Original URL: http://www.theregister.co.uk/2013/04/12/race_for_space_2/

Space elevators, vacuum chutes: What next for big rocket tech?

How boffins hope to escape long shadow of the V2

By Lester Haines

Posted in SPB, 12th April 2013 12:37 GMT

Pic special We recently suggested that even the most advanced rocket currently slipping the surly bonds of Earth is nothing more than glorified V2, over 70 years since Hitler's Vergeltungswaffe 2 first lifted off the pad at Peenemünde.

Today, we'll have a look at some technologies that may one day allow us to escape V2 designer Wernher von Braun's long shadow - the extent of which is clearly demonstrated by a quick look at how far we've advanced since the V2.

Von Braun's rocket was powered by a couple of big tanks of liquid ethanol/water mix and liquid oxygen (LOX)...

A cutaway view of a V2 rocket

The V2 laid bare (click for a bigger version)

...while the Saturn V's first stage used a couple of enormous tanks of kerosene and LOX to get off the ground:

Cutaway view of the Saturn V. Pic: NASA

The Saturn V

Cutaway view of the Ariane 5 on the launchpad. Pic: ESA

The Ariane 5 (click for a bigger version)

The mighty lifter's second and third stage were propelled Moon-wards by liquid hydrogen (LH2) and LOX. Fast forward 40 years, and Europe's Ariane 5's two stages similarly rely on one LH2/LOX unit each.

Looking at cutaways of the three vehicles, it's clear the chap in his cavalry riding breeches admiring the V2 would, if confronted with an Ariane 5, find little has changed in the basic design.

Unlike the V2 and Saturn V, however, the Ariane gets an initial kick from two solid rocket boosters (SRBs), providing 92 per cent of thrust at lift-off.

These burn ammonium perchlorate composite propellant (APCP) - a block of oxidising ammonium perchlorate and aluminium powder, held in a binder such as hydroxyl-terminated polybutadiene (HTPB).

It sounds impressive, although solid-rocket tech is fundamentally unaltered since the Chinese had the bright idea of strapping a cylinder packed with gunpowder to an arrow, thereby creating the fire-arrow projectile.

Solid-fuel rockets are simple and reliable - so simple that while it took a thousand years go from this...

An engraving of a soldier launching a fire-arrow

The fire-arrow: Light blue touchpaper and stand well back

...to this...

A cutaway view of a space shuttle SRB. Pic: NASA

A bigger fire-arrow: The space shuttle's SRB (click for a bigger version)

...even the most advanced SRB can hardly be considered a major scientific advance.

Wings above the world

By the time the last Saturn V thundered heavenwards in May 1973, carrying the orbiting platform Skylab aloft, rocket scientists were eyeing alternative methods for getting payloads - and people - into orbit.

The prototype space shuttle Enterprise rolled off the production line in September 1976, and in April 1981, Columbia reached low Earth orbit. The world's first crewed reusable orbital spacecraft then blazed the trail for 135 missions until the final mission of Atlantis in 2011.

Space shuttle Atlantis blasts off from Kennedy Space Center in May 2009. Pic: NASA/Sandra Joseph-Kevin O'Connell

Space shuttle Atlantis blasts off from Kennedy Space Center in May 2009

For all its advanced design and materials, the shuttle was lifted by two SRBs and the vehicle's trio of LH2/LOX engines, and was arguably nothing more than a reentry-proof glider strapped to a V2 and a pair of massive fire-arrows.

That it was an impressive technological achievement isn't in doubt, but the 1986 Challenger disaster - caused by a failed SRB sealing ring - highlighted the inherent dangers of existing propulsion systems.

This and the 2003 loss of Columbia seriously compromised the US space programme's intended aim of a rapid series of affordable launches, with quick turn-around provided by the reusable vehicle.

The Russian equivalent - Buran - made just one unmanned orbital flight in 1988, in the process becoming the first vehicle to land automatically upon returning from space. It too depended on old-school lifting tech: an Energia rocket boasting four kerosene/LOX-burning boosters around four central core LH2/LOX powerplants.

Buran on the launch pad

Buran on the launchpad

Buran was canned in 1993 due to a lack of cash following the dissolution of the Soviet Union. The US space shuttle similarly succumbed to financial constraints as NASA looked to cheaper methods to reach orbit.

The rocket-launched manned shuttle path to orbit ultimately proved to be a cul-de-sac, although the spaceplane idea lives on in the Boeing X-37.

