Lecture 3.2: Hybrid Rocket Throttling
Liquid Rocket Engine
Space Shuttle SSME and RSRM Rockets
Solid Rocket Motor
Hybrid Rocket Motor
1! MAE 6430 - Propulsion Systems, II! Chemical Rockets! Solid Rockets ! Oxidizer and fuel are chemically mixed together at the molecular level to form a solid fuel grain. Once ignited, they cannot be stopped, throttled, or Solid Rocket restarted.! Motor!
Liquid Rockets ! In a bi-propellant liquid rocket, an oxidizer and fuel are mixed in the combustion chamber. Oxidizer is usually maintained at cryogenic temperatures, typically requiring turbo pumps. Mono-propellant liquid rockets use a material which combusts in the presence of a catalyst. Liquid rockets can be throttled, stopped, and restarted.! Liquid Rocket Engine! Hybrid Rockets ! Possess features of both liquid and solid rockets. A hybrid consists of a solid fuel grain made from a polymeric material. The oxidizer is stored in a tank separate from the fuel grain, which is stored in a combustion chamber. Both propellants are inert and only combust when the fuel is converted to gaseous state and mixed with oxidizer in the combustion chamber. Like liquid Hybrid rockets, hybrid rockets have the potential for throttle and multiple restarts.! Rocket Motor!
2! MAE 6430 - Propulsion Systems, II! How about Hybrid Throttling?!
• * #Th % & - mox = A C 2ρ × p − P ox dox ox , $ ox ' 0 / + % 100 ( . • Only the Oxidizer rate can be modulated … ! … no direct means to control fuel massflow and burn O/F ratio!
… First lets revisit bi-propellant liquid rocket throttling!
3! MAE 6430 - Propulsion Systems, II! Hybrid Motor Throttling
In theory a rocket engine can be throttled back until the throat is no longer ! sonic by reducing propellant flow rate (injector pressure)!
Difficult problem in practice for conventional liquid rockets! • Essential for pressure drop across injector > 25% of chamber pressure! !-- Pressure ratio insures propellant flow rates ! !are independent of fluctuations in chamber pressure. !
• Fixed geometry injectors – Injector Feed-Coupling Instability! !Reduction of Propellant flow rates causes injector !pressure to drop faster than the chamber pressure ! !… until injector pressure becomes so low that !coupling between chamber and propellant feed system !occurs … causing combustor instability (a.k.a explosion or !more likely flameout)!
• Typically, liquid rocket engine with fixed injector geometry can be throttled down to 60-70% of nominal thrust without stability problems ! 4! MAE 6430 - Propulsion Systems, II! 27! Mechanism for Bi-Prop Pressure Throttling!
* γ +1 - ∂P , A* $ 2 ' (γ −1) / 0 = −P γ R T + ∂t 0 , V g 0 %& γ + 1() / , c / + . * - RgT0 $ 2Th % 5 ' $ 2Th % 5 ' ,AfuelCd 2ρ f 3 × p f 6 − P0 + AoxCd 2ρox 3 × pox 6 − P0 / V f %& 100 () ox %& 100 () c +, 4 7 4 7 ./
• By modulating the injector pressures, we can throttle! the rocket engine … must properly Phase propellant feed pressures .. !
… Injector pressure ratio > 1.25 for combustion stability!
… Inherent Latency in massflow and thrust response!
5! MAE 6430 - Propulsion Systems, II! A Mechanism for Throttling (cont’d)!
60%! Throttle!
Injector pressure! Ratio ---> 1!
6! MAE 6430 - Propulsion Systems, II! Hybrid Motor Throttling!
Typical Chamber Pressure Throttled Thrust Profile!
Liquid Bi-Prop engine! Stability limit at! Injector pressure ratio ~ 1.25! Injector Feed- Coupling Instability!
Commanded Throttle = Actual Thrust/Max Thrust! 7! 28! MAE 6430 - Propulsion Systems, II! Bi-Prop Injector Area Throttling!
