The North Star.
The North Star
Safety is Virgin Galactic's North Star. It is at the heart of the design of our new vehicles and will be engrained in the culture of our space line operation. For nearly 30 years, Scaled Composites, the designers and builders of Virgin Galactic's space launch system, has been responsible for some of the world's most innovative experimental aircraft and has a safety record that is second to none. It has established a culture and an approach to design which like Virgin Galactic, is predicated on safety. Scaled's design philosophy is based on simplicity. The rationale is that a complex system has an increased chance of failure, therefore risk reduction has been the most important design element in the new vehicles.
From the mid 1990's, Virgin looked at many plans for potential space launch vehicles. All were rejected, primarily on the basis that they were fundamentally unsafe by design.
Our excitement in 2002 on discovering Burt Rutan's plans for SpaceShipOne focused on a number of design features which we believed in their own right could make the vehicles many thousands of times safer than any manned space craft of the past. Overlay that with Virgin's experience in transportation operations and there potentially existed a unique opportunity to transform levels of safety from day one; a pre-requisite for any responsible operator and in particular for Virgin Galactic.
Safer by Design
Air Launch not Ground launch.
SpaceShipTwo is a rocket powered space plane and launches horizontally from an aircraft at around 50,000ft rather than the vertically from the ground as has been normal for space craft of the past. Ground launch comes with intrinsic dangers. The craft has to pass through the lower, denser regions of the atmosphere whilst rocket motor's exhaust is ejected at very high velocity and for the motor to work efficiently the spacecraft velocity must also be very high. Of course, traveling at very high speeds in the lower atmosphere creates a great deal of drag, produces high structural loads and needs a stronger heavier fuselage. Large quantities of fuel are required for the longer duration burn, meaning a bigger-still fuselage, leading to yet more weight, leading in turn to a requirement for more fuel to lift the extra weight, and so on. Effectively detonating a huge bomb at ground level means everything has to go right first time - if it doesn't there are generally few options for those inside.
Burt Rutan calculated that the safest and most efficient strategy was to air-launch his spacecraft from around 50,000ft, a height at which it is already above most of the Earth's atmosphere. This also meant that the rocket motor had to burn only for a very short time in order to reach space and that if there were any problems during the boost phase, the rocket motor could simply be shut down and the spaceship would return as a glider to the runway.
Carbon Composite Construction
Scaled Composites - and the clue is in the name - builds its vehicles with the maximum use of composite construction techniques. Both WhiteKnightTwo and SpaceShipTwo are no exception. In WhiteKnightTwo we have produced the largest all-composite aircraft ever built. Carbon fiber composite is a marvellous material; four times the strength of steel and a quarter of its weight, meaning less energy is required to propel both vehicles. However, not only is it very light and strong, but it also has a virtually unlimited fatigue life; as long as the stresses are kept below the ultimate, it does not deteriorate in use in the same way that metal fatigues. It is also easy to modify, as was proved during the SpaceShipOne program when significant aerodynamic changes were made during the flight test program, simply by bonding on additional pieces. Scaled's unique understanding of carbon composite construction techniques in aerospace design is key to the safer by design philosophy that has been central to the Virgin Galactic project.
Hybrid Rocket Motor Technology
Because a rocket has to operate in the very thin upper atmosphere, where oxygen for fuel combustion is scarce, and in space, where there really isn't any, it has to carry its own oxidizer. There are two main types of rocket propulsion: liquid engines and solid motors.
Unsurprisingly, liquid engines mix two liquids together and ignite them to produce thrust. Typically these may be liquid hydrogen and liquid oxygen, both potentially volatile substances that need careful separate storage and highly specialized pumps to supply them to the combustion chamber. Liquid engines have the advantage of high efficiencies; they are throttle-able and can be shut down early if necessary. But they are relatively complex and expensive to build.
Solid rocket motors are like fireworks, a solid mixture of fuel and oxidiser contained in a tube. Just like fireworks,
you light them and off you go. Their great advantage is that they are very simple. But the big disadvantage is that, once lit, they can't be stopped; they burn until all the propellant is used up.
However, there is a third type of rocket propulsion known as a hybrid motor. Here the fuel is in solid form and the oxidizer is a liquid. The passage of the oxidizer over the fuel is controlled by a valve which allows the motor to be throttled or shut down as required.
Hybrid motors offer both simplicity and safety. This is the type of motor that SpaceShipTwo will employ and that was used by SpaceShipOne. It means that the pilots will be able to shut down the SpaceShipTwo rocket motor at any time during its operation and glide safely back to the runway. The oxidizer is Nitrous Oxide and the fuel a rubber compound; both benign, stable as well as containing none of the toxins found in solid rocket motors.
Wing Feathering for Re-Entry
Perhaps the most radical safety feature employed by SpaceshipOne and now SpaceShipTwo is the unique way it returns into the dense atmosphere from the vacuum of space. This part of space flight has always been considered as one of the most technically challenging and dangerous and Burt Rutan was determined to find a failsafe solution which remained true to Scaled Composite's philosophy of safety through simplicity. His inspiration for what is known as the feathered re-entry was the humble shuttlecock, which like SpaceShipTwo relies on aerodynamic design and laws of physics to control speed and altitude.
Once out of the atmosphere the entire tail structure of the spaceship can be rotated upwards to about 65º. The feathered configuration allows an
automatic control of attitude with the fuselage parallel to the horizon. This creates very high drag as the spacecraft descends through the upper regions of the atmosphere. The feather configuration is also highly stable, effectively giving the pilot a hands-free re-entry capability, something that has not been possible on spacecraft before, without resorting to computer controlled fly-by-wire systems. The combination of high drag and low weight (due to the very light materials used to construct the vehicle) mean that the skin temperature during re-entry stays very low compared to previous manned spacecraft and thermal protection systems such as heat shields or tiles are not needed. Following re-entry at around 70,000ft, the feather lowers to its original configuration and the spaceship becomes a glider for the flight back to the spaceport runway.
- Feathered Re-Entry
- The aerodynamics of SS2's pivoted wings act like a shuttlecock, slowing and controlling the spaceship's re-entry.