Artist's impression of the X-37. Pic: NASA/Marshall

Artist's impression of the X-37

Originally designed to be carried in the space shuttle's payload bay, the robotic vehicle's first foray into space was atop a workhorse Atlas V in April 2010. With its own hydrogen peroxide/kerosene motor it is, like the Buran, able to fly itself back to base, following classified missions of several months' duration.

It's an impressive piece of kit, but it falls short of the ultimate goal: to develop a "single-stage-to-orbit" (SSTO) vehicle which powers itself to orbit without the need for a hefty leg-up.

On paper, this seems a simple enough proposition. However, to achieve a stable low Earth orbit (between around 160km and 2,000km altitude), you need to hit a very nippy 7.8km/s (28,080km/h).

This requires tremendous amounts of thrust and, with conventional rocket/jet motor tech, massive amounts of fuel. Before you've even got off the ground, your design is compromised by the huge powerplant and propellant weight.

Here are three very speedy ships, none of which came even close to LEO velocities, and two of which needed conventional aircraft assistance to leave the runway:

The Bell X-1, Lockheed SR-71 and North American X-15. Pics: NASA

The Bell X-1, Lockheed SR-71 and North American X-15

The Bell X-1 required the services of a modified B-29 Superfortress to get up to launch altitude, at which point it was released from the mothership and its Reaction Motors XLR-11 kicked in. Like the V2, this burned ethyl alcohol mixed with water, and LOX, propelling the X-1 through the sound barrier in October 1947, when Chuck Yeager piloted it to 1,126km/h.

The X-1A variant later breached Mach 2, hitting 2,608km/h - again with Yeager at the controls.

The Lockheed SR-71 has since 1976 held the record for the fastest air-breathing manned aircraft, at 3,529.6km/h. Its pair of Pratt & Whitney J58 engines featured intake shock cones designed to keep air flowing into the engine at subsonic speeds, even when the SR-71 hits at least Mach 3 - a vital requirement for conventional jet engines.

The SR-71 gulped a special fuel - JP-7 - with a low flashpoint and high thermal stability, to prevent aircraft skin heating at high speeds and igniting the propellant in the tanks.

This exotic brew is "composed primarily of hydrocarbons, including alkanes, cycloalkanes, alkylbenzenes, indanes/tetralins, and naphthalenes", and is so reluctant to ignite that triethylborane (TEB) must be injected into the engine to start the party. TEB was also used to light the Saturn V's F-1 motors, while Space X's Falcon 9 uses a triethylaluminum/triethylborane combination as an igniter for its first-stage Merlin engines.

The North American X-15 boasts the record for the fastest speed ever reached by a manned aircraft - a whopping 7,274km/h. Early models carried a couple of XLR-11 motors, as used on the Bell X-1, later replaced by two ammonia/LOX Reaction Motors XLR-99s.

Depending on the variant, the X-15 carried enough fuel for between 80 and 150 seconds of motor burn following the drop from a B-52 Stratofortress mothership. Two 1963 flights, with Joseph A. Walker in the cockpit, edged into space at altitudes of 105.9km and 107.8km - a tad over the official boundary at 100km.

Mere mortals wishing to share that experience will soon be able to climb aboard Virgin Galactic's SpaceShipTwo, described as "an air-launched glider with a rocket motor and a couple of extra systems for spaceflight".

Virgin Galactic spaceship and mothership on the tarmac in Mojave, California. Pic: Virgin Galactic

SpaceShipTwo under the Virgin Mothership Eve

After uncoupling from the White Knight Two, aka "Virgin Mothership Eve", at 15,200m, SpaceShipTwo will be propelled to 4,200km/h and a suborbital apogee of 110km.

The glider's engine is worthy of comment. It's a hybrid unit burning solid hydroxy-terminated polybutadiene (HTPB) and liquid nitrous oxide. A similar powerplant - packing HTPB and high-test peroxide (HTP) oxidiser - intended for use in the Bloodhound SuperSonic Car, rattled Cornwall last year during a test of the "biggest rocket fired in the UK for over 20 years".

Air-breathing monsters

So far we have, with the exception of the SR-71, considered purely rocket-powered vehicles. The advantage of the air-breathing jet engine is that it doesn't require an on-board oxidiser, instead relying on atmospheric oxygen to do the job of oxidising the fuel.

