What is an under-jet?
I saw it in 'A Martian Odyssey' written by Stanley Weinbaum
Context:
the wings haven't much lift in this air at less than a hundred miles per hour, and even then I had to use the under-jets.
Question:
What is an under-jet? Just like the jet engines under the wings we see from the jet aircraft?

Question

Answer

I think this is a phrase to imply that extra strength from an engine was needed. Yes, I think this is the regular engine that hangs under the wing. I am not well educated in aviation words. I looked for the phrase "under-jet" in trustworthy on-line sources and it is not there.  The author may have used a word combination to imply engine strength.