High ISP Vac Rocket Engine

[Roy_H]

I am not a rocket engineer, but I know there are several of this web site.
Is it possible to modify a conventional, in this case Raptor 2, engine for much higher ISP and efficiency in space?
Intuitively, to me, an expanding nozzle reduces speed of exhaust and ISP. But a long barrel should accelerate the exhaust particles to some theoretical limit.
Also if as much of the barrel is of insulating material as possible, this would not cool the exhaust by convention but the exhaust should only cool by expansion as it increases speed.

Does this idea have any merit? Would it be a more efficient drive for in space use?

[edzieba]

Intuitively, to me, an expanding nozzle reduces speed of exhaust and ISP.

Your intuition is wrong. Look up de Laval nozzles.

[Roy_H]

Intuitively, to me, an expanding nozzle reduces speed of exhaust and ISP.

Your intuition is wrong. Look up de Laval nozzles.

Your cryptic response doesn't help me much, I am clearly in over my head. So I find this formula attached


And find that exhaust velocity is highest when k > 1 and as close to 1 as possible
K is defined as Cp/Cv or isentropic expansion factor
None of the other terms relate to any physical dimensions of the nozzle but I thought maybe this expansion factor does.
However when I look that up it tells me Cp is Constant pressure and Cv is constant volume. This only leads me to more confusion. I need a formula that relates to the physical dimensions of the nozzle.

[Gliderflyer]

Try this: https://www.grc.nasa.gov/www/k-12/rocket/nozzle.html

[Roy_H]

Try this: https://www.grc.nasa.gov/www/k-12/rocket/nozzle.html

Thank you for the link. I will have to spend some time to absorb this but I think this statement:
"Then an increase in the area (dA > 0) produces an increase in the velocity (dV > 0). This is exactly the opposite of what happens subsonically."
Goes to the root of my confusion.

[aero]

Unfortunately it can be confusing, but also, fortunately, it is true. Your nozzle would have subsonic flow the whole length, and only reach sonic velocity at the exit. It is just a very, very long rocket engine throat. Subsonic because gas friction with the walls would cause back  pressure which would keep the flow from reaching sonic velocity until the gas was release at the exit.

[Robotbeat]

Unfortunately it can be confusing, but also, fortunately, it is true. Your nozzle would have subsonic flow the whole length, and only reach sonic velocity at the exit. It is just a very, very long rocket engine throat. Subsonic because gas friction with the walls would cause back  pressure which would keep the flow from reaching sonic velocity until the gas was release at the exit.

Actually, it could choke and then expand supersonically even in a long tube. Armadillo Aerospace tested the idea. You would think it'd be very inefficient, but apparently it's not too bad.

[rakaydos]

Unfortunately it can be confusing, but also, fortunately, it is true. Your nozzle would have subsonic flow the whole length, and only reach sonic velocity at the exit. It is just a very, very long rocket engine throat. Subsonic because gas friction with the walls would cause back  pressure which would keep the flow from reaching sonic velocity until the gas was release at the exit.

Actually, it could choke and then expand supersonically even in a long tube. Armadillo Aerospace tested the idea. You would think it'd be very inefficient, but apparently it's not too bad.

Link to video or reading?