That air must be sucked into the engine poses particular problems as speeds increase. As noted, the SR-71's Pratt & Whitney J58's had intake shock cones to keep the flow of air inside the engine at subsonic speeds.

The engineering challenges associated with rotary air compressors and turbines, added to their upper speed limit of about Mach 2.5, make the ramjet an attractive alternative. The incoming air is simply slowed to subsonic speeds by the intake and fed directly into the combustion chamber.

The scramjet (supersonic-combustion ramjet) differs in that the airflow inside the engine is supersonic, offering the possibility of really hitting the gas pedal to above Mach 6, where ramjets' performance is seriously impaired by the dramatic fall in efficiency in slowing the intake air to subsonic speeds.

There is plenty of brainpower currently being dedicated to the scramjet. After a shaky start, NASA's hypersonic X-43 programme hit a high in 2004 when the hydrogen-powered vehicle reached 10,617km/h.

The X-43 and X-51 scramjet vehicles. Pics: Boeing/NASA/DARPA

The X-43 and X-51 scramjet vehicles

In May 2010, Boeing's X-51 "Waverider", burning a JP-7 and ethylene igniter mix and then purely JP-7, thundered to approximately Mach 5 in the "longest supersonic combustion ramjet-powered hypersonic flight to date".

During a second flight in June 2011, the engine fired but didn't transition to pure JP-7 operation. In August 2012, the third launch test ended when the aircraft lost control and crashed into the Pacific.

The evident problems associated with scramjets confirm the assertion that their operation is comparable to "lighting a match in a hurricane and keeping it burning".

There are other impediments to viable scramjet vehicles, not least of which is that the engines require a kick-start. They won't fire unless they've got a good blast of incoming air, which is why the X-43 needed a modified Pegasus rocket first stage - itself carried skywards by a B-52 - to accelerate it to ignition speed.

The X-51 suspended under the wing of a B-52

The X-51 hitches a ride

The X-51 was likewise taken to test altitude under a B-52, where an adapted solid-propellant Lockheed Martin MGM-140 missile drove it to operational velocity.

For the foreseeable future, then, the dream of a scramjet-powered SSTO vehicle, or indeed the fabled London to New York in 30 minutes airliner, will remain just that.

Back on the drawing board, meanwhile, a cunning combination of jet/rocket propulsion is making its pitch for orbital glory. The Skylon team proposes to use "combined air-breathing and rocket cycles" to lift 15 tonnes from a conventional runway into LEO.

Artist's representation of the Skylon. Pic: Reaction Engines

The Skylon

The Skylon's SABRE engines will use LH2 mixed with atmospheric air in a jet phase up to Mach 5.5, at which point they will go into LH2/LOX rocket mode, shoving the aircraft into space at Mach 25.

The SABRE has its roots in the Rolls Royce RB545 engine, developed for deployment in the British HOTOL (Horizontal Take-Off and Landing) vehicle.

Artists impression of HOTOL in flight

HOTOL: It looked good on paper

It remains to be seen if the Skylon goes the way of the HOTOL, and indeed the Lockheed Martin X-33.

Failure of the lightweight composite fuel cells designed to hold the X-33's LH2/LOX led to its cancellation in 2001, although the planned aerospike engine showed promise.

Artists impression of the X-33. Pic: NASA

The X-33

An aerospike relies on ambient air pressure to form one side of a virtual exhaust nozzle, as the rocket exhaust exits across the surface of a wedge-shaped "spike", in the case of the X-33.

The advantage of the design is the elimination of the traditional bell nozzle "point design" restriction, in that they're "designed to provide optimum performance at one certain altitude or pressure".

NASA summarises that "since the combustion gasses only are constrained on one side by a fixed surface - the ramp - and constrained on the other side by atmospheric pressure, the aerospike's plume can widen with the decreasing atmospheric pressure as the vehicle climbs, thus maintaining more efficient thrust throughout the vehicle's flight".

As scientists continue to push the envelope of jet and rocket performance to the limit, there are other options for achieving orbit, some more plausible than others.

More bangs for your bucks

In the 1960s, ballistics engineer Gerald Bull persuaded the Canadian and US defence departments that it might be possible to fire a payload into space from a very large gun.

Cue Project HARP (High Altitude Research Project), which rolled out a US Navy 16in (406mm) piece to lob projectiles over the Atlantic from a base in Barbados.

