Question F1B Engine development.

[Michael_Chr]

Back in 2013 ARS Technica released an article about development of the Apollo era famous Rocketdyne F1 engine.

Since Im working on a new Velcro design using an F1B engine I'm interested if the work with the F1B include an improvement in ISP over the original F1 design.

The original ISP for the F1 was 304sec (Vac) and 265sec (SL) (seen on Astronautix). There must be room for improvement here since the RD170 is quoted for 337sec/309sec respectively.
Has anyone seen a source quoting a "uprated" ISP for a possible future F1B?

Best regards
Michael

http://arstechnica.com/science/2013...es-apollo-era-deisgn-with-1-8m-lbs-of-thrust/

 

[Urwumpe]

The original ISP for the F1 was 304sec (Vac) and 265sec (SL) (seen on Astronautix). There must be room for improvement here since the RD170 is quoted for 337sec/309sec respectively.
Has anyone seen a source quoting a "uprated" ISP for a possible future F1B?

You compare apples and oranges there.

The F-1 is a gas generator cycle engine, which is limited to 69 atm chamber pressure for peak ISP - that is what the F-1 maximal achieved without turning out to be a completely new engine. The F-1B is the same engine cycle, so there can't be a big step upwards. The RD-170 is a staged combustion cycle engine with almost 250 atm chamber pressure - higher chamber pressure means higher specific impulse, also staged combustion engine have no turbine exhaust that does not contribute to the thrust, which again increases the specific impulse.

Actually, the simplifications mean, that the ISP could even drop a bit with the F-1B

 

[Michael_Chr]

Hi Urwumpe...Thx for the post...I learned something there and will have to go and study some more about gas generator cycle vs. staged cumbustion.
But the 69 atm chamber pressure limit.... What is it that governs that?...and why exactly 69 atm?

 

[Urwumpe]

But the 69 atm chamber pressure limit.... What is it that governs that?...and why exactly 69 atm?

Well, it is actually very simple:

For pumping the propellants from tank head or tank outlet pressure to injection pressure (which is about 30%-50% higher than chamber pressure), you need energy. This energy is produced by burning some fuel and oxidizer in the gas generator and sending the exhaust into a turbine for driving the pump. The less useful this exhaust is for producing thrust, the better it is for pumping the propellants. Or: The more energy you extract from the exhaust for driving the pumps, the less energy you have for propulsion. A lot of energy gets vented overboard with the exhaust so that you can still get some mass flow. (hypothetically, if you would have no energy left in the exhaust, it would just clog the exhaust pipe)


So: the higher your chamber pressure should be, the more energy you need for pumping the fuel to this pressure and the more propellants you need to burn in the gas generator for doing so and the more energy you will need to waste in the turbine exhaust without producing thrust.

This describes a typical optimization problem: If you want to achieve more than 69 atm with the same fuel combination, you will loose specific impulse, since you need much more propellant for pumping and will have less fuel available for propulsion.

That is why it is most effective to operate such engines at 69 atm chamber pressure - you can use less or more of course, but this will lower the specific impulse, meaning a small increase in thrust would result in a bigger increase in fuel consumption.

 

[Michael_Chr]

OK...very informative. I understood the gasgeneration process...but didn't know that there was an optimizing "cut-off" at 69 atm. But it makes sense. So in that respect I now understand why the F1 design is fully optimized with regards to ISP vs. Turbopump limitation.
Thx a lot.

 

[Urwumpe]

OK...very informative. I understood the gasgeneration process...but didn't know that there was an optimizing "cut-off" at 69 atm. But it makes sense. So in that respect I now understand why the F1 design is fully optimized with regards to ISP vs. Turbopump limitation.
Thx a lot.

Yes - that is why the F1B optimized for the remaining free dimensions:
Thrust, Weight and Cost.

 

[Michael_Chr]

yep...just checked out some other exotic designs. Seems like SpaceX is leading the way with the Raptor design. Full flow staged combustion with ISP of 380/321, 8.2/6.9MN Thrust and exoctic Liq.Methane/LOX combo. Even reusable. Read that SpaceX is financing modification of Stennis Space center to allow for future testing of Methane Powered engines...Interesting. But current (available) technology seems to favour RP1/LOX combo for the three dimensions you list above - so thats the way to go for a 1st stage these days I guess.