College of Engineering Honors Program

12-10-2016

Combustion Tap-Off Cycle

Nicole Shriver Embry-Riddle Aeronautical University, [email protected]

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Honors Directed Study: Combustion Tap-Off Cycle

Date of Submission: December 10, 2016

by

Nicole Shriver [email protected]

Submitted to Dr. Michael Fabian Department of Aerospace Engineering College of Engineering In Partial Fulfillment Of the Requirements Of Honors Directed Study Fall 2016

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1.0 INTRODUCTION

The combustion tap-off cycle is also known as the “topping cycle” or “chamber bleed cycle.” It is an open liquid bipropellant cycle, usually of liquid hydrogen and liquid oxygen, that combines the fuel and oxidizer in the main combustion chamber. Gases from the edges of the combustion chamber are used to power the engine’s turbine and are expelled as exhaust. Figure 1.1 below shows a picture representation of the cycle.

Figure 1.1: Combustion Tap-Off Cycle

The combustion tap-off cycle is rather unconventional for rocket engines as it has only been put into practice with two engines. The most unique aspect of this cycle is that it allows for the engine to go into “idle mode” with a much lower thrust than maximum. The wide range of thrust options allows for this cycle to be throttled, which is very appealing for rockets designed to carry people.

This report will discuss the two applications of the combustion tap-off cycle, the J-2S and BE-3. J-2S is a technology demonstrator and has never been flown before, but has been tested extensively. BE-3 has been tested and flown a number of times and promises to fly the first space tourists by 2018.

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2.0 J-2S

The first combustion tap-off cycle engine made was the J-2S which was developed between 1965 and 1969. This engine was developed by heritage Rocketdyne and although it was never used in flight, it was tested extensively. NASA considered using the engine for many future projects due to its throttling capability, but it was never flown.

The J-2S was designed as a part of the J-2 series which was designed as a replacement for the upper stages of the Saturn vehicle. The S stands for simplified as the J-2S was an upgraded and simplified version of the J-2X (experimental) engine. The J-2S program tested six different flight configurations in sea level, high-altitude, and vacuum conditions. Over the course of 273 tests, the engine gained 30,858 seconds of operational experience.

The most unique part of the J-2S engine was the Idle Mode feature. This feature allowed the engine to operate at low thrusts for a period of time. In Idle Mode, the engine had a nominal thrust of 5,000 pound-force and a mixture ratio of 2.5, relative to the main-stage operation with a thrust between 230,000 and 256,000 pound-force and a mixture ratio of 5.5. A basic schematic of the J- 2S engine can be seen below in Figure 2.1.

Figure 2.1: J-2S Basic Engine Schematic

The tap-off cycle can be seen in the figure above as some of the hot gases are channeled from the edges of the combustion chamber and into a fuel turbopump. The gases power the turbopump and then are expelled as exhaust. With the tap-off cycle, this engine has an Isp of 436.

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3.0 BE-3

BE-3 (Blue Engine-3) is the first combustion tap-off cycle engine ever to have flown on a rocket. It was developed by Blue Origin and first announced in 2013. The engine was designed to fly on their New Shepard vehicle which will bring up to six astronauts at a time to altitudes over 100 kilometers.

Like J-2S, BE-3 operates on liquid hydrogen and liquid oxygen. However, it only provides 110,000 pounds of thrust and can throttle down to 20,000 pounds which allows it to make vertical landings and be reused multiple times. Even though this engine does not have as large of a range of operational thrusts as the J-2S, it works well for the rocket that it has been designed for.

Figure 3.1 below shows a test firing of the BE-3 engine from afar at Blue Origin’s test facility in West Texas.

Figure 3.1: BE-3 Engine Test Firing

The figure above does not show much of the engine, which is due to Blue Origin’s tendency to be secretive about their work. Not much information is available to the public regarding the performance of the engine. All that is known regarding the Isp is that it is “relatively high” as a result of using the combustion tap-off cycle.

The tap-off cycle is a unique one for Blue Origin to choose for its reusable rockets as the cycle had only been built and tested once before with the J-2S engine. Blue Origin states that the cycle is suited for human spaceflight as it has a single combustion chamber and a “graceful” shutdown

4 mode. The single combustion chamber allows for the system to be simpler and therefore less likely to experience a failure. The graceful shutdown mode stems from the throttling capabilities of the tap-off cycle.

Even though the tap-off cycle works for BE-3, the engine does not come without its challenges. The tap-off cycle has a complex start-up system and high temperature turbine gases that drive the combustion process which makes the system a little more complicated. The nature of the cycle also causes it to tend to shut down rather than continue the combustion process. This is due to the fact that it only has one combustor and no pre-burners which is different from many other engine cycles.

Blue Origin is also developing modifications to the BE-3 engine that can be used for an upper stage engine called the BE-3U. A shorter nozzle works well for deep throttling and landing while a large expansion ratio nozzle works well at higher altitudes. These modifications will allow for the engine to be used on the upper stages of rockets for ULA or other contractors.

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4.0 CONCLUSION

The J-2S and BE-3 are both successful examples of the combustion tap-off cycle. The cycle was proven to be both viable and useful by the J-2S which paved the way for BE-3. Even though J-2S never flew, it showed that the tap-off cycle was useful for its throttling capabilities and had a relatively high Isp. The BE-3 engine proved that the cycle could be successfully flown on a rocket, carry people comfortably, and land safely after flight.

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5.0 REFERENCES

J-2 Engine Fact Sheet (1968). In Marshall Space Flight Center. Retrieved December 6, 2016, from https://www.nasa.gov/centers/marshall/pdf/499245main_J2_Engine_fs.pdf

Norris, G. (2013, December 3). Blue Origin Demos New Shepard Mission Engine Cycle. In Aviation Week Network. Retrieved December 6, 2016, from http://aviationweek.com/space/blue-origin-demos-new-shepard-mission-engine-cycle

Our Approach to Technology (2016). In Blue Origin. Retrieved December 6, 2016, from https://www.blueorigin.com/technology

Pillow, C. E. (1970, September). Altitude Developmental Testing of the J-2s Rocket Engine. In Defense Technical Information Center. Retrieved December 6, 2016, from http://www.dtic.mil/dtic/tr/fulltext/u2/874400.pdf

Wade, M. (2016). J-2S. In Encyclopedia Astronautica. Retrieved December 6, 2016, from http://www.astronautix.com/j/j-2s.html

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