The HARP gun firing, and as it is today, abandoned in Barbados

The HARP gun in action, and abandoned today

With an initial barrel length of 20m, later extended to 40m, HARP eventually managed to propel a 180kg slug at 3,600m/s to an altitude of 180km.

The project was cancelled shortly after this 1966 high, and Bull went on to develop the Iraqi-funded "Project Babylon" supergun, ostensibly a means of getting satellites into orbit.

Bull's 1990 assassination brought Project Babylon to an abrupt halt, but US scientists continued to work on the spacegun concept.

In 1992, the US fired up its Super High Altitude Research Project (SHARP) light gas gun. SHARP worked by igniting methane to drive a piston which in turn compressed hydrogen, all within a pump tube. At the required moment, the pressurised hydrogen was released behind a projectile sitting in the launch tube, which exited the 10cm diameter, 47m-long "barrel" at tremendous speed.

Artist's impression of the Quicklaunch system

Quicklaunch

SHARP was able to accelerate a 5kg projectile to 3,000m/s. A proposed giant version of the gun - the "Jules Verne Launcher" named in honour of the man whose novel From the Earth to the Moon described a cannon-launched manned Moon mission - never got off the drawing board.

The light gas gun isn't entirely dead, though, as SHARP scientist Dr John Hunter continues to fly the flag with the aquatic Quicklaunch.

The Quicklaunch system will overcome one hurdle facing ground-launched projectiles - that of air resistance slowing their progress through the atmosphere - by deploying rocket motor boost at altitude.

What it won't be able to do, though, is put people into space. Jules Verne's vision didn't encompass the effects of enormous G-forces on the human body, which would relegate any space gun to lifting inanimate objects.

What we need, therefore, is a gentler way to fly, and what better than to float skywards using the lifting power of gas?

This sedate approach relies on the lighter-than-air properties of hydrogen or helium to provide lift. Last year, the Red Bull Stratos mission saw Felix Baumgartner soar to 39,068.5m under a substantial helium-filled globe, so what's to prevent a balloon drifting ever upwards into orbit?

The launch of the Red Bull Stratos. Pic: Red Bull

The Launch of the Red Bull Stratos in 2012

Well, for starters, you've got the fact that as you ascend, the lifting gas expands, meaning you've either got to have a gargantuan envelope to start with, or an envelope capable of stretching to gargantuan proportions, or you'll have to vent some gas, meaning loss of lift.

Wind conditions in the lower atmosphere are severe, so the larger your balloon at ground level, the more vulnerable it becomes as it rises.

To address these issues, the volunteers who form the US's JP Aerospace have drawn up a three-stage "Airship to Orbit" concept, with each of the system's stages designed to operate in its particular operating environment.

Firstly, The "Ascender" hybrid "atmospheric airship", using a combination of buoyancy and aerodynamic lift, is steered by electric-driven propellers to around 42,000m.

There, it docks with a floating "Dark Sky Station", which is the jumping-off point for the third stage: "an airship/dynamic vehicle that flies directly to orbit".

In order to get any kind of lift at that altitude, the orbital vehicle needs to be quite large - over 1.6km long, according to JP aerospace. Once it's ascended to 60,000m, electric motors gradually accelerate it to orbital velocity.

It's an audacious plan, but hardly more bold than the Lofstrom loop - a 2000km-long maglev orbital launch track theoretically capable of flinging manned vehicles of up to five tonnes into orbit.

Artists impression of the loop. Pic: Keith Lofstrom

The Loftsrom Loop

The loop is formed by a sheath containing an iron tube which spins longitudinally within the sheath, causing it to rise from the ground into a tethered arc. When a payload is placed onto the loop, it's magnetically levitated and driven forwards, gradually gaining speed until it's lobbed into orbit.

Related maglev concepts include a circular configuration, around which payloads are accelerated until they can be projected spacewards.

On a more realistic note, NASA has mulled the use of magnetic levitation in hybrid launch technologies, where it might be deployed to give vehicles that first kick.

NASA concept drawing of a maglev launcher. Pic: NASA

NASA's concept of a maglev launch platform

These might propel scramjet-powered vehicles to the velocity necessary for their engines to cut in, without the need for conventional rocket-powered assist.

The StarTram is an advanced version of the maglev-assist idea, in which spacecraft would accelerate in a vacuum up a tube to an altitude of 22km, after which conventional motors would do the rest.

