Next Generation Launch Vehicles

The Space Shuttle has been the workhorse of NASA for the last twenty years and will probably remain so for at least the next ten. But at roughly $600 million a launch, that's one expensive horse. In fairness, the shuttle's achievements should not be overlooked. Its effectiveness at returning payloads to Earth and carrying out in-orbit servicing missions - such as the one that saved the Hubble Space Telescope - are perhaps its biggest asset. (Even at a launch price of $600 million, it was still a lot cheaper than launching a replacement telescope for $15 billion.)

The shape of things to come? From Jumbo Jets to RLVs

	Air Industry	Space Shuttle	Reusable Launch Vehicle
Timeframe	Today		10 Years time?
Launch Costs	$1/lb	$10,000/lb	$1000/lb
Catastrophic Failure	1 in 2,000,000 flights	1 in 200 flights*	1 in 10,000 flights
Crew Escape	Not Required	None	Yes
Fleet Flights Per Year	Millions	10	100
Turnaround Time	1 hour	5 months	1 week
People Required to Launch	None	170	10
Range Safety	Air Traffic Control	Flight Unique	Mission Class Unique

*Estimates for this figure vary from 1 in 100 to about 1 in 450 depending on which source you want to trust. With an ongoing program of upgrades and safety improvements, NASA are hoping to improve this figure to somewhere in the region of 1 in 10,000.

So what are the alternatives to the Shuttle? The technology and politics behind the ever-changing world of launch vehicles could probably fill a web-site in itself, so here instead is a general overview.

Staging (2STO, 3STO etc.)

Staging isn't pretty. A jumbo jet doesn't drop bits of itself when it leaves the runway (if it did, you would have cause to be seriously worried), so why should a rocket?

The reason is that multi-stage rockets remain the only way to effectively get heavy payloads into orbit. Once emptied, a fuel tank is just dead weight to a rocket, so it makes sense to jettison it rather than lug it all the way into orbit. But then again, building a new rocket from scratch every time you want to launch a payload into space is hardly cost effective.

One way round this conundrum is to make all the stages reusable, though this brings a new set of problems to the table; namely: getting an empty fuel tank (with roughly the aerodynamic properties of a giant beer-can) to make a soft landing, controlling where it lands, and getting it ready to fly again within a week. A more elegant solution to the staging problem is to use a jet engine for the first stage and a rocket for the second stage (as discussed below under Spaceplanes). However, elegance is rarely associated with sheer power and such vehicles would probably only be useful for suborbital flights and lifting small satellites to orbit.

Ungainly though it seems, staging isn't going to go away for a while. Even after SSTO vehicles become a reality, multi-staged expendable rockets will still be the only way to lift heavy payloads to Earth orbit and beyond.

Spaceplanes (HTOL)

The traditional science-fiction scenario of a single-staged winged vehicle taking off horizontally from a runway and powering straight into space isn't really practical when you think it through. For a start, jet (air-breathing) engines won't work above a certain altitude (because there's no air!), so the plane would have to carry an additional set of rocket engines, adding greatly to the mass of the vehicle. Similarly, its wings would be of no use in space (again, no air), and so would waste mass that could have been used for payload.

A solution would be to leave the "plane" part of the vehicle behind in the atmosphere and deploy an upper stage that can fire itself into space using rocket engines. In effect this is like a reusable two-stage vehicle where the first stage flies back to a runway instead of falling to Earth like a stone. This strategy is being explored by a number of companies, including Britain's own Bristol Spaceplanes and Mitchell Clapp's Pioneer Rocketplane. The latter company's Black Horse (single stage) and Pathfinder (two stage) designs ingeniously get round the weight problems at launch by taking off with nearly empty tanks and refuelling in mid-air, much like fighter planes do.

Single Stage To Orbit (SSTO)

The Holy Grail of rocket science is to build a launch vehicle that takes off and lands in one piece and can be made ready to fly again within a few days. The challenge is formidable. Long-distance airliners take off with approximately half their mass devoted to fuel. In contrast, a rocket has to be about 90 percent propellant (fuel and oxidiser) by mass to reach orbit. The remaining 10 percent would need to include, among other things, the engines, a thermal protection system for atmospheric re-entry, the overall structure or frame, electronics and a landing gear. This doesn't leave an awful lot of mass for payload.

However, the concept of SSTO was at least partly validated in 1993 when the DC-X (for Delta-Clipper Experimental) managed to complete several subsonic test flights. Put together for just $60 million, the DC-X was a scale prototype for a potential SSTO vehicle, employing a vertical-take-off-and-landing (VTOL) conical, wingless design. Its success prompted NASA to invite bids to build its own SSTO test vehicle - the X-33.

Lockheed Martin won the contract ahead of bids from McDonnell-Douglas (who proposed an enlarged version of the DC-X) and Rockwell. Their vehicle (right), resembling (in appearance at least) an evolved version of the Space Shuttle, was to take off vertically and land horizontally (VTOHL) and was supposed to have led to a commercial follow-on called the "Venture Star".

So where does that leave SSTO? The leading aerospace companies are reluctant to invest too much money into developing reusable launch vehicles as it would make a large portion of their existing fleets of expendable rockets redundant. Therefore, it's up to the private entrepreneurs to lead the way. There are plenty of schemes already on the drawing board, including Rotary Rocket Company's ROTON, a VTOL rocket which uses rotating engines at launch and deploys helicopter blades after re-entry to control its descent.

For more descriptions and pictures of some of the proposed launch vehicles, have a look at the SpaceFuture site. (It takes a while to load, so be patient - or put the kettle on!) There are a multitude of ideas (some exotic, others based on tried-and-tested designs) and given the large investments required, it's likely that the majority of these schemes will never even be built, let alone make it into space. However, the first ones to succeed could find themselves opening the floodgates to a whole new era of space exploration. The risks are high, but the incentive is huge and, as you'll see on the next page, there are more than just business contracts at stake...

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