[AR] Re: engine life (was Re: Nozzles for Amateur Solids)
- From: rebel without a job <rebelwithoutajob@xxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Tue, 13 Mar 2018 02:56:58 +0000
On Mar 11, 2018, at 10:14 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
Which is actually good news in some ways: cooling with liquids is a whole
lot easier and more effective than cooling with gases. If there's a torrent
of propellant going into the chamber, using it for cooling is an awfully
obvious thing to do. (In fact, aircraft capable of sustained supersonic
speed often use their fuel for cooling too, but this works even better for
rockets, cooling a compact chamber with a high fluid flow.)
Actually all modern jets use fuel for cooling. Internal cooling is done by oil,
and the oil is cooled by an air cooler and then a fuel-cooled oil cooler. Of
course that’s just air cooling by proxy - the fuel is cold because it sits out
on the wing surrounded by cool air. When the temperature is higher, the cooling
systems tap off more fuel flow and open a few air vanes.
This is the great benefit of the air engine - it can separately meter the flow
of fuel, reaction mass, and cooling flow.
Chamber cooling is substantially harder when Isp gets so high that there's no
longer enough propellant flow to accept all the heat, but that happens well
above the chemical-rocket range. (Fusion-rocket designs tend to be extreme
cases of this. In the "Firefly" concept for a fusion starprobe, which has
the fusion plasma pretty much out in the open with the vehicle intercepting
as little as possible of the emissions, 3/4 of the dry mass of the vehicle is
the cooling system.)
As you alluded to earlier, much of the solution here is to keep the heat away
from the walls. The simplest way is to take a page from the marine diesel
industry, have enormous combustion chambers, keep the hottest part of the flame
far from the walls, and most importantly let the cube-square law work in our
favor by having less surface area to cool.
Of course, while this is a big win from the cooling perspective, having large
combustion chambers introduces losses in other ways.
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