From Hypersonics to Space Planes: A New Era

The US Defense Department's DARPA is resuming efforts to develop a hypersonic "X-plane (HX). DARPA's plans are ambitious, and will require very rapid innovation and development in order to meet its schedule -- achieving a rocket launched hypersonic craft by the year 2016.

Wired

The hypersonic X-plane (HX) will launch using a disposable rocket stack, unlike previous models which have used space launch rockets, then begin its hypersonic glide. The plan is for the "highly manoeuvrable" vehicle to be recoverable, meaning it will either return to Earth with the help of a parachute, or possibly land on a runway.

Darpa is reigniting the hypersonic flight research with the intention of launching future missions ranging from "space access to survivable, time-critical transport [troop deployment] to conventional prompt global strike." _Wired

Wired

Development of the Skylon spaceplane has passed a critical milestone following tests on the key component for its Sabre engine.

The engine, being developed by Reaction Engines Ltd, looks set to revolutionise not only space travel but also air transport around the world.

It promises to allow a new generation of aircraft to fly from one side of the Earth to the other, e.g. the UK to Australia, in just four hours instead of the 22 or so needed nowadays.

But it will also provide sufficient boost to send the Skylon spaceplane into orbit where it could deliver satellites or link up with the International Space Station.

What makes Sabre different from other aircraft engines is a revolutionary ability to switch from an air-breathing mode to that of a rocket engine.

This hybrid function will allow it to power aircraft at up to five times the speed of sound within the atmosphere or directly into Earth orbit at 25 times the speed of sound. _Wired

Hybrid engines of this type may provide a crucial weight savings in the early stages of the development of workable hypersonic craft. Engineers need to be cautious in avoiding excessive complexity in this new type of engine. KISS (Keep it simple, stupid!) is the key, as far as it is possible.

The type of craft pictured above has been discussed by Brian Wang in multiple articles. It is powered in multiple ways: by chemical rocket, air-breathing propulsion, by MHD, and by dense plasma focus fusion -- all of which should combine to allow it to travel at very high velocities. It will require significant development of several components, including the focus fusion module, before it becomes practical.

The Stratolaunch air launch system pictured above allows for a very versatile approach to orbital insertions. The huge carrier craft serves as the "first stage" of the launch system, carrying the launch craft to altitude and providing a wider range of opportunistic launch attitudes than is conveniently possible for fixed launch sites.

Ideally, all components of the system would be reusable, to save on costs. There are several tradeoffs to be made in terms of launch platforms, launch attitudes, propulsion strategies: pure vs. hybrid vs multiple etc., numbers of propulsion stages, orbital vs sub-orbital flight, method of terrestrial return, and so on.

Hypersonic flight inside the Earth's atmosphere is very punishing, physically. Better types of outer covering materials are needed to provide greater durability for multiple flights. Alternatively, inexpensive spray-on outer skins may be applied prior to each flight, which will naturally ablate in the course of the flight.

An earlier Al Fin article on this topic