Recently Vector space from USA has said that propylene is a better choice of fuel than RP-1. See Ars Technica's After a decade of testing, propylene rocket fuel may be ready for prime time

Why is propylene a better fuel than RP-1? At what density is it used for that purpose?

edit: while some level of comparison can be found in answer(s) to Vector-R's LP-1 and -2 engines use liquid propylene as fuel with LOX, advantages and distinctions from kerosene?, I'd like to know more details on how the two fuels compare quantitatively.

I tried to check the density of propylene on internet but it is less dense than RP-1 then how does it provide more performance, and roughly how large is this difference in performance?

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    $\begingroup$ Side note: NASA contracted the inventor of ProPoly in July 2017 to build a ProPoly50 engine (electric fed lower stage/pressure fed upper stage) for a small satellite launcher. Seems that propylene is at least interesting. See here. $\endgroup$ – Christoph Sep 28 '18 at 12:18
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    $\begingroup$ @uhoh yes ikr. Its a crucial part. But how can we know that? $\endgroup$ – user167195 Sep 28 '18 at 13:07
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    $\begingroup$ @uhoh thankyou 🙏 $\endgroup$ – user167195 Sep 28 '18 at 14:22
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    $\begingroup$ I've added the quantitative information from my answer here to my answer on the other question, which I feel makes this Q a closable dupe. $\endgroup$ – Russell Borogove Sep 30 '18 at 22:11

Density is one of two important figures of merit for first-stage rocket fuels; the other is mass-specific impulse, the product of force and time that is yielded from a given mass of fuel.

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, according to a chart originally posted on nasaspaceflight.com (the chilled propylene/RP-1 callouts are mine.)

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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.

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    $\begingroup$ Nice answer. Maybe you could add in the rocket equation and show how 2% more impulse results in around 6% less lift-off weight for a typical launch to low earth orbit (I think of a delta-V of 10 km/s). I can provide performance data from cpropep like in this answer later today if anyone requests it. This would show e.g. the high chamber temperature compared to the alkanes. Propylene is quite known for its polymere so I could imagine that this might be an issue with cooling the chamber. Anyone got information on that? $\endgroup$ – Christoph Oct 1 '18 at 7:24
  • $\begingroup$ @Christoph if RB is too busy, then in a few days you could consider adding an answer of your own as well. $\endgroup$ – uhoh Oct 2 '18 at 9:05

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