[AR] Re: Orion-13
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 19 Jun 2018 12:55:15 -0400 (EDT)
On Sun, 17 Jun 2018, Craig Fink wrote:
For Oxygen and Hydrogen, if a fuel cell is added for backup energy,
water generation, and breathing oxygen out of the cryogenic tank, would
mean that there is a slow uses rate of the Cryogenic Oxygen and
Hydrogen...
Fuel cells unfortunately tend to be heavy, complex, and cranky. If the
goal is to use boiloff that's happening anyway, a generator driven by a
turbine or piston engine may be preferable. And it's pretty definitely
not worth deliberately using up fuel to produce power, not when modern
solar arrays are relatively lightweight.
Long missions will need water recycling anyway -- humans need wash water
(for people, clothes, dishes) as well as drinking water, and the mass
involved is high enough that closing the water cycle is important. This
reduces the payoff for water production (and not all fuel-cell types
produce *drinkable* water, by the way).
This propellant use rate if implemented properly can give additional
cooling to the propellants during the long trip. Essentially a flash
evaporator inside the tank so that the propellant to be used for
whatever purpose, breathing, drinking, energy, attitude thrusters and
small burns will cool the bulk of stored propellants.
Yes, people thought of this fifty years ago. It's an obvious thing to do
for almost any cryo system; the Apollo oxygen tanks did it, if I recall
correctly.
For the design of this system, the flash evaporator is mounted inside
the tank away from the walls, it's a vacuum pipe like the evaporator
coil on an air conditioner. The propellant is fed through a capillary
tube to flash from a low pressure liquid to a near vacuum colder gas to
absorb heat before it exits the tank...
Basically the right idea, but you *don't* put it in the middle of the
tank. You want to cool the *walls*. That's where outside heat comes into
the tank, so cooling the walls avoids any need to directly cool the
propellant; this avoids complications like having to circulate the liquid.
And wall cooling is easy -- just run the lower-pressure outlet plumbing
along the outside of the wall (inside the insulation), perhaps by having
the tubing brazed/welded to the wall. You don't need a particularly large
pressure reduction, by the way, so there's no need for a compressor after
that.
Henry
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