Wednesday, January 29, 2014

Transitioning SpaceShipTwo to liquid fueled engines: a technology driver to reusable orbital launchers.

 Copyright 2014 Robert Clark

 A new book by Tom Bower on Richard Branson, “Branson: Behind the Mask”, claims the hybrid engine on SpaceShipTwo still does not have enough power to get the vehicle to the altitude for suborbital flight. Doug Messier on his blog has been reporting on the technical problems developing the hybrid engine for some time.

 There has been much speculation actually that Virgin Galactic will have to transition to a liquid fueled engine to achieve suborbital flight. In point of fact, independent studies have shown that SS2 by switching to liquid fueled propulsion, can be suborbital on its own without even needing the carrier craft WhiteKnightTwo:

SpaceShipTwo could be single stage to suborbit says ESA firm

Reusable Space Plane Idea Intrigues Europeans.
Rob Coppinger, Contributor
Date: 01 May 2012 Time: 04:30 PM ET
The Vinci suborbital space plane's structure and cryogenic fuel and oxidizer tanks are depicted in this illustration.
Credit: ESA

  This would be by using the hydrogen-fueled Vinci engine. The Vinci is soon to be introduced on the Ariane 6. However, the existing HM-7 engine used on the cryogenic version of the upper stage of the Ariane 5 could also be used. The advantage of this is that it has been in use for decades and is well-characterized. You would probably need to place an extra one on the Ariane 5 upper stage to be able to lift the SS2. Still the engine and the stage are already developed and the cost of the addition of an extra engine should be comparatively small. The development cost of the SS2+WK2 combo has reportedly reached into the few hundred million dollars range. In contrast, the addition of an already existing engine to an already existing stage should be simpler, quicker and far cheaper than creating a new engine, hybrid, from scratch.

 The reason for the choice of the hybrid for the SS2 rather than a higher performance liquid-fueled engine was the idea that a hybrid engine could not explode. However, the accident in 2007 at Scaled Composites due to a nitrous oxide explosion has destroyed that misperception. Indeed because of the instability of nitrous oxide one team involved in developing a rocket propelled car suggests nitrous should not be used for passenger flight:

Observations and comments on Cal/OSHA report (Inspection No: 31081103) on fatal accident at Mojave test site of Scaled Composites at the Mojave Air and Space Port, 26th July 2007.
While it is most advisable to apply the established safety protocols relating to liquid oxygen, such protocols, in themselves are not sufficient to ensure the safe handling of Nitrous Oxide. The unique physical properties of N2O require further protocols above and beyond those used for liquid oxygen.
Safety protocols for N2O, in a rocket motor system, should include (in addition to the protocols used for Liquid Oxygen)

   * The detailed study of materials compatibility of all components in the system
   *  Avoidance of high temperatures at all points in the system
   * Stirring of large tanks
   * Avoidance of the gaseous phase both during apparatus filling and in use
   *  Purging of lines and valves immediately prior to ignition
   * Not using any component that may have previously absorbed N2O –   especially fuel grains

We are not confident that, even with these additional precautions, that we yet know enough about N2O to consider it a safe oxidiser for use in passenger flight. In the light of what we do know, safety must remain a major concern.

  Then the SS2 hybrid engine should no longer be considered to have an advantage over a liquid fueled engine. Then the fast and low cost development possible, especially with using an already existing engine, should push the decision to using liquid fuel. In fact by doing so SS2 probably could already have been flying by the originally announced date of the first suborbital test flights of 2007.

 The importance of their making that decision then and of their making that decision now goes far beyond that of just suborbital rockets however. If you look at the specifications of the cryogenic Ariane 5 upper stage, you see it could be propelled, with the SS2 aeroshell around it, well above the speed needed for suborbital flight. In fact it could be in the high Mach range envisioned for example for the X-33. A stage like that though could be used for a reusable first stage booster for a two-stage to orbit system.

 Now, since the first stage is generally much larger and costlier than the upper stage, a reusable first stage could significantly cut the cost to orbit of a two stage system. This in fact is what DARPA wants with its reusable spaceplane program.

 So Virgin Galactic giving SS2 a liquid fueled propulsion system could have a system to satisfy the requirements for DARPA's reusable spaceplane. In fact, it could already have had such in 2007.

   Bob Clark