General Discussion => General Discussion => Topic started by: leaflion on 12/04/2015 06:52 AM

Title: Why ORSC?
Post by: leaflion on 12/04/2015 06:52 AM
I'm not a rocket scientist, but I can grasp that high temp oxygen in the ORSC cycle must be difficult.  Why then is ORSC and attractive option?  Why not just go fuel rich?

Sorry if this is already a topic elsewhere, couldn't find anything that answered my question via search
Title: Re: Why ORSC?
Post by: Proponent on 12/04/2015 10:05 AM
I believe ORSC is attractive when the chemical or thermodynamic properties of the fuel make it unsuitable.  The combustion products of a very fuel-rich oxygen-kerosene mixture are no doubt very dirty.  The larger mass of oxidizer (in almost all cases) means that the oxidizer flow has a larger potential for powering turbopumps too, so if you're going to do single-propellant staged combustion, ORSC is more attractive than FRSC.
Title: Re: Why ORSC?
Post by: baldusi on 12/04/2015 11:26 AM
Yep. First you have to be able to have a fuel rich preburner to trade it. So, for example, RP-1 would polymerize and thus it would not go though nicely through the turbines. It would probably clog some of the injectors, too, which could bring catastrophic combustion instabilities.
And even when it is possible, like the methane case, you have to remember that the turbine is (mostly) a thermal machine. Thus, the massflow x specific heat capacity x temperature drop is a good first approach number for your power budget at the turbine (which is actually the whole point of doing staged combustion).
If you do the numbers for LOX/methane, you will notice that you are mostly even with each case. The O/F is around 3.5~3.6. But the specific heat (kJ/kg.K at 750K) for methane is 3.766 vs 1.043 for LOX, which is a 3.61 ratio. So you are pretty much ambivalent. This is interesting because Energomash had proposed methane engines fuel rich, Blue Origin is doing ORSC, SpaceX is doing full staged and KhKbA is doing ORSC with methane expander cycle.
In the LH2/LOX case it is clearly a better case of going fuel rich, thou. The O/F is usually 6.0 and the H2 specific heat is 14.65. So you can get roughly twice the power from running fuel rich than oxidizer rich.
Title: Re: Why ORSC?
Post by: notsorandom on 12/04/2015 01:02 PM
In the case of methane since ORSC and FRSC are so similar what is the reason one would choose to do ORSC over FRSC? There has to be a reason why Blue is dealing with the hassle of dealing with high temperature and pressure oxygen over the simpler metallurgy of methane in a FRSC engine.
Title: Re: Why ORSC?
Post by: baldusi on 12/04/2015 03:07 PM
You still get roughly the same temperature. It's the hot O2 corrosiveness that limits you. I sort of cheated, if you look at the O/F ratio and the preburner output temperature, you could end up with advantage to either ORSC or FRSC for methalox. So I guess they traded on the case where ORSC was simpler.
It might also happen that they are taking a bit of advantage of the expander cyclo on the methane side, or are tryin to reduce the pressure losses on the regen side (for which methane has just 30% of LOX viscosity).
And if you already have the metallurgy, there's probably more experience on oxidizer rich SC than methane rich one. Methalox is so balanced that each designer is using a different approach.
Title: Re: Why ORSC?
Post by: Proponent on 12/04/2015 04:19 PM
And even with methane as the fuel, the output of an ox-rich preburner will be noticeably cleaner than that of a methane-rich one (