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HiHo / Hi-Hoe / NOTS-EV-2 Caleb

In the late 1950s, a significant technical effort to develop and launch a vehicle into space was undertaken at China Lake. A series of designs was developed and included Caleb, Notsnik, and Hi-Ho [also seen in authoritative sources as Hi-Hoe]. These vehicles were fabricated in the Weapons Prototype facilities. The NOTS-EV-2 Caleb, also known as NOTS-500, Hi-Hoe and SIP was an expendable launch system, which was later used as a sounding rocket and prototype anti-satellite weapon.

The missile design team of the Naval Ordnance Test Station made six attempts to air-launch a satellite in the summer of 1958. Plans for additional NOTSNIK flights were not approved and development efforts instead shifted towards upgrading the existing rocket design. One project, called Caleb, sought to build an improved air-launch system but was eventually cancelled because of political pressure from the USAF who wanted to monopolize military space launches. While it would not launch payloads into orbit, Caleb did fly as part of the Navy's secret high altitude "Hi-Hoe" program, with the last flight in 1962 reportedly reaching an altitude of 1,167.3 kilometers (725.5 miles).

The HiHo ASAT project, which unlike most other schemes went beyond the discussion phase, was based on the concept of an air-launched missile. The concept was tested, using a Navy F4D fighter and Caleb rockets, at the Pacific Missile Test Range. There were two tests in 1962. The first in march 1962 ended in failure. During one test on 25 June 1962, a zoom-climbing F4D fired a rocket to an altitude of 1,000 miles, in theory able to reach any non-US intelligence satellite in orbit at that time. Interestingly, both the Bold Orion and Hi-Ho ASAT test programs of the early 1960s used the Altair as a second stage, the same upper stage as the later Miniature Homing Vehicle ASAT. A follow-on program, called NOTSNIK II, sought to develop an anti-satellite capability. The NOTS-EV-2 rocket was used in the SIP (Satellite Interceptor Program) project, an attempt to develop an air-launched ASAT (anti-satellite) missile. Two NOTS-EV-2 vehicles were ground-launched in SIP-related tests in October 1961 and May 1962, respectively. The ASAT project was possibly also known as "NOTSNIK II".

There were any number of missiles carried by and launched from carrier aircraft while in flight, including air-to-air and air-to-ground missiles. Such missiles, however, were not designed to leave the atmosphere, do not achieve either orbital speed or altitude, and attain only about 5% of the energy needed to achieve an orbital trajectory. Furthermore, in such missiles the wings and other aerodynamic control surfaces are not jettisoned after ascending to beyond the atmosphere.

Conventional ground-launched ballistic (i.e., non-lifting) booster rockets are the most common approach to payload launch. However, such rockets require complex vertical takeoff facilities, including launch pad apparatus, and are subject to severe operational and geographical restrictions necessitated by the hazards of propellants and flight over populated areas. Moreover, conventional ground-launched boosters suffer from inherent inefficiencies resulting from a compromise of competing design and operational considerations. These inefficiencies necessarily increase the size, complexity and cost of such systems, making them uneconomic or otherwise undesirable for certain applications.

One such set of competing considerations is the compromise between thrust direction losses and drag losses in conventional ground-launched ballistic boosters. In particular, because the final flight attitude for circular and elliptical orbits, as well as most other missions of interest, is horizontal or substantially horizontal, conventional, vertically launched rockets must pitch over from their initial vertical ascent to a near-horizontal ascent to achieve final orbital flight attitude. Achieving orbit requires high velocity and near-horizontal flight. To minimize losses associated with such thrust direction change (i.e., "thrust direction losses"), pitch-over should ideally occur while the vehicle is ascending at a relatively low velocity, resulting in a near-horizontal ascent early in the trajectory. A shallow ascent profile of this nature was utilized, for example, under zero atmosphere conditions (i.e., in vacuum) by the US Apollo Program Lunar Module to achieve lunar orbit after liftoff from the moon's surface.

