, water only flows down on earth; in space, it won’t come down. Nice.

2. So how to solve that problem, making the liquids going down through the straws, even in space?

She said, put a vacuum cleaner there to suck the liquids down. Holy! that’s exactly what a turbo-pump is.

3. Now I said vacuum cleaner uses electricity, on earth you can plug it into wall, and electricity goes from power plant to it. But in space, you can’t dangle a power cord there. How to solve this problem?

She said, she would liquefy electricity and store the liquid electricity in a 3rd tank on board the rocket.

(She chose liquefying because in another day I talked to her about the difficulty of storing lots of gaseous oxygen on board so we do LOX)

She’s right, this is the basic design of Rutherford engine on Electron rocket, where the turbo-pump is powered by electricity.

4. We also talked about pressure fed engine design. I said, what if you can push the liquids down through the pipes, instead of sucking them out.

She first said she would spin the tank so a whirl would form and by her observation, whirling liquids go down the drain :) Nice try.

Then she said, she would put a lid in the tank (essentially a piston) and push the lid down. Nice idea, but doesn’t usually work, since building such a big piston against a not so rigid tank isn’t always practical, plus the weight, and mechanism.

Lastly, she tried, how about putting a fan, and blowing air into the tank so the liquid gets pushed down by the wind.

This time, she’s pretty much close: that we use pressured noble gas (usually helium) to push fuel/LOX down.

5. We finally talked about open gas generator cycle for medium/large rockets where electric pump isn’t powerful enough.

Here she needed a lot more hand-holding, and wasn’t able to come up an idea close to the pre-burner configuration.

Oh well, good enough for a 9yo for a night!

Can't wait for the future discussion on staged combustion cycle and the latest and shiniest full flow staged combustion cycle.