Sketch of the StarTram tube. Pic: StarTram

StarTram

The big downside of maglev launchers is the quantity of juice required to drive them. At Loftsrom Loop scales, just keeping the thing elevated would require an estimated 200MW, meaning quite shocking electricity bills for the operator.

Tripping the light fantastic

Proponents of laser-assisted launch systems might also like to consider their electricity consumption. Dr Jordin T Kare's "modular laser launcher" would require "hundreds or thousands" of ground-based lasers, "each of which transmits a relatively small amount of power to a laser-powered rocket vehicle".

Diagram of the modular laser launcher system

Dr Jordin T Kare's modular laser launch system

Scientists have various flavours of laser propulsion systems on the menu. These include: ablative, which uses a pulsed laser aimed at a solid metal fuel, burning off a thrust-producing plasma; pulsed plasma, where the breakdown of a gas such as air, and the resulting expanding plasma, provides thrust; and laser electric propulsion - the conversion of the beam's energy into electricity, for example via photovoltaic cells.

In 2003, NASA hailed the world's first laser-powered aircraft, which used just such photovoltaic cells collecting energy from a remote laser to drive a small electric motor.

NASA's laser-powered aircraft in flight. Pic: Tom Tschida, Dryden Flight Research Center

NASA's laser-powered aircraft

Such aircraft will fly as long as their panels are targeted by the laser, but that's not really practical for high-altitude operation at long distance. Dr Kare's hybrid solution is to use laser energy to heat a launch vehicle's fuel via a heat exchanger. His multi-laser approach would appear to give a good chance of continuous power delivery, albeit at the cost of a massive ground operation.

Still, the potential environmental impact of vast laser farms spreading across the landscape probably wouldn't be as great as that of using nuclear power to get into space, as suggested by Project Orion scientists.

Artist's impression of the proposed Orion spacecraft heading away from Earth. Pic: NASA

Orion

In 1958, the US started work on a nuclear pulse propulsion system - simply a series of nuclear detonations delivering massive thrust - was theoretically capable of lifting a 3,629 tonne Orion vehicle plus a 1,451 tonne payload into LEO.

The spaceship's massive size was dictated by the smallest nuclear device available, of which 800 would be required to reach LEO. Bombs would be continually ejected at the rate of around one per second through a "pusher plate", explode, and propel the vehicle. The damped pusher plate smoothed the delivery of thrust to the spacecraft above.

Diagram of the Orion propulsion system. Pic: NASA

Very hot stuff: The Orion propulsion system

The yield of each bomb was 0.14 kt - a modest amount but the total for a launch was 112 kt, greater than that of a W76 thermonuclear warhead.

Concerns over fallout led to loss of political support for the programme; the 1963 Partial Test Ban Treaty finally killed it. The technology still has its supporters as a means for interplanetary travel, on the assumption it's used once spacecraft are a safe distance from Earth.

We'll wrap up this look at space launch technologies with what is very much flavour of the month in get-me-off-this-planet circles: the space elevator.

Requiring nothing more than a cable tethered to the Earth's surface and extending beyond geostationary orbit with a "counterweight" at the end, the current space tether concept is the offspring of the space tower proposal to build our way into orbit.

As gravity decreases up the length of the cable, the centrifugal forces increase. The balance of the two keeps the cable taut and the upper end in a fixed position above the surface.

Having got our long rope to the stars in place, we can then ride elevators up the cable into orbit, from where our conquest of the galaxy is assured.

NASA concept picture of a space elevator

Going up: NASA's concept for a space elevator

NASA has taken a keen interest in space elevators, as has the private sector.

Japan's Obayashi Corporation has declared it will be able to whisk passengers upwards by 2050, thanks to carbon nanotube technology.

The Obayashi Corporation concept for a space elevator

The Obayashi Corporation's space elevator

The carbon nanotube is vital to the space elevator. When Russian scientist Konstantin Tsiolkovsky first considered extending the Eiffel Tower into space in 1895, he was doubtless aware that the weight of the beast would simply crush it at the base.

Now that this compression structure plan has given way to a tensile structure model, the weight of the cable still poses a big problem. Lightweight carbon nanotubes are the solution, scientists say, and perhaps within a decade or so we'll be in a position to begin weaving our space cable.

Until that happens, we'll leave you with one last theoretical means of escaping Earth's pull and Wernher von Braun's long shadow, which is to our minds at least as feasible as the space elevator...

Bones, Kirk and Spock prepare to transport in Star Trek

Beam me up, Wernher...

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