Description
RocketMotorTwo is a hybrid rocket engine utilizing solid hydroxyl-terminated polybutadiene (HTPB) fuel and liquid nitrous oxide oxidizer – sometimes referred to as an N2O/ HTPB motor – providing of thrust. The design makes use of lessons learned during the development of the SpaceShipOne hybrid rocket motor.Background
, Sierra Nevada had performed over 300 hybrid rocket test firings. The company also developed the rocket engine for the first private spacecraft to reach space, SpaceShipOne, which won the Ansari X-Prize in 2004. Sierra Nevada is also developing a similar hybrid engine, the Dream Chaser rocket engine.History
Related test program accident
On 26 July 2007, during the early vehicle subsystem testing phase, an explosion occurred during a propellant flow test by Scaled Composites at the Mojave Air and Space Port. The test included filling the oxidizer tank with of nitrous oxide, followed by a 15-second cold flow injector test. The test did not ignite the engine and no solid rocket fuel was involved. Three Scaled employees were killed and three injured, two critically and one seriously, by flying shrapnel when the nitrous oxide oxidizer spontaneously ignited and exploded (dissociation into N2 and O components, releasing energy).Abdollah, Tami and Silverstein, Stuart (27 July 2007)First-generation engine
Hot-fire ground tests
Scaled Composites performed a series of subscale rocket hot-firings between June 2005 and April 2009, before choosing a full-scale rocket motor design. News reports in early 2009 reported that the hot fire tests had been "completed" and that Spacedev (later acquired by Sierra Nevada Corporation) had been contracted by Scaled Composites to assist Scaled in developing SS2. By December 2011, 21 full-scale hot-fire ground tests had been carried out on RocketMotorTwo. "Virgin Galactic successfully completes SpaceShipTwo glide flight test and rocket motor firing on same day"Extended delays in flight testing
Despite statements in early 2009 that flight testing of RocketMotorTwo was planned for later in 2009, no flight tests took place in 2009, 2010 or 2011, and only glide flight tests—with the engine installed in SS2—occurred in 2012. In July 2012,2014 change of engine
In May 2014, Virgin Galactic announced a change to the hybrid engine to be used in SpaceShipTwo, and took the development effort in-house, terminating the contract with Sierra Nevada and halting all development on the first-generationSecond-generation engine
The engineering and development work of the second-generation engine was done in-house, by Virgin Galactic. The work began on the new formulation for the hybrid engine in 2013, and by May 2014—when SNC's involvement with SS2 propulsion using the first-generation rocket motor was ended—the new engine formulation had already completed full-duration burns of over 60 seconds in ground tests on an engine test stand.New fuel formulation
Rather than use rubber-based HTPB in the solid portion of the hybrid rocket motor—which had experienced serious engine stability issues on firings longer than approximately 20 seconds with the first-generation engine—the Virgin Galactic-developed SS2 hybrid rocket engine would now use thermoplastic polyamide (i.e., nylon) as the solid fuel component of the propellant. The plastic fuel was projected to have better performance (by several unspecified measures) and was expected to allow SpaceShipTwo to make flights to a higher altitude.Related airframe and subsystem modifications to support the new engine
The second-generation engine design also required the modification to the SS2 airframe to fit additional tanks in the wings of SpaceShipTwo—one holdingGround test regime
Even though Virgin Galactic has run a number of ground tests on the new engine by May 2014, it was stated then that four additional ground tests of the polyamide-fueled engine were anticipated before the SpaceShipTwo flight tests could resume with the new-fuel rocket motor.PF04 flight test malfunction
On 31 October 2014, the new polyamide engine fuel formulation was used in flight for the first time in the powered test flight no. 4 (PF04) of SpaceShipTwo. At 10:12am PDT, VSS Enterprise suffered a malfunction, and subsequently broke up in mid-flight. The inflight mishap resulted in the death of one test pilot and a "serious shoulder injury" to the other test pilot, and a total loss of the vehicle.Mojave Air and Space Port press conference on October 31, 2014, at 2:00 p.m. PDT — involving: the Spaceport, Sheriff's Department, County Fire Department, Scaled Composites, Virgin Galactic The accident investigation revealed that the "ship’s fuel tanks and its engine were recovered intact, indicating there was no explosion ... no signs of burn-through, no signs of being breached". While the US government National Transportation Safety Board (NTSB), which is leading the investigation, has released only "statements of fact", space journalist Irene Klotz has stated more explicitly that " wasn’t SpaceShipTwo’s hybrid rocket motor – which was flying ... with a new type of fuel – that caused the fatal crash." The NTSB final report attributed that the cause was 1) pilot error and 2) inadequate cockpit and procedure design that tended to make a certain high-stress situation more consequential for the pilot than it perhaps should have been.Production
The HTPB hybrid rocket motor and its oxidizer valve system were produced in SNC's manufacturing facility in Poway, California in conjunction with Scaled Composites. In 2013, the Poway facility was reported to be "currently producing motors for both SpaceShipTwo and SNC’s own Dream Chaser orbital crew vehicle". SNC closed the Poway facility in late 2014. The polyamide hybrid rocket motor is a modified version of the polybutadiene version, with different oxidizer valve arrangement. Most of the engine remains the same, but for changing the fuel cartridge from rubber to plastic fuel.See also
* Dream Chaser rocket engineReferences
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