General principle of turbomachinery: true also for rocket turbopumps?

[RobertGC]

TL;DR

Discussion of an approach to high reusability of rocket engines.

A rule of thumb of aircraft engineers is every 10% increase in the power level engines are run at corresponds to a 50% decrease in engine lifetime. This is a general phenomenon of turbomachinery. Then it is likely it also holds for rocket turbopumps.

Then quite key is the rule also holds in reverse, every decrease in power level by 10% can result in doubling the lifetime of the engine. Then by running the engine at 0.9^5 = 0.60 power level can result in 2^5 = 32 times longer lifetime. For a rocket engine, this would bring the rocket engine lifetime into the range of jet engines of over a thousand reuses. It is notable as well that rocket engines can commonly be safely run at the reduced thrust level of 60%, i.e., this is within the safe throttlebility range.

So has this been tested in rocket turbopumps?

 

[Astronuc]

TL;DR: Discussion of an approach to high reusability of rocket engines.

So has this been tested in rocket turbopumps?

It probably has been done, e.g., Space Shuttle Main Engine. One would have to query the team at NASA Stennis Space Center. I believe the SSME power head was disassembled and rebuilt after each flight for inspection, but it's been years since I looked into those details.

A Defining Era: NASA Stennis and Space Shuttle Main Engine Testing​

https://www.nasa.gov/centers-and-facilities/stennis/a-defining-era-ssme/#hds-sidebar-nav-3

SPACE SHUTTLE MAIN ENGINE TESTING: T PLUS 30 YEARS AND COUNTING​

https://www.nasa.gov/news-release/space-shuttle-main-engine-testing-t-plus-30-years-and-counting/

Since the first test on May 19, 1975, the NASA/contractor team at SSC has conducted more than 2,200 tests on SSMEs, including the ones that helped propel Space Shuttle Discovery on STS-114, NASA’s Return to Flight mission.
A total of 10 tests were conducted in the first two months to help establish fuel preburner, oxygen preburner and main combustion chamber ignition. Three years later, test teams at SSC were firing the Main Propulsion Test Article: the three-engine cluster that helps propel the Space Shuttle into orbit. On Jan. 21, 2004, SSC celebrated 1 million seconds of successful SSME engine firings, both in testing and flight operations. This milestone is a testament to the employees and to the engine itself, which has never experienced a major anomaly.
“The unmatched reliability and durability of the SSME serves as an enormous credit to the NASA/contractor teams that have manufactured and tested the engine for three decades,” said Ronnie Rigney, SSME Project Manager at SSC. “They’ve done an extraordinary job.”
There have been 114 missions since the first Space Shuttle took flight from Kennedy Space Center (KSC) on April 12, 1981, all powered by SSMEs tested at SSC.

History of Space Shuttle Main Engine Turbopump Bearing Testing at the Marshall Space Flight Center
https://llis.nasa.gov/lessons/24006...rd Gisbon et al released NTRS 20100023061.pdf

How NASA Tested the Space Shuttle: Ground Testing, Avionics, and Data Acquisition Explained​

https://dewesoft.com/blog/how-nasa-tested-space-shuttle

Summary of Results from Space Shuttle Main Engine Off-Nominal Testing
https://ntrs.nasa.gov/api/citations/20120001440/downloads/20120001440.pdf

SPACE SHUTTLE MAIN ENGINE THE FIRST TEN YEARS
https://gandalfddi.z19.web.core.win...ine The First Ten Years - Robert E. Biggs.pdf

Next-Generation RS-25 Engines for the NASA Space Launch System
https://ntrs.nasa.gov/api/citations/20170008958/downloads/20170008958.pdf

NASA, Aerojet Rocketdyne plan busy RS-25 test schedule for 2021​

https://www.nasaspaceflight.com/2020/12/nasa-aerojet-plan-busy-2021-test-schedule/

SSME to RS-25: Challenges of Adapting a Heritage Engine to a New Vehicle Architecture
6TH EUROPEAN CONFERENCE FOR AERONAUTICS AND SPACE SCIENCES (EUCASS) (2015)
https://ntrs.nasa.gov/api/citations/20150016499/downloads/20150016499.pdf

The RS-25 is pump-fed staged-combustion rocket engine burning liquid oxygen (LOX) and liquid hydrogen (LH2) to produce 2279 kN of vacuum thrust. Primary components involve two low-pressure turbopumps feeding into two
high-pressure turbopumps supplying propellants to the combustion devices, including two preburners, the main combustion chamber and nozzle. The preburners are independently controlled to provide variable thust and mixture ratio. In addition, the system was designed to be reusable, providing a certified service life of 55 starts and 27,000 seconds. The fuel-rich staged combustion cycle provides high performance, making it an attractive candidate in many vehicle trades for the SLS and prior conceptual vehicle studies

I do not know the programs for Space X or Blue Origin, and the details may be proprietary.

 

[DaveC426913]

Then quite key is the rule also holds in reverse, every decrease in power level by 10% can result in doubling the lifetime of the engine. Then by running the engine at 0.9^5 = 0.60 power level can result in 2^5 = 32 times longer lifetime. For a rocket engine, this would bring the rocket engine lifetime into the range of jet engines of over a thousand reuses. It is notable as well that rocket engines can commonly be safely run at the reduced thrust level of 60%, i.e., this is within the safe throttlebility range.

