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Reusable launch vehicle

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A reusable launch system (or RLV: reusable launch vehicle) is a launch vehicle which is capable of launching into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded.

No true orbital reusable launch system is in use as of August, 2006. The closest example is the partially reusable Space Shuttle. The orbiter, which includes the main engines, and the two solid rocket boosters, are reused after several months of refitting work for each launch. The external fuel drop tank is discarded.

Orbital RLVs are thought to provide the possibility of low cost and highly reliable access to space. However, reusability implies weight penalties such as reentry shielding and possibly a stronger structure to survive multiple uses, and given the lack of experience with these vehicles, the actual costs and reliability are yet to be seen.

History

File:Destination Moon DVD.jpg
Destination Moon (film)

As usual, science fiction preceded science fact in this area. In the early 1950s popular science fiction often depicted space launch vehicles as either single-stage reusable rocketships which could launch and land vertically (SSTO VTVL), or single-stage reusable rocketplanes which could launch and land horizontally (SSTO HTHL).

The realities of early engine technology with low specific impulse or insufficient thrust-to-weight ratio to escape our gravity well, compounded by construction materials without adequate performance (strength, stiffness, heat resistance) and low weight seemingly rendered that original single-stage reusable vehicle vision impossible.

However advances in materials and engine technology have rendered this concept potentially feasible.

Before VTVL SSTO designs came the partially reusable multi-stage NEXUS launcher by Krafft Ehricke. The pioneer in the field of VTVL SSTO, Philip Bono, worked at Douglas. Bono proposed several launch vehicles including: ROOST, ROMBUS, Ithacus, Pegasus and SASSTO. Most of his vehicles combined similar innovations to achieve SSTO capability. Bono proposed:

  • Plug nozzle engines to retain high specific impulse at all altitudes.
  • Base first reentry which allowed the reuse of the engine as a heat shield, lowering required heat shield mass.
  • Use of spherical tanks and stubby shape to reduce vehicle structural mass further.
  • Use of drop tanks to increase range.
  • Use of in-orbit refueling to increase range.

Bono also proposed the use of his vehicles for space launch, rapid intercontinental military transport (Ithacus), rapid intercontinental civilian transport (Pegasus), even Moon and Mars missions (Project Selena, Project Deimos).

In Europe, Dietrich Koelle, inspired by Bono's SASSTO design, proposed his own VTVL vehicle named BETA.

Before HTHL SSTO designs came Eugen Sänger and his Silbervogel ("Silverbird") suborbital skip bomber. HTHL vehicles which can reach orbital velocity are harder to design than VTVL due to their higher vehicle structural weight. This led to several multi-stage prototypes such as an suborbital X-15. Aerospaceplane being one of the first HTHL SSTO concepts. Proposals have been made to make such a vehicle more viable including:

  • Rail boost.
  • Use of lifting body designs to reduce vehicle structural mass.
  • Use of in-flight refueling.

Other launch system configuration designs are possible such as horizontal launch with vertical landing (HTVL) and vertical launch with horizontal landing (VTHL). One of the few HTVL designs made is Hyperion by Philip Bono. X-20 Dyna-Soar is one example of an early VTHL design.

The late 1960s saw the start of the Space Shuttle design process. From an initial multitude of ideas a two-stage reusable VTHL design was pushed forward. That eventually ended up as a reusable orbiter with an expendable drop tank and reusable solid rocket boosters to reduce design expenses.

During the 1970s further VTVL and HTHL SSTO designs were proposed for solar power satellite and military applications. There was a VTVL SSTO study by Boeing. HTHL SSTO designs included the Rockwell Star-Raker and the Boeing HTHL SSTO study. However the focus of all space launch funding in the United States on the Shuttle killed off these prospects. The Soviet Union followed suit with Buran. Others preferred expendables for their lower design risk, and lower design cost.

Eventually the Shuttle was found to be expensive to maintain, even more expensive than an expendable launch system would have been. The cancellation of a Shuttle-Centaur rocket after the loss of Challenger also caused an hiatus that would make it necessary for the United States military to scramble back towards expendables to launch their payloads. Many commercial satellite customers had switched to expendables even before that, due to unresponsiveness to customer concerns by the Shuttle launch system.

Then the Soviet Union imploded and the cost of Buran became untenable. Russia has only used pure expendables for space launch since.

The 1990s saw interest in developing new reusable vehicles. The military Strategic Defense Initiative ("Star Wars") program "Brilliant Pebbles" required low cost, rapid turnaround space launch. From this requirement came the McDonnell Douglas Delta Clipper VTVL SSTO proposal. The DC-X prototype for Delta Clipper demonstrated rapid turnaround time and that automatic computer control of such a vehicle was possible. It also demonstrated it was possible to make a reusable space launch vehicle which did not require a large standing army to maintain like Shuttle. From the commercial side, large satellite constellations such as Iridium were proposed which also had low cost space access demands. This fueled a private launch industry, including partially reusable vehicle players, such as Kistler, and reusable vehicle players such as Rotary Rocket.

The end of that decade saw the implosion of the satellite constellation market with the bankruptcy of Iridium. In turn the nascent private launch industry collapsed. The fall of the Soviet Union eventually had political ripples which led to a scaling down of ballistic missile defense, including the demise of the "Brilliant Pebbles" program. The military decided to replace their aging expendable launcher workhorses, evolved from ballistic missile technology, with the EELV program. NASA proposed riskier reusable concepts to replace Shuttle, to be demonstrated under the X-33 and X-34 programs.

