University of Strathclyde in cooperation with Reaction Engines Ltd) compared to NASA’s space shuttle.
Rockets are expensive. If they weren’t, we’d be flying to space all the time. But what exactly is it that makes going to the moon so dang pricey? Aside from enormous fuel costs, a large part of the price of space travel can be blamed on the single-deployment designs currently in use. The idea behind rocket staging (the practice of dropping hardware off a rocket as if flies) is this: the less mass you have, the easier it is to accelerate enough to get to and stay in the orbit you want. On top of this, dropping mass actually increases your speed. For these reasons, multi-stage rockets have been used in every successful orbital flight to date.
This strategy has its flaws though. Multi-stage rockets typically employ single use boosters, which means the inclusion of expensive hardware that can only be used once. Even if dropped stages are recovered, they require someone to recover them, which would be a huge problem for planet hopping. Recoverable boosters also require extensive servicing before they can be used again.
Which is why space planes are awesome. They would allow for multiple space flights without the hassle of booster recovery or the cost of disposable boosters. If a successful space plane was built, it would also have a lower overall cost than its staged counterparts. Still, they present an enormous challenge in terms of fuel cost and design. Such a plane would need to generate enough thrust to move around both in earth’s gravity and out of earth’s gravity well into space. Not only this, but it would have to withstand the enormous stress of moving freely into and out of earth’s atmosphere multiple times.
The challenges of SSTO flight haven’t discouraged everyone though. NASA attempted the feat with their X-33 space plane program, which got discontinued in 2001 due to lack of funding. The plane was to use a liquid-hydrogen propellant, stored in tanks that were carefully integrated into the fuselage to reduce drag. An innovative exterior made partly with a honeycombed metal alloy helped protect the plane from the heat of re-entry, as well as basically everything else you could think of including massive vibrations and high-velocity impacts. While the plane was on track to make it to the sky, interior disagreements about the materials for the fuel tanks eventually led to delays and, finally, program cancellation. However, the programs initial success showed that the SSTO vehicle, while elusive, was certainly still possible.
More recently, several companies have been taking their own shots at building a successful SSTO craft. Among them is ARCA, the designer of the Hass 2CA. The main selling feature of this aircraft is its use of an innovative aerospike engine that could maintain efficiency even at low altitudes (this is a problem for most aircraft for reasons explained here). Currently the design is only expected to be able to deliver 220 lbs of payload to orbit, significantly less than modern staged rockets. However, its re-usability means it could theoretically launch multiple times in a single week.
Finally, and possibly most excitingly because it looks awesome, is British company Reaction Engines LTD’s Skylon space plane. Skylon is designed to be a completely reusable launch vehicle, capable of flying in and out of atmosphere with equal ease and of delivering up to 37,000 lbs of payload to low earth orbit and 24,000 lbs even farther to the ISS. This incredibly innovative design will rely on the new, in-development synthetic air breathing rocket engine (SABRE) concept, also a product of Reaction Engine LTD. This engine has two modes, air breathing and rocket mode. In air breathing mode, the plane can reach Mach 5.4 (conventional jet engines can only accelerate a plane to Mach 3 at full throttle). In rocket mode, it not only has the ability to accelerate a craft to Mach 25, but exhibits fantastic efficiency. For obvious reasons, the company is somewhat secretive about their developing technologies. However, it is known that one of the major design features of the SABRE engine is its advanced pre-cooler, which cools the intake air to -150C (while keeping it in gaseous form) then uses the captured heat to turbo-charge the engine. This cooling system overcomes one of the major problems that modern jets face, which is overheating. There is as of yet no timeline for the development of Skylon, but the first tests of the fully assembled SABRE engine are expected to occur sometime in 2020.
On the Horizon 