In reality, this doesn't work so well. To lift a given mass of rocket/payload while using only 60% (3/5) thrust will require engines that are 5/3 more powerful, and - in the best case - proportionally more massive. The challenge is to make up the additonal 2/5 of power without the corresponding increase in mass - i.e. more efficient, next gen tech.

Unlike jets, mass is still at a premium, over and above reusability.



* I think. I am no aerospace engineer.

 

[RobertGC]

It probably has been done, e.g., Space Shuttle Main Engine. One would have to query the team at NASA Stennis Space Center. I believe the SSME power head was disassembled and rebuilt after each flight for inspection, but it's been years since I looked into those details.

A Defining Era: NASA Stennis and Space Shuttle Main Engine Testing​

https://www.nasa.gov/centers-and-facilities/stennis/a-defining-era-ssme/#hds-sidebar-nav-3

SPACE SHUTTLE MAIN ENGINE TESTING: T PLUS 30 YEARS AND COUNTING​

https://www.nasa.gov/news-release/space-shuttle-main-engine-testing-t-plus-30-years-and-counting/


History of Space Shuttle Main Engine Turbopump Bearing Testing at the Marshall Space Flight Center
https://llis.nasa.gov/lessons/24006/History of SSME Turbopump Bearing Testing Howard Gisbon et al released NTRS 20100023061.pdf

How NASA Tested the Space Shuttle: Ground Testing, Avionics, and Data Acquisition Explained​

https://dewesoft.com/blog/how-nasa-tested-space-shuttle

Summary of Results from Space Shuttle Main Engine Off-Nominal Testing
https://ntrs.nasa.gov/api/citations/20120001440/downloads/20120001440.pdf

SPACE SHUTTLE MAIN ENGINE THE FIRST TEN YEARS
https://gandalfddi.z19.web.core.windows.net/Shuttle/SSME_MPS_Info/Space Shuttle Main Engine The First Ten Years - Robert E. Biggs.pdf

Next-Generation RS-25 Engines for the NASA Space Launch System
https://ntrs.nasa.gov/api/citations/20170008958/downloads/20170008958.pdf

NASA, Aerojet Rocketdyne plan busy RS-25 test schedule for 2021​

https://www.nasaspaceflight.com/2020/12/nasa-aerojet-plan-busy-2021-test-schedule/

SSME to RS-25: Challenges of Adapting a Heritage Engine to a New Vehicle Architecture
6TH EUROPEAN CONFERENCE FOR AERONAUTICS AND SPACE SCIENCES (EUCASS) (2015)
https://ntrs.nasa.gov/api/citations/20150016499/downloads/20150016499.pdf



I do not know the programs for Space X or Blue Origin, and the details may be proprietary.

Thanks for those references. I’ll try to contact the authors on the topic.

Robert Clark

 

[RobertGC]

In reality, this doesn't work so well. To lift a given mass of rocket/payload while using only 60% (3/5) thrust will require engines that are 5/3 more powerful, and - in the best case - proportionally more massive. The challenge is to make up the additonal 2/5 of power without the corresponding increase in mass - i.e. more efficient, next gen tech.

Unlike jets, mass is still at a premium, over and above reusability.



* I think. I am no aerospace engineer.

Thanks for responding. Running the engines at reduced power would certainly reduced the payload possible. But the effect of increasing the reuse range of rockets to thousands of flights, comparable to that of aircraft, would radically reduce their costs, finally making spaceflight, even passenger spaceflight routine.

Robert Clark

 

[DaveC426913]

Thanks for responding. Running the engines at reduced power would certainly reduced the payload possible. But the effect of increasing the reuse range of rockets to thousands of flights, comparable to that of aircraft, would radically reduce their costs, finally making spaceflight, even passenger spaceflight routine.

Robert Clark

That may sound plausible but is it true? These factors dont scale linearly. Only doing the math will tell you.

And you cant do the math without a pretty deep dive into a feasibility study, as NASA and other orgs do frequently.

 

[256bits]

TL;DR: Discussion of an approach to high reusability of rocket engines.

A rule of thumb of aircraft engineers is every 10% increase in the power level engines are run at corresponds to a 50% decrease in engine lifetime. This is a general phenomenon of turbomachinery. Then it is likely it also holds for rocket turbopumps.

Is there a valid reference for the claim other than from a chat site.

Aircraft engines are rated more with Low Cycle Fatigue - ie changes in engine power - than sustained engine power. Of course there is a limit to maximum engine power, and one will surely find out quite soon when that is exceeded as the engine flies apart, but they are not to go there due to the immediate catastrophic results.

De-rating an engine can prolong its life to some extent, as creep on rotating parts will be lessened, Start - stops affect bearing lifetimes. Changing power levels affect crack growth.

Since one is interested in optimization of all components of the vehicle ( itself subject to vibrations and induced stresses from air friction and acceleration forces ) so that all have similar lifetimes, or lifetimes to failure, beefing up one component adds unnecessary weight and cost if none of the other components are correspondingly beefed up as well, so as to produce a corresponding vehicle payload.

 

[RobertGC]

A fair question. Google AI cited some references: https://share.google/aimode/lO1qBndTcbhOXQYWG

However, since AI programs are known to have "hallucinations" any references they provide should always be checked. I have confirmed the referenced sources do indeed suggest high increase in engine lifetimes by derating an engine.

 

[berkeman]

Google AI cited some references: https://share.google/aimode/lO1qBndTcbhOXQYWG

Using AI as a technical reference at PF is not allowed. Thread is done.