[AR] Re: Disadvantages

  • From: "John Dom" <johndom@xxxxxxxxx>
  • To: <arocket@xxxxxxxxxxxxx>
  • Date: Tue, 15 Sep 2015 16:21:39 +0200

The amount of solid propellant allowed on the road is only 2 kg per car over
here, depending on composition. Requiring several cars for multiple
segments.
I so far never came across such transport limits for liquid propellants. As
to hypergols, I suppose 2 cars is sufficient.

jd

-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Bill Claybaugh
Sent: dinsdag 15 september 2015 15:33
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Disadvantages

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


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