RocketMotorTwo is the name given by Scaled Composites

Scaled Composites

Scaled Composites is an aerospace company founded by Burt Rutan and currently owned by Northrop Grumman that is located at the Mojave Spaceport, Mojave, California, United States...

 to the rocket engine

Rocket engine

A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

 of their SpaceShipTwo spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, to be used for sub-orbital spaceflight

Sub-orbital spaceflight

A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....

. The rocket motor is being designed and built by Sierra Nevada Corporation

Sierra Nevada Corporation

Sierra Nevada Corporation is an electronic systems provider and systems integrator specializing in microsatellites, energy, telemedicine, nanotechnology, and commercial orbital transportation services. The company contracts with the US military, NASA and private spaceflight companies. The company...

 (SNC).

Test program

Scaled Composites performed different subscale hot-firings between June 2005 and April 2009, before choosing a full-scale rocket motor system design.

During the early test phase of SpaceShipTwo's rocket system, on 26 July 2007, an explosion occurred during a fuel flow test at the Mojave Air & Space Port. The test included filling the oxidizer tank with 4,500 kg (10,000 pounds) of nitrous oxide

Nitrous oxide

Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

 followed by a 15 second cold flow injector test. Although the tests did not ignite the gas, three employees were killed and three injured, two critically and one seriously, from shrapnel wounds.

As of June 2011, six hot-fire tests have been carried out on RocketMotorTwo.