Rocket sled launch
A rocket sled launch is a method of launching space vehicles. A rail or maglev track and a rocket or jet booster is used to accelerate a sled holding a vehicle up an eastward facing mountain slope. Substantial fuel savings can be gained since the spacecraft does not need to use its engines for the initial acceleration, allowing a single-stage-to-orbit reusable vehicle.
Effectively a 'sky ramp' would make the most expensive, first stage of a rocket fully reusable since the sled is returned to its starting position, to be refueled and may be reused in the order of hours after use. Present launch vehicles have performance-driven costs of thousands of dollars per kilogram of dry weight; sled launch would aim to reduce performance requirements and amortize hardware expenses over frequent, repeated launches. Designs for mountain based inclined rail 'rocket' sleds often use jet engines or rockets to accelerate the spacecraft mounted on it. Electromagnetic methods (such as Bantam, Maglifter, and StarTram) are another technique investigated to accelerate a rocket before launch, potentially scalable to greater rocket masses and velocities than air launch.
Overview of the problem
NASA studies have shown that the Space Shuttle used more than a third of its fuel just to reach 1,000 mph (1,600 km/h). If a rocket was already moving at launch, with corresponding reduced propellant needs, a greater fraction of liftoff mass could be payload and hardware.
Due to factors including the exponential nature of the rocket equation and higher propulsive efficiency than if a rocket takes off stationary, a NASA Maglifter study estimated that a 270 m/s (600 mph) launch of an ELV rocket from a 3000 meter altitude mountain peak could increase payload to LEO by 80% compared to the same rocket from a conventional launch pad. Mountains of such height are available within the mainland U.S. for the easiest logistics, or nearer to the Equator for a little more gain from Earth's rotation. Among other possibilities, a larger SSTO could be reduced in liftoff mass by 35% with such launch assist, dropping to 4 instead of 6 engines in one case considered.
At an anticipated efficiency close to 90%, electrical energy consumed per launch of a 500-ton rocket would be around 30 GJ, 8000 kilowatt hours (each kilowatt-hour costing a few cents at the current cost of electricity in the United States), aside from any additional losses in energy storage. It is a system with low marginal costs dominated by initial capital costs Although a fixed site, it was estimated to provide a substantial net payload increase for a high portion of the varying launch azimuths needed by different satellites, with rocket maneuvering during the early stage of post-launch ascent (an alternative to adding electric propulsion for later orbital inclination change). Maglev guideway costs were estimated as $10 – $20 million per mile in the 1994 study, which had anticipated annual maglev maintenance costs on the order of 1% of capital costs.
Benefits of high altitude launches
Rocket sled launch helps a vehicle gain altitude, and proposals commonly involve the track curving up a mountain. Advantages to any launch system that starts from high altitudes include reduce gravity drag (the cost of lifting fuel in a gravity well). The thinner air will reduce air resistance and allow more efficient engine geometries. Rocket nozzles have different shapes (expansion ratios) to maximize thrust at different air pressures. (Though NASA's aerospike engine for the Lockheed Martin X-33 was designed to change geometry to remain efficient at a variety of different pressures, the aerospike engine had added weight and complexity; X-33 funding was canceled in 2001; and other benefits from launch assist would remain even if aerospike engines reached flight testing).
For example, the air is 39% thinner at 2500 meters. The more efficient rocket plume geometry and the reduced air friction allows the engine to be 5% more efficient per amount of fuel burned.
Another advantage to high altitude launches is that it eliminates the need to throttle back the engine when the "Max Q" limit is attained. Rockets launched in thick atmosphere can go so fast that air resistance may cause structural damage. Engines are throttled back when Max Q is reached, until the rocket is high enough that they can resume full power. The Atlas V 551 gives an example of this. It reaches its Max Q at 30,000 feet. Its engine is throttled back to 60% thrust for 30 seconds. This reduced acceleration adds to the gravity drag the rocket must overcome. Additionally, space craft engines concerned with Max Q are more complex as they must be throttled during launch.
A launch from high altitude need not throttle back at Max Q as it starts above the thickest portion of the Earth's atmosphere.
Debora A. Grant and James L. Rand in: "The Balloon Assisted Launch System - A Heavy Lift Balloon" wrote: "It was established some time ago that a ground launched rocket capable of reaching 20 km would be able to reach an altitude of almost 100km if it was launched from 20km." They suggest that small rockets are lifted above the majority of the atmosphere by balloon in order to avoid the problems discussed above.
Compatibility with reusable launch vehicles
Rocket sleds at China Lake testing ground have reached Mach 4 while carrying 60,000 kg masses. A sled track that gave a Mach 2 or greater launch assist could reduce the fuel to orbit by 40% or more, while helping counter the weight penalty when aiming to make a fully reusable launch vehicle. Angled at 55 degrees to vertical, a track on a tall mountain could allow a single stage to orbit reusable vehicle with no new technology.
Rocket sled launches in fiction
- Robert A. Heinlein used a lunar maglev launcher in The Moon is a Harsh Mistress. One on Earth was built by the novel's end.
- Dean Ing used a similar system in his 1988 novel "The Big Lifters".
- Fireball XL5 was launched on a sled from sea level.
- When Worlds Collide (film) used a rocket sled to launch the Ark, but the book did not.
- Laser propulsion
- Non-rocket spacelaunch
- Orbiting skyhooks
- Hopper (spacecraft)
- U.S. Air Force: "Test Sets World Land Speed Record". Archived from the original on June 4, 2012. Retrieved April 24, 2011.
- "Transformational Technologies to Expedite Space Access and Development". SPESIF, Dr. John Rather, prior Assistant Director for Space Technology Program Development at NASA. Retrieved 28 April 2011.
- "Maglifter Tradeoff Study and Subscale System Demonstrations". NASA. CiteSeerX: 10.1.1.110.9317.
- NASA: "Space Shuttle Basics". Retrieved April 28, 2011.
- "The Maglifter: An Advanced Concept Using Electromagnetic Propulsion in Reducing the Cost of Space Launch". NASA. Retrieved 24 May 2011.
- "RS-2200". Retrieved April 28, 2011.
- "The Aerospike Nozzle". Retrieved April 28, 2011.
- "Altitude Compensation". Retrieved April 28, 2011.
- "Dynamic Pressure". Retrieved April 28, 2011.
- "Atlas Launch System Mission Planner's Guide". Retrieved April 28, 2011.
- "The Balloon Assisted Launch System". Retrieved April 28, 2011.
- "Sky Ramp Technology". Retrieved April 28, 2011.
- A website discussing "Skyramps": http://www.g2mil.com/skyramp.htm
- "A Light Gas Gun Approach To Achieving 'First Stage Acceleration' for the Highly Reusable Space Transportation System" 1997 M. Frank Rose, R .M. Jenkins, M. R. Brown, Space Power Institute, Auburn University, AL, 36849
- Link to Lockheed Proposal for a sled based reusable launch vehicle. http://www.astronautix.com/lvs/recstics.htm
- Europe's Phoenix: Test Craft Sets Stage For Reusable Rocketry http://www.space.com/missionlaunches/europe_phoenix_020621.html
- Holloman Air Force Base: http://www.holloman.af.mil/photos/index.asp?galleryID=2718
- NASA Studies pneumatic rocket boosts: http://www.techbriefs.com/content/view/2257/32/
- Describes rocket efficiency at various air pressures & aerospike engine: http://www.aerospaceweb.org/design/aerospike/compensation.shtml