I had an idea hit me out of the blue for a high quality parachute. A childs
nylon umbrella. just remove the metal tines attach some shroud lines and you
have a great chute. Is this a good idea or a bad one?
Whoa... Too much good tech info at one time. My brain can't handle this.
There's enough good info here for 20 good rmr threads.
Thank you for the post. Whole new things for me to explore... after I get
though the other 100 rocket things I must know more about.
Thanks to everyone who helped me on this.
Now I've decided not to use the idea, I'll say what it was all about:
There are a few competitions such as the above where hang-glider pilots
try to fly as far as possible. Human power is allowed, but for various
reasons propellors don't seem to work too well. They catch on the surface
of the sea shortening the flight, and cause drag at the start of the
flight when the pilot is not pedalling.
I wondered whether a really big water rocket might work, with the pilot
using some kind of pedal-powered foot pump. Power is limited to about 1kw,
so having a high specific impulse is a bad idea - you wouldn't be able to
get enough thrust to stay airborne.
Using a lower exhaust velocity means that the pilot needs to expend less
effort in order to produce the same amount of thrust, but this means that
he has to carry more water. The problem is that even for a 6-10 second
flight the mass of water the pilot would have to carry becomes
prohibitive: the glide angle would decrease, but more importantly take-off
It wasn't a daft idea until I'd done the sums,
Just a friendly nit ...
The term 'specific' in engineering circles implys 'mass'. So the term
specific impulse implies impulse per unit mass. Similarly specific fuel
consumption implies performance per unit of fuel mass.
Therfore mass-specific impulse adds no additional information and is
redundant in meaning.