* γ +1 - ∂P , A* $ 2 ' (γ −1) / 0 = −P γ R T + ∂t 0 , V g 0 %& γ + 1() / , c / + . * - RgT0 $ 2Th % 5 ' $ 2Th % 5 ' ,AfuelCd 2ρ f 3 × p f 6 − P0 + AoxCd 2ρox 3 × pox 6 − P0 / V f %& 100 () ox %& 100 () c +, 4 7 4 7 ./
• Can directly throttle by modulating the injector feed area!
• Variable geometry injectors complex to implement in practice!
… practice? … Pintle Injectors !
8! MAE 6430 - Propulsion Systems, II! Deep-Throttle Rocket Engines" (cont’d)! • Pintle Injectors!
* γ +1 - ∂P , A* $ 2 ' (γ −1) / 0 = −P γ R T + ∂t 0 , V g 0 %& γ + 1() / , c / + . * - RgT0 $ 2Th % 5 ' $ 2Th % 5 ' ,AfuelCd 2ρ f 3 × p f 6 − P0 + AoxCd 2ρox 3 × pox 6 − P0 / V f %& 100 () ox %& 100 () c +, 4 7 4 7 ./ • By moving the pintle shaft! Propellant mass flow is modulated! by changing the effective injector area!
• Injection pressure remains relatively! constant … reducing combustion instability! problem …. Complex jack-screw mechanism! 9! MAE 6430 - Propulsion Systems, II! Deep-Throttle Rocket Engines" (cont’d)!
• Low Cost Pintle Engine (LCPE)!
The key element of the LCPE's design is its ! single element coaxial pintle injector, used to ! introduce propellants into the combustion chamber.!
• Moveable pintle injector attributes include ! deep throttle capability, 10:1 turndown ratio! (LCPE) TRW has tested more than 50 different pintle injector engines, ! using more than 25 different propellant combinations with ! complete combustion stability!
Range in size from the 100-pound thrust liquid apogee engine ! used on NASA's Chandra X-ray Observatory to the ! 10,0-pound thrust Delta and LMDE engines.!
10! MAE 6430 - Propulsion Systems, II! Deep-Throttle Rocket Engines" (cont’d)!
11! MAE 6430 - Propulsion Systems, II! Deep-Throttle Rocket Engines" (cont’d)! • SSME Throttle Simulation using Pintle Injector (Area Modulation)!
12! MAE 6430 - Propulsion Systems, II! Applications of Deep Throttling (I)! • Lunar Lander … Throttling key element in ability! to perform precision landing … Apollo --> Open loop throttle! under control of mission commander! • Apollo Lunar Module Descent Engine • Highest Operational (LMDE)! - The key element of the LMDE's design “turndown ratio” Engine! was its single element coaxial pintle injector, used to introduce propellants into the •Lunar Module descent engine, 10:1 !combustion chamber.! turndown ratio!
•Even with a variable-geometry injector !there were stability Problems!
•Thrust levels between 100% and 65% were never used because mixture ratio was so hard to properly control.!
13! MAE 6430 - Propulsion Systems, II! Applications of Deep Throttling (II)" Throttling for Applications Cruise/Glide Weapons Potential for throttling and motor restart is also very advantageous for cruise or guided glide munitions.! • Throttle operation can reduce airframe loads during launch.! -Thrust levels can be reduced as airframe loads (and aero control effectiveness) increases. ! • The ability to shutdown and restart upon command is very desirable for allowing multiple target access or range extension. !
14! MAE 6430 - Propulsion Systems, II! 26! Hybrid Motor Throttling!
Pressure–Throttled Hybrid Motor, a less complex alternative to the Pintle engine!
• Pintle injector rocket engines are complex devices with many movable parts, applicable to both liquid and solid rockets.!
!- Solids do not have restart capability! !- Liquid systems have long term storability issues! • Hybrid motors offer lower specific impulse compared to liquid rockets; however, !
!- Hybrids will operate at very low injector pressure ratio without ! combustion instability! !- Well suited for packaging in “long-skinny” high fineness ratio bodies! !- Hybrid motor throttling currently has modetate-to-low low Technology ! Readiness Level (TRL) < 5 !
15! MAE 6430 - Propulsion Systems, II! 30! Hybrid Motor Throttling!
