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TSTO spaceplanes
by IKE - 19-4-2006


This system is presented in an article of Aviation Week & Space technology and has a lot of similarities not only to Boeings TSTO described above but also to the XB-70 'Valkyrie' experimental trisonic bomber. The later was designed by North American Aviation, which later -as Rockwell- merged with Boeing. The information about Blackstar is blurry and of course unofficial. Its action is mostly placed in the 90s and the system consist from the carrier aircraft named SR-3 and the orbiter named XOV from 'experimental orbital vehicle'.

The carrier aircraft has many similarities with the XB-70 bomber. It is possible that not only the equipment and infrastructure for producing the XB-70 but also the 3rd unfinished XB-70 prototype was used as well for the development of the SR-3.

The main external difference is at the wingtips. The XB-70 used a revolutionary movable at almost 90 degrees angle delta wingtips which produced a substantial amount of lift due to compression of the flow. The SR-3 has instead simple winglets which are quite large and stabilize the aircraft instead of the conventional vertical tails of the XB-70.This difference is probably due to the lower maximum speed of the SR-3 which rendered the XB-70 layout useless.

SR-3 used at least four turbojets (possibly six General Electric YJ-93-3 like XB-70). The engines were placed in two groups leaving a cavity between them where the orbital vehicle nested conformally. The XOV has a length of about 180-200ft which is approximately the length of XB-70's engine bay.

While XB-70 used a wing made from composites with a honeycomb core, SR-3's were constructed more conventionally from aluminum with a simpler structure due to a smaller maximum airspeed of 2 mach (instead of XB-70's 3.3+ mach) which stresses a lot less both thermally and structurally the wing and frame.

In the front there are two canard wings which are reported to be seen both inclined front or rear. It is believed that the canards could alter the sweep angle (from front sweep to rear) to compensate for the change of the relative distance between the center of gravity and the center of aerodynamic pressure when the XOV is released.


The orbiter is a lifting body with small blended wings. It has dropped winglets possibly to take advantage of compression lift as -in quite larger scale- did the original XB-70. Its fuselage was constructed from advanced composite materials with a sandwich honeycomb core and it has a length of 97.5ft. There probably was a smaller, possibly unmanned, version also.

The vertical stabilizer was quