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Quaoar

Member

Registered: 2013-12-13

Posts: 665

O/F ratio for maximize specific impulse

I have seen very different O/F ratio in LOX-LH2 rocket engine: from 6 of the SSME to the 5.8 of the ESA Vinci. I have also read that most engine can change the O/F ratio during the burn to maximize thrust or specific impulse.
GW's mars mission study 470 s specific impulse rocket for example use an O/F ratio of 4.25 (I calculate it from the mass of LOX and LH2 in the tanks).

Which is the best O/F ratio for a deep space orbit-to-orbit spaceship needing the highest specific impulse?

Thank and merry Xmass for all.

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,431

Re: O/F ratio for maximize specific impulse

I am thinking that it is about the expansion ratio and pressure in the nozzle bell but I could be way off...

Quaoar

Member

Registered: 2013-12-13

Posts: 665

Re: O/F ratio for maximize specific impulse

I am thinking that it is about the expansion ratio and pressure in the nozzle bell but I could be way off...

Deep space engine usually have very high expansion ratio: 250:1 or 280:1, but I'm very curious about O/F ratio

RobertDyck

Moderator

From: Winnipeg, Canada

Registered: 2002-08-20

Posts: 7,934

Website

Re: O/F ratio for maximize specific impulse

No idea. Could be oxidizer rich for complete combustion, or fuel rich to maximize specific impulse. I could calculate stoichiometric ratio. According to periodic table, atomic mas of hydrogen is 1.008, while oxygen has 15.999. They burn to produce water, H2O, so the ratio 2*1.008 : 15.999 reduces to 1:7.936. If you represent that as oxidizer:fuel then 7.936:1. So all the numbers you posted are fuel-rich.

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,431

Re: O/F ratio for maximize specific impulse

http://www.astronautix.com/p/propellants.html

https://space.stackexchange.com/questio … f-a-rocket

http://www.braeunig.us/space/propel.htm

https://www.mpoweruk.com/rockets.htm
Rocket equations present on the page

Quaoar

Member

Registered: 2013-12-13

Posts: 665

Re: O/F ratio for maximize specific impulse

http://www.astronautix.com/p/propellants.html

https://space.stackexchange.com/questio … f-a-rocket

http://www.braeunig.us/space/propel.htm

https://www.mpoweruk.com/rockets.htm
Rocket equations present on the page

Very interesting
tanks and happy new year

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,406

Re: O/F ratio for maximize specific impulse

Quaoar, are you still around?

Your's is the only topic that contains the word "impulse"

GW Johnson just wrote an email indicating he may have decoded the recent transcripts from ChatGPT(4).  GW thinks they ** may ** be trying to tell us to look at "Impulse engines" to improve ISP and thrust at the same time. 

His comments reminded me of Startrek's fictional "Impulse Engines" for the Enterprise. I don't think those were ever explained, but fans might have made guesses about what they were after the series finished.

(th)