Only the Oxidizer rate can be modulated! Throttling not as “crisp” as with liquid rockets, but still works!
… Oxidizer flux … feedback mechanism effects the fuel burn rate or mixture! ratio of combustion .. “self adjusting” O/F ratio .. Proportional to oxidizer mass flow! to 1/5th power! $ A '4 /5 0.23 cport $ h ' !& ) "solid 2/3 & fuel ) % Aburn ( 1/5 M 21.2766 P m O/F = ! r !& ) ! 1/5 ! ( ! ox ) &#hflame ) $ µe ' % fuel ( &Aburn ! ) % L (
16! MAE 6430 - Propulsion Systems, II! 31! Hybrid Throttling Simulation
Fixed area injector!
100% to 60% throttle!
17! MAE 6430 - Propulsion Systems, II! Rocket Crafters™, All Rights Reserved, 2011 PROPRIETARY & CONFIDENTIAL! 32! Hybrid Throttling Simulation! Motor Mass flow, kg/sec! Fixed area injector! 100% to 60% throttle!
Fuel regression rate, cm/sec!
18! MAE 6430 - Propulsion Systems, II! 33! USU Throttled Hybrid Research Topics • How low can we go with injector pressure ratio and still achieve adequate combustion?! … can be done with fixed geometry injector and! Ox feed line regulator! • Develop feedback laws for “achieved” thrust versus injector input pressure.!
• Methods for variable geometry injector:! !- Pintle, variable port size, number injector! !- Variable port injector can be coupled with! - Throttled injector pressure (coarse and fine control)!
• What is the response fidelity, repeatability?!
19! MAE 6430 - Propulsion Systems, II! 36! USU Throttled Hybrid Research (2) • See linked papers of web page, section 3!
•Whitmore, S. A., Peterson, Z. W., and Eilers, S. D., "Closed- Loop Precision Throttling of a Hybrid Rocket Motor," J. Propulsion and Power, Vol. 30, No 2., March 2014, pp. 325-336.!
• Whitmore, S. A., Peterson, Z. W., and Eilers, S. D., " Deep Throttle of a Nitrous Oxide and Hydroxyl-Terminated Poly- butadiene Hybrid Rocket Motor," J. Propulsion and Power, Vol. 30, No 1., January 2014, pp. 78-86.!
20! MAE 6430 - Propulsion Systems, II! 36! USU Throttled Hybrid Research (3)
Test Apparatus, Piping and Instrumentation (P&ID)!
Motor Layout!
21! MAE 6430 - Propulsion Systems, II! 36! USU Throttled Hybrid Research (4) PID Control Law!
Control Signal Logic and! Functional Block Diagram!
• Design used a “less than Ideal” cheap industrial actuated! ball valve with very narrow response band !
22! MAE 6430 - Propulsion Systems, II! Throttle Valve Response Curve! USU Throttled Hybrid Research (5)
Chamber Pressure Response to % of Maximum Valve Travel (%MVT)!
Still very effective system! response to valve command! Thrust and chamber pressure from typical Open-loop throttle test 23! MAE 6430 - Propulsion Systems, II! USU Throttled Hybrid Research (6) Closed Loop Throttle Response!
Ramp and Step Command, Step and Ramp Command, Thrust Feedback! Chamber Pressure Feedback!
24! MAE 6430 - Propulsion Systems, II! USU Throttled Hybrid Research (5)
Injector-Feed Coupling Instability!
Deep Throttle Test 25! MAE 6430 - Propulsion Systems, II! 36! USU Throttled Hybrid Research (6)
Injector-Feed Coupling Instability!
Deep Throttle Test 26! MAE 6430 - Propulsion Systems, II! 36! USU Throttled Hybrid Research (7)
Injector-Feed Coupling Instability!
27! MAE 6430 - Propulsion Systems,Deep II! Throttle Test USU Throttled Hybrid Research (8) Vapor cavitation through injector! Injector-Feed For % MVT < 30%! Coupling Instability! Likely cause of feed instability!
Still … !
Deep Throttle Test
28! MAE 6430 - Propulsion Systems, II! Questions??
MAE 6430 - Propulsion Systems, II! 29