Structural stress and aerodynamic heating considerations, however, preclude the implementation of this ideal flight path in applications where the vehicle is being launched through an atmosphere. Aerodynamic forces, including drag and lift forces, increase with the atmospheric density and vehicle velocity. Accordingly, for a given velocity, greater drag forces are experienced at lower altitudes than at higher altitudes. As the vehicle accelerates during booster rocket flight, it is desirable to ascend as near to vertical as possible until the dynamic pressure reaches a maximum value, thereby minimizing the peak aerodynamic load on the vehicle. Accordingly, unlike the zero atmosphere ascent of the Lunar Module, aerodynamic load considerations dictate that conventional ground-launched boosters be launched vertically. Consequently, pitch-over occurs at a point reducing aerodynamic load on the vehicle at the expense of substantial excess propellant usage attributable to thrust direction losses.

In addition, because the conventional ballistic booster spends a significant portion of its flight time in a vertical or near-vertical attitude, the force of gravity directly counteracts the vehicle thrust forces, resulting in other losses, commonly referred to as "gravity losses." Although gravity losses are reduced as a vehicle approaches horizontal flight, the aerodynamic load considerations preclude substantial horizontal flight of the vehicle. Consequently, the conventional booster vehicle incurs substantial gravity losses for a significant portion of its ascent trajectory.

Furthermore, booster rocket motor efficiency increases with increasing exhaust nozzle expansion ratio or nozzle exit area. However, ambient atmospheric pressure forces acting upon the rocket motor nozzle exit are reduce net engine thrust as nozzle area increases. This thrust loss, commonly referred to as "atmospheric pressure-induced thrust reduction," necessitates the design of conventional boosters with nozzle exit areas or expansion ratios providing less than peak motor propulsive efficiency in order to reduce atmospheric pressure-induced thrust reduction and to maximize the net thrust in the denser (lower) regions of the atmosphere.

Various concepts for horizontal takeoff launch systems have been suggested. These concepts avoid most of the problems associated with vertical takeoff systems, but have other problems associated with their proposed implementation. In general, these concepts cannot be made truly operational by use of existing technology. Therefore, the expected development costs of systems based on these concepts are quite high and dates of completion of operational systems would be relatively far into the future. Another problem associated with suggested horizontal takeoff concepts is the inability to meet the ever present need for a "positive" payload. For a launch system to be capable of providing a positive payload, it must be capable of launching into orbit a gross weight that is greater than the weight of the orbiter vehicle itself plus the fuel required by the orbiter vehicle.

The gross weight minus the combined vehicle and fuel weight is the potential payload. In a fully operational practical launch system, the potential payload is not only positive but is also above a practical minimum. Finding a solution to the problem of providing a horizontal takeoff system capable of launching a positive payload that equals or exceeds a practical minimum has proved very difficult but is crucial to the success of any such system.

Some of the problems of ground launching can be overcome by launching the rocket launch vehicle from an aircraft in flight. An air launch offers several advantages over a ground launch, such as the avoidance of weather related delays, the simplification of operations, and increases in safety, both for the crew by simplifying abort options, and for the public because of the ability to avoid the overflight of populated areas. In addition, air launching presents design options that simplify the operation of the launch vehicle engine.

In particular, it is known that high area ratio nozzles for a given engine pressure increase performance. Thus if a lower engine pressure is used to take advantage of the high area ratio nozzle, a lower cost solution is possible. A lower pressure engine can be pressured fed which means that no turbo-pumps or gas generators are needed, resulting in a less complex solution. A lower pressure engine is also a safer solution because no operational pressure fed rocket has ever exploded. The air launched, captive-on-bottom vehicle offers a good compromise between vehicle size, payload amount and operational complexity.

Examples of air launched, captive-on-bottom vehicles include the Pegasus vehicle developed by Orbital Sciences Corporation. Pegasus is a Rocket-Powered, Air-Deployed, Lift-Assisted Booster Vehicle for Orbital, Supraorbital and Suborbital Flight. It is a multistage air-launched vehicle that can place hundreds of pounds into low earth orbit. Air-launching allows independent selection of launch point and launch azimuth, which in turn provides for the independent specification of orbital inclination and longitude of the ascending node. Such orbits are called tailored orbits, and they allow repeated overflight of regions on the earth's surface.

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