Vector-R's LP-1 and -2 engines use liquid propylene as fuel with LOX, advantages and distinctions from kerosene or propane?

Question

@gwally's answer highlights the company Vector Launch Inc. and their Vector-R rocket, which uses three LP-1 18,300 lbf engines on the first stage and one LP-2 1,000 lbf second stage engine to put small payloads such as cubesats into LEO.

I think the "LP" in the engine names stands for liquid propylene, also called propene or C3H6 (one double bond).

What are some advantages to using propene over the more common and longer-chained kerosene such as RP-1? In what way would the design and operation of these L-propene/LOX engines be different than similarly-sized Kerolox engines?

With only three carbons, it's going to need to be cryogenic to be a liquid without extreme pressure, but not as cold as methane. I also wonder why liquid propene was selected over liquid propane, or other short hydrocarbons.

above: Images from here, here, and here.

above: Vector-R's three 1st stage LP-1 engines, from Spaceflight Insider.

Answer 1

According to a pair of posts on the nasaspaceflight.com forum, liquid propene/propylene yields substantially better specific impulse than RP-1, while being substantially denser than methane. Thus, it seems sort of a compromise propellant between the two. Garvey Spacecraft Corp's site makes the same points about that fuel combination, with a 2009 flight test of a small engine based on it.

It's also both slightly more specific-impulsive and a bit denser than propane and the other short alkenes.

Unlike a kerolox stage, a propylox (?) stage has both cryogenic fuel and oxidizer. The fuel doesn't want to go quite as cold as LOX (melting point 88K, but gets waxy below 100K, versus LOX's boiling point of 90K), but that might still be close enough for an insulated-common-bulkhead tank instead of separate fuel and oxidizer tanks -- I have no idea if Vector actually does that, though. In general it would be much more like a methalox stage than a kerolox, just a bit more compact in the fuel tankage.

Specific impulse varies with engine design and other factors, but all other things being equal, propylene's specific impulse is about 2% higher than that of RP-1. When both fuels are cooled until they reach a viscosity of 3.3 cP (which is the point at which Falcon 9 FT keeps its RP-1), RP-1 is only 3% denser:

Because the tanks themselves are extremely thin and lightweight, the mass penalty incurred by needing a 3% larger tank for propylene is less significant than the specific impulse advantage gained.