Bob clark. can you please stop using guesswork about the skylon design? It would please me greatly if you actually checked the facts before making claims about how something is suposed to work.
The skylon engine does not gimbal. pitch and roll is done with flight controll surfaces. pitch is controlled by forward canards. (those little wing like thingies on the nose of the vehicle you know) and roll is controlled with an all moving tailfin. once out of the atmosphere controll is maintained with the use of fixed forward and aft RCS thrusters.
The main reason for using hydrogen as fuel in skylon is not ISP. it's the fact that a vehicle of this kind needs an enourmous amount of very cold coolant. Luck has it that hydrogen is excelent as both. Denser fuels cannot act as a coolant in this manner, and is hence limited to an air breathing speed below mach 3,5 barring any revolutions in high temperature alloys.
The second advantage to using hydrogen is that it fits with a large volume low density vehicle of the kind that get's to have a fairly benign re-entry. This allows the use of much more dammage resistant materials than what the shuttle is allowed to use. This is of critical importanse to the safety and practicality of skylon as a launch vehicle.
If you actually read the 2004 documents on REL's website you will find the exact trades done when selecting fuel type. ISP was a minor factor. so minor it's hardly even mentioned.
You are the one doing the asuming here. not REL. You are asuming that RP1 is a superior fuel because it's denser. but that is just one factor in the kind of trades you need to do when selecting fuel types.
@Danscope. If everyone was thinking like you we would still be living in caves and eating our food raw. I'f you havent actually noticed yet the skylon vehicle is dependant on a critical item of brand new technology. without whitch a vehicle like skylon would not be capable of working at all. Namely preecooled air breathing rocketry. nobody has made that work all that well before. REL is changing that trend as we speak. so how about waiting with the doom and gloom til they have concluded their technology demonstration programme hey? We will know for sure in 2012 or thereabouts.