Some other disadvantages of liquids are lower reliability and--given design for
low cost in both cases--higher costs.
Nice idea, however.
Bill
Sent from my Commodore 64
On Sep 14, 2015, at 5:14 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
One of the few disadvantages liquids have over solids is propellant
depletion. A solid burns nearly all its propellant, though some of it tends
to be at lower than nominal pressures during the tail off. For a liquid stage
to get maximum impulse it is imperative that it deplete both propellants
simultaneously. The last kilogram of propellant is the most important it. A
real world example of this is that Stiga flew to ~90km but had 30lb of fuel
remaining after it burned out the LOX; with equal depletion it would have
reached roughly 120km.
Centaur accomplishes this with capacitive sensors in the propellant tanks. A
rod is inside and electrically insulated from a tube that is open at bottom
and top. This forms a capacitor. As the level of the propellant goes down,
more of the capacitor contains gas than liquid and its capacitance changes.
This is used as an input to the mixture ratio controller, which drives a
harmonic drive on a throttle valve to slightly tweak the mixture. With this
control a stage carrying thousands of kilograms of propellant is able to
deplete down to tens of kg, helping give the stage it's incredible (and
consistent) performance.
One downside of this system is that it is analog and it requires some extract
structures and mass to hold up the tube and wire, and keep them electrically
isolated.
Cryo and electronics geeks know that some types of light emitting diodes
(LEDs) change their emission wavelength when they are cooled to cryogenic
temperatures.
An optocoupler is a solid state semiconductor device essentially composed of
an IR LED mechanically coupled to an IR transistor. The two components aren't
electrically coupled, so it's a way to send information between two isolated
DC systems without them needing to share a ground. I used optocouplers to run
some of the solenoids on Xombie and Xoie; it was a convenient way to amplify
a TTL signal to coil current while isolating the noise associated with
quickly turning on and off inductive loads.
Background complete. You could make a chain of optocouplers inside a
cryogenic tank, with them turned on all the time. It would consume just a few
mA per sensor. If the wavelength alteration trick works on IR LEDs, the
optocouplers would only pass a signal when they're above the level of the
cryogen.
By watching the timing as the sensors toggle, you can then measure the
depletion rate of the propellant in the tank and adjust mixture ratio
accordingly. With SMD optocouplers, the whole thing could be quite light.
There is a similar concept with small heaters coupled to temperature sensors,
where sensors that get warmer are above the liquid level. But I think my
concept would put less heat into the tank and has digital outputs, rather
than analog.
So that's my idea of the afternoon.
Ben
