I see that the new Vector-R launch vehicle is LOX/propylene (first stage,
https://vectorspacesystems.com/vector-r/). I wonder how they handle the
polymerization issues.
On Sat, Apr 1, 2017 at 5:07 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Sat, 1 Apr 2017, Brian Feeney wrote:
Anthony Ceasaroni wrote:Sulphur is not the dominating issue.
What is / are the dominating issue(s)?
Depends on the fuel and the chamber materials.
For kerosene and suchlike -- that is, complex mixtures of higher
hydrocarbons -- you get classical coking, which is essentially reactive
components of the fuel polymerizing into solid organic gunk, and then
gradually baking down to mostly carbon, under heat and pressure. Sulfur
might perhaps help this along a bit, but isn't at all necessary. What
distinguishes RP-1 (and such) from (say) Jet A, is that RP-1 has
essentially none of the troublemaking components, and so it doesn't
polymerize easily.
(There has been speculation in the past that things like ULS -- Ultra Low
Sulfur -- diesel fuel might be similarly heat-resistant, not because of the
lack of sulfur but because the treatments used to get rid of sulfur
impurities also tend to get rid of the reactive hydrocarbons in the mix.
And if you want CO2 brownie points, peanut oil is quite heat-resistant and
might be suiable...)
Simple hydrocarbons like methane and propane, when reasonably pure, simply
don't polymerize on their own. For them, the big issue is sulfur
corrosion, especially in copper chambers. It doesn't take much -- the
Aerojet guys found heavy deposits of copper sulfides with 10ppm of CH3SH in
methane. (CH3SH, methyl mercaptan, is a common "stenchant" in methane,
natural gas, etc., added to make them smellable.)
(RP-1 is officially limited to 50ppm of sulfur, but normally has very
little. They tried adding 50ppm of a mercaptan to it, and it showed major
sulfur corrosion too, at rather lower temperatures than needed to make it
show classical coking.)
One problem with commercial fuel "propane", and to a lesser extent with
LNG as well, is that they're not pure propane and methane respectively.
"Propane" usually has some propylene (which most definitely does
polymerize), and especially in hot climates it may also have a fair bit of
butane and pentane to reduce its vapor pressure. LNG is mostly methane but
has small amounts of other hydrocarbons too, and if it has ethylene or
propylene, you may again be in trouble. Possibly the biggest headache with
heating-fuel grades is simply that their compositions aren't well
controlled -- even the choice of stenchant can change -- so good results
with one batch don't guarantee anything about the next batch.
(I'm told that propane sold as vehicle fuel has *much* tighter specs, but
I haven't verified this.)
Henry
