### Author Topic: RS-25/SSME - Calculation  (Read 2569 times)

#### Aeneas

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##### RS-25/SSME - Calculation
« on: 05/14/2020 07:01 am »
Dear all,

I'm new to space exploration and I only gathered infos over the last couple of months. At the moment, I'm trying to estimate possible modern launchers in the super heavy class, similar to Starship/Superheavy and model the launch to LEO in my simulator (MS Excel, time step flexible, simplified aerodynamics). Being aware of the challenges with hydrolox, I still try to get my mind around a second stage powered by hydrolox engines. with a payload of ~500 tons and delta-v for ~6,100 m/s for the stage, I likely need a thrust of ~26 MN. Since it's a serial staging process, the upper stage ignites above 45 km, therefore can be vacuum optimised. Due to sizing, RL10s are out of the option and RS-25s are optimised for parallel staging. I wondered what a hydrolox engine (> 2.5 MN) could be capable to delivery if applying Raptor technology. So full flow staged combustion, 300 bar Pc and a vacuum optimised nozzle. To have a comparison, I tried to recalculate the RS-25, searched a long time for possible "k"s and finally entered everything to just realise, that my ideal calculation is still about 16 s Isp short of the actual 452 s. I thought, I come around 490 for the ideal calculation and then there's probably some friction losses and some losses for the turbo pump. Where am I wrong? Maybe it's because k changes through the process? Maybe I got the wrong k? Maybe I got the wrong exit pressure and throat diameter? I don't know. Those are my numbers. Mostly with the help of www.braeunig.us.

SSME/RS-25
O/F mix   6
#   k   1.1955
K   Tc   3,630.00
J/kmol-K   R*   8,314.46
g/mol   MM   13.6
bar   Pe   0.220000
bar   Pc   206.00
bar   Pt   116.46
K   Tt   3,306.76
kg/s   fuel flow (q)   514
m²   At   0.0574
m   throat diameter   0.2703
Nm²   Nm²   21.0885
m²   Ae   3.9617
m   mouth diameter   2.2459
m/s   Ve   4,275.0
s   Isp   435.9
#   nozzle ratio   69.0
kN   thrust   2,197.4

s   Isp actual   452.0
kN   vacuum thrust   2279

Taking the risk to bore you, can you help me?

#### envy887

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##### Re: RS-25/SSME - Calculation
« Reply #1 on: 05/14/2020 09:06 pm »
Try RPA software, the lite version is free but quite capable: http://propulsion-analysis.com/index.htm

Hydrolox doesn't gain much from FFSC like Raptor, as opposed to FRSC like RS-25, because the hydrogen is the vast majority of the volume of flow. Also, for a vacuum-optimized engine, very high chamber pressures doesn't improve ISP very much.

And unless you are limited by gross liftoff mass, having a hydrogen upper stage may not be worthwhile at all. Hydrocarbon stages are cheaper to build, fly, and maintain, and they offer much better mass ratios.

#### Fequalsma

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##### Re: RS-25/SSME - Calculation
« Reply #2 on: 05/15/2020 06:53 pm »
Great post!  Have you checked your spreadsheet against the Saturn V or Shuttle trajectories?  And can you post your spreadsheet here?

f=ma

#### Aeneas

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##### Re: RS-25/SSME - Calculation
« Reply #3 on: 05/15/2020 07:30 pm »
Try RPA software, the lite version is free but quite capable: http://propulsion-analysis.com/index.htm

Hydrolox doesn't gain much from FFSC like Raptor, as opposed to FRSC like RS-25, because the hydrogen is the vast majority of the volume of flow. Also, for a vacuum-optimized engine, very high chamber pressures doesn't improve ISP very much.

And unless you are limited by gross liftoff mass, having a hydrogen upper stage may not be worthwhile at all. Hydrocarbon stages are cheaper to build, fly, and maintain, and they offer much better mass ratios.

Wow! RPA is awesome! I can see the changing "k"s! Thank you! Nonetheless, I first have to understand how to implement FFSC to see that it doesn't bring anything to the table. Gotta fiddle a bit.

Well, Isp isn't much improved but apparently it has a huge effect on the nozzle diameter. That's helpful to enable a TWR for the second stage of 1...

#### Aeneas

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##### Re: RS-25/SSME - Calculation
« Reply #4 on: 05/16/2020 08:56 am »
Great post!  Have you checked your spreadsheet against the Saturn V or Shuttle trajectories?  And can you post your spreadsheet here?

f=ma

Attached you'll find the model. But it's total chaos. I tried to match braeunings Apollo 11, didn't match exactly but's the right direction. Just recently I found out how the Space Shuttle was shot into orbit. I'm not there yet to try to model that. At the moment, the model just shuts down engines when it reaches an acceleration threshhold. I'll add a throttle, a cut off, some form of Isp change to model parallel staging and some form of better pitch steering.

#### Fequalsma

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##### Re: RS-25/SSME - Calculation
« Reply #5 on: 05/19/2020 01:45 am »
Thanks Aeneas!  Do keep us posted on your progress!
F=ma

#### Aeneas

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##### Re: RS-25/SSME - Calculation
« Reply #6 on: 05/27/2020 07:43 am »
Try RPA software, the lite version is free but quite capable: http://propulsion-analysis.com/index.htm

Hydrolox doesn't gain much from FFSC like Raptor, as opposed to FRSC like RS-25, because the hydrogen is the vast majority of the volume of flow. Also, for a vacuum-optimized engine, very high chamber pressures doesn't improve ISP very much.

Ok, I don't get it. Do you know, how to implement FFSC in the program? Are there any example files? Maybe even for a Raptor engine?

#### envy887

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##### Re: RS-25/SSME - Calculation
« Reply #7 on: 05/27/2020 11:05 pm »
Try RPA software, the lite version is free but quite capable: http://propulsion-analysis.com/index.htm

Hydrolox doesn't gain much from FFSC like Raptor, as opposed to FRSC like RS-25, because the hydrogen is the vast majority of the volume of flow. Also, for a vacuum-optimized engine, very high chamber pressures doesn't improve ISP very much.

Ok, I don't get it. Do you know, how to implement FFSC in the program? Are there any example files? Maybe even for a Raptor engine?

FFSC mostly affects combustion efficiency (because of gas-gas mixing) and chamber pressure, and you can tweak both of those manually in the engine definition settings in RPA. For Raptor those are about 99.5% and and 26-30 bar.
« Last Edit: 05/27/2020 11:06 pm by envy887 »

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