The Prototype X-Racer
Origins and Overview
The Rocket Racing League contracted with XCOR Aerospace to design and build a prototype Rocket Racer aircraft called the X-Racer. The Prototype X-Racer (referred to here as the X-Racer) is a logical evolution in the design of XCOR's previous flight vehicle, the EZ-Rocket. The X-Racer features a larger, single pump-fed engine with a visually spectacular kerosene exhaust plume. The X-Racer is based on the Velocity SE airframe, a home built aircraft offered by Velocity of Sebastian, Florida. The Rocket Racing League acquired Velocity Aircraft in 2008.
The Velocity SE airframe is modified to carry the XCOR XR-4K14, a 1,500 pound thrust rocket engine, which burns liquid oxygen (LOX) and kerosene. The rear seats are removed to accommodate a LOX tank near the center of gravity (CG), and fuel is stored in the strake tanks. XCOR has integrated its rocket propellant piston pump in the X-Racer, eliminating the need for the pressurized fuel belly tank seen on the EZ-Rocket.
The X-Racer is a single-pilot vehicle with an empty weight of roughly 1,500 lbs and a propellant weight of 1,500 lbs, yielding a gross takeoff weight of 3,000 pounds. The X-Racer has an airframe-limited maximum speed of around 230 mph. This is limited by airframe safety, not engine power.
The X-Racer is the first XCOR vehicle to utilize XCOR's proprietary rocket propellant piston pump. The pump, a result of 10 years of research and development, is a key system component that allows the elimination of heavy, pressurized fuel tanks as well as heavy, high pressure gas bottles. This is one of several proprietary weight saving technologies that enable vehicles such as the X-Racer and other XCOR vehicles to meet exacting performance goals. The lack of a second pressurized fuel tank also allows for easier packaging and integration into a standard airframe, eliminating CG and fit problems associated with multiple pressurized tanks in a small aircraft.
Rapid Turnaround and Non-Toxic Propellants
The X-Racer has the ability to be rapidly refueled, with a demonstrated refueling time of eight and half minutes from chocks in to chocks out. (click for video) XCOR developed and demonstrated a new rapid LOX, kerosene and helium loading procedure for this purpose. The equipment required in each pit stop primarily consists of ground handling equipment for the propellants and standard maintenance equipment for the vehicles. XCOR's engines use non-toxic propellants, making them safer to maintain, operate and handle.
The largest number of flights in one day during our test program was seven, with a rapid turnaround between each flight.
Reliability and Restartability
The XCOR 4K14 engine system features the same ignition interlock technology as all previous and currently envisaged future engines from XCOR; the main fuel valve will not open unless a successful ignition is detected from the electrical spark-torch igniter. Main fuel and LOX valves are coupled together in a single, XCOR-designed bi-propellant valve that prevents one propellant from entering the chamber without the other, further eliminating pooling of propellants. With this design in place, we have never experienced a hard start or other catastrophic engine failure in over 5,000 cumulative engine tests at XCOR.
These same features also allow the X-Racer engine to stop and re-start in flight as often as desired (or as fuel in the tanks will allow). This can be used to prolong flight time, shut down instantly in case of emergency, or to provide an extra margin of safety for glider approaches. As is demonstrated in this static test video, the engine can undergo rapid and successive re-starts without difficulty.
Before the flight test program began, the engine was tested on our traditional ‘5K’ test stand until reaching a level of maturity acceptable for integration. This phase began in May of 2006 and concluded in mid-2007.
Over the summer of 2007, the engine was then mounted on a ‘firewall’ test stand, with the entire aircraft-ready engine assembly, from the firewall back, mounted to the stand. The test operator sat in the front part of the stand, in front of the aircraft instrument panel. It was during this phase of the program that the 4K14 engine was first run with the XCOR rocket propellant piston pump.
In parallel with the engine test program, the Velocity SE airframe was modified to accept the rocket propulsion system and related elements. A large circular access hatch was added into the roof to accommodate the insulated spherical LOX tank that replaced the rear seats. The tank was located for minimal impact on the CG of the aircraft. With the fuel being pump-fed, the standard aircraft strake tanks could be used for the kerosene, eliminating the need for a belly tank.
The piston engine was removed, and the firewall prepped to receive the custom-built truss that housed all of the rocket engine plumbing, pump, valves and control components.
After running reliably on the firewall stand, the engine system was then mounted to the aircraft, and static testing of the engine on the aircraft commenced. A short time later, the X-Racer took its first tentative roll down Runway 30 at the Mojave Air and Space Port. Starting with a two-second burn of the engine, our experienced test pilot (and former astronaut) Richard Searfoss gradually worked up to fast taxis, and then a runway hop. The first flight occurred on October 25, 2007. From there the flight test program continued in a gradual envelope expansion phase, moving on to an up-and-away flight on October 29, 2007. Further flights included positive-G aerobatics such as Immelmans, wing-overs and rolls. These were practiced in anticipation of our exhibition flights.
Flight Test Engineer
The X-Racer has two seats. The right seat is reserved for a flight test engineer. During the flight test program, the flight test engineer’s primary responsibility is to monitor the health of the propulsion system. This job is accomplished through various sensing devices in the engine system as well as a miniature camera mounted in the left vertical stabilizer, aimed at the engine exhaust. There are a total of four cameras mounted on the X-Racer that provide the on-board views seen in our videos, and all four can be viewed in-cockpit via a selector switch.
Primary Flight Test Engineers during the X-Racer program were Mark Street and Douglas Jones. However, in total there were 23 different flight test engineers over 40 flights, to give the broadest number of program staff experience with first-hand rocket operation, but more importantly, to provide a critical evaluation of training materials and monitoring criteria for the X-Racer as developed by the engineering teams. By having numerous staff go through the flight test engineering training program, and then utilize the written and taught processes and procedures, we were able to identify aspects of the process that the original “expert” flight test engineers did as a matter of course during normal and anomalous conditions, and document them for future use.
The test program for the X-Racer has successfully concluded with 40 total flights, including three flights at EAA AirVenture 2008 and 37 test flights in Mojave, seven of which were conducted in a single day.
Oshkosh AirVenture 2008
In 2008, the X-Racer travelled to EAA AirVenture, the largest airshow in the world, held each year in Oshkosh, Wisconsin. (videos) (photos) The X-Racer completed three successful flights, each one starting and finishing absolutely on time. These three flights were conducted under the FAA’s experimental/exhibition certification, and were fully insured by an aviation insurance company. XCOR remains the only company in the world to have exhibited and flown a rocket powered aircraft at an airshow in the western hemisphere. This is XCOR’s second rocket vehicle that has been certificated to perform before the public.
During AirVenture flight #2, the XCOR team was asked 20 minutes before our scheduled flight time to move our flight up by 15 minutes. We were easily able to accommodate the schedule change. Our propulsion systems and vehicles are designed with practicality and flexibility in mind, being able to both hold short or move up our flight time with a minimum of effort. Our later X-Racer test flights in Mojave were scheduled as part of the airport’s normal traffic, including general aviation and other experimental class aircraft. Our vehicles take off as powered aircraft and land as gliders, with re-start capability for go-arounds provided sufficient fuel on board.
The X-Racer holds claim to several other records, including most flights in one day on a manned rocket powered aircraft, and fastest turnaround time for a manned rocket powered vehicle. Flight #40, the last flight, was an up-and-away flight to 12,800 feet lasting over 20 minutes, setting an XCOR company record. It is also the only known rocket powered airplane to have performed several “touch-and go” maneuvers, including with and without a person in the right seat.
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