The 21st century saw rising costs and teething problems lead to the cancellation of both X-33 and X-34. Then the Space Shuttle Columbia disaster and another grounding of the fleet. The Shuttle design was now over 20 years old and in need of replacement. Meanwhile the military EELV program churned out a new generation of better expendables. The commercial satellite market is depressed due to a glut of cheap expendable rockets and there is a dearth of satellite payloads.

Against this dire backdrop came the Ansari X Prize contest, inspired by the aviation contests made in the early 20th century. Many private competitors disputed the Ansari X Prize, the winner being Scaled Composites with their reusable HTHL SpaceShipOne. It won the ten million dollars, by reaching 100 kilometers in altitude twice in a two week period with the equivalent of three people on board, with no more than ten percent of the non-fuel weight of the spacecraft replaced between flights. While SpaceShipOne is suborbital like the X-15, some hope the private sector can eventually develop reusable orbital vehicles given enough incentive. SpaceX is a recent player in the private launch market which has partially reusable vehicles.

Reusability concepts

Single Stage

Single stage to orbit requires very lightweight structures, high efficiency engines and usually implies small margins.

Two or more stages to orbit

Two stage to orbit requires designing and building two independent vehicles and dealing with the interactions between them at launch. Usually the second stage in launch vehicle is 5-10 times smaller than the first stage, although in bimese and trimese approaches each vehicle is the same size.

In addition, the first stage needs to be returned to the launch site for it to be reused. This is usually proposed to be done by flying a compromise trajectory that keeps the first stage above or close to the launch site at all times, or by using small airbreathing engines to fly the vehicle back, or by recovering the first stage downrange and returning it some other way (often landing in the Sea, and returning it by ship.) Most techniques involve some performance penalty; these can require the first stage to be several times larger for the same payload, although for recovery from downrange these penalties may be small.

The second stage is normally returned after flying one or more orbits and reentering.

Horizontal landing

In this case the vehicle requires wings and undercarriage (unless landing at sea). This typically requires about 9-12% of the landing vehicle to be wings; which in turn implies that the takeoff weight is higher and/or the payload smaller.

Vertical landing

In this approach rockets are typically used to softland the vehicle on the ground from the subsonic speeds reached at low altitude. This typically requires about 10% of the landing weight of the vehicle to be propellant.

A slightly different approach to vertical landing is to use an autogyro or helicopter rotor. This requires perhaps 2-3% of the landing weight for the rotor.

Horizontal takeoff

The vehicle needs wings to takeoff. For reaching orbit, a 'wet wing' would often need to be used where the wing contains propellant. Around 9-12% of the vehicle takeoff weight is perhaps tied up in the wings.

Vertical takeoff

This is the traditional takeoff regime for pure rocket vehicles. Rockets are good for this regime, since they have a very high thrust/weight ratio (~100).

Airbreathing

Airbreathing approaches use the air for propulsion during ascent. The most commonly proposed approach is the scramjet, but turborocket, Liquid Air Cycle Engine (LACE) and precooled engines are also proposed to be used.

In all cases the highest speed that airbreathing can reach is far short of orbital speed (about mach 15 for Scramjets and mach 5-6 for the other engine designs) and rockets would be used for the remaining 10-20 mach for orbit.

The thermal situation for airbreathers (particularly scramjets) can be awkward; normal rockets fly steep initial trajectories to avoid drag, whereas scramjets would deliberately fly through relatively thick atmosphere at high speed generating enormous heating of the airframe. The thermal situation for the other airbreathing approaches is much more benign, although is not without its challenges.

Hydrogen fuel

Hydrogen is often proposed since it has the highest exhaust velocity. However tankage and pump weights are high due to insulation and low propellant density; and this wipes out much of the advantage.

Still, the 'wet mass' of a hydrogen fuelled stage is lighter than an equivalent dense stage with the same payload and this can permit usage of wings, and is good for second stages.

Dense fuel

Dense fuel is sometimes proposed since, although it implies a heavier vehicle, the specific tankage and pump mass is much improved over hydrogen. Dense fuel is usually suggested for vertical takeoff vehicles, and is compatible with horizontal landing vehicles, since the vehicle is lighter than an equivalent hydrogen vehicle when empty of propellant.

Tripropellant

Dense fuel is optimal early on in a flight, since the thrust to weight of the engines is better due to higher density; this means the vehicle acclerates more quickly and reaches orbit sooner, reducing gravity losses.

However, for reaching orbital speed, hydrogen is a better fuel, since the high exhaust velocity counts more later in the flight.

Therefore tripropellant vehicles start off burning with dense fuel and transition to hydrogen. (In a sense the Space Shuttle does this with its combination of solid rockets and main engines, but tripropellant vehicles usually carry their engines to orbit.)

Reentry heat shields

As a rough rule of thumb, Robert Zubrin has said that 15% of the landed weight of a vehicle needs to be aerobraking reentry shielding.[1]

Reentry heat shields on these vehicles are often proposed to be some sort of ceramic and/or carbon-carbon heat shields, or occasionally metallic heat shields (possibly using water cooling or some sort of relatively exotic rare earth metal.)

Some shields would be single use ablatives and would be discarded after reentry.

Orbital reusable launchers

Currently in use

Planned

Historical

Cancelled

  • Kliper (Russian-European partially reuseable spacecraft that was to be launched around 2011 for the first time. It has been reported, however, that Energia is still working on the craft, and the Russian space program plans to review the decision to cancel the Kliper once they produce their now-planned modernized version of the Soyuz spacecraft. Thus, the Russian space program may decide to use it after all, and even so Energia may be able to market it to other space programs if they finish it)
  • Roton Commercial launch vehicle- ran out of funds.

Suborbital reusable launchers

Planned

Historical

References

  1. ^ [1]
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