A rocket sled launch, also known as ground-based launch assist, catapult launch assist, and sky-ramp launch, is a proposed method for launching space vehicles. With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain while an externally applied force is used to accelerate the launch vehicle to a given velocity. Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit. This allows the launch vehicle to carry a larger payload and reduces the cost of getting to orbit. When the amount of velocity added to the launch vehicle by the ground accelerator becomes great enough, single-stage-to-orbit flight with a reusable launch vehicle becomes possible.
For hypersonic research in general, tracks at Holloman Air Force Base have tested, as of 2011, small rocket sleds moving at up to 6453 mph (2,885 m/s; Mach 8.5). [1]
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 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. [2] [3]
Rockets carrying their own propellant with them use the vast majority of that propellant at the beginning of their journey to accelerate most of that very same propellant, as enshrined in the rocket equation. For example, the Space Shuttle used more than a third of its fuel just to reach 1,000 mph (1,600 km/h). [4] If that energy was provided without (yet, or at all) using a propellant the rocket carries, its propellant need would be much reduced, and its payload could be a larger fraction of its liftoff mass, increasing its efficiency.
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 low Earth orbit by 80% compared to the same rocket from a conventional launch pad. [5] 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 single-stage-to-orbit (SSTO) could be reduced in liftoff mass by 35% with such launch assist, dropping to 4 instead of 6 engines in one case considered. [5]
At an anticipated efficiency close to 90%, electrical energy consumed per launch of a 500-ton rocket would be around 30 gigajoules (8,300 kWh) (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 [3] 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. [5]
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). [6] [7]
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. [8]
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. [9] 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. [10] 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", [11] 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.
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° to vertical, a track on a tall mountain could allow a single stage to orbit reusable vehicle with no new technology. [12]
A rocket is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely from propellant carried within the vehicle; therefore a rocket can fly in the vacuum of space. Rockets work more efficiently in a vacuum and incur a loss of thrust due to the opposing pressure of the atmosphere.
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program name was Space Transportation System (STS), taken from the 1969 plan led by U.S. Vice President Spiro Agnew for a system of reusable spacecraft where it was the only item funded for development.
A single-stage-to-orbit (SSTO) vehicle reaches orbit from the surface of a body using only propellants and fluids and without expending tanks, engines, or other major hardware. The term exclusively refers to reusable vehicles. To date, no Earth-launched SSTO launch vehicles have ever been flown; orbital launches from Earth have been performed by either fully or partially expendable multi-stage rockets.
A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. Existing and proposed mass drivers use coils of wire energized by electricity to make electromagnets, though a rotary mass driver has also been proposed. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move due to momentum.
The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes. It belongs to the class of altitude compensating nozzle engines. Aerospike engines were proposed for many single-stage-to-orbit (SSTO) designs. They were a contender for the Space Shuttle main engine. However, as of 2023 no such engine was in commercial production, although some large-scale aerospikes were in testing phases.
A reusable launch vehicle has parts that can be recovered and reflown, while carrying payloads from the surface to outer space. Rocket stages are the most common launch vehicle parts aimed for reuse. Smaller parts such as rocket engines and boosters can also be reused, though reusable spacecraft may be launched on top of an expendable launch vehicle. Reusable launch vehicles do not need to make these parts for each launch, therefore reducing its launch cost significantly. However, these benefits are diminished by the cost of recovery and refurbishment.
Rotary Rocket Company was a rocketry company that developed the Roton concept in the late 1990s as a fully reusable single-stage-to-orbit (SSTO) crewed spacecraft. The design was initially conceived by Bevin McKinney, who shared it with Gary Hudson. In 1996, Rotary Rocket Company was formed to commercialize the concept. The Roton was intended to reduce costs of launching payloads into low Earth orbit by a factor of ten.
The Lockheed Martin X-33 was a proposed uncrewed, sub-scale technology demonstrator suborbital spaceplane that was developed for a period in the 1990s. The X-33 was a technology demonstrator for the VentureStar orbital spaceplane, which was planned to be a next-generation, commercially operated reusable launch vehicle. The X-33 would flight-test a range of technologies that NASA believed it needed for single-stage-to-orbit reusable launch vehicles, such as metallic thermal protection systems, composite cryogenic fuel tanks for liquid hydrogen, the aerospike engine, autonomous (uncrewed) flight control, rapid flight turn-around times through streamlined operations, and its lifting body aerodynamics.
A solid rocket booster (SRB) is a large solid propellant motor used to provide thrust in spacecraft launches from initial launch through the first ascent. Many launch vehicles, including the Atlas V, SLS and Space Shuttle, have used SRBs to give launch vehicles much of the thrust required to place the vehicle into orbit. The Space Shuttle used two Space Shuttle SRBs, which were the largest solid propellant motors ever built and the first designed for recovery and reuse. The propellant for each solid rocket motor on the Space Shuttle weighed approximately 500,000 kilograms.
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Non-rocket spacelaunch refers to theoretical concepts for launch into space where much of the speed and altitude needed to achieve orbit is provided by a propulsion technique that is not subject to the limits of the rocket equation. Although all space launches to date have been rockets, a number of alternatives to rockets have been proposed. In some systems, such as a combination launch system, skyhook, rocket sled launch, rockoon, or air launch, a portion of the total delta-v may be provided, either directly or indirectly, by using rocket propulsion.
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StarTram is a proposed space launch system propelled by maglev technology. The initial Generation 1 facility is proposed to launch cargo only from a mountain peak at an altitude of 3 to 7 kilometres using an evacuated tube remaining at local surface level. Annual orbital lift was estimated at approximately 150,000 tons. More advanced technology is required for a Generation 2 system for passengers, with a longer track instead gradually curving up at its end to the thinner air at 22 kilometres (72,000 ft) altitude, supported by magnetic levitation, reducing g-forces when each capsule transitions from the vacuum tube to the atmosphere. A SPESIF 2010 presentation stated that Generation 1 could be completed by the year 2020 or later if funding began in 2010, and Generation 2 by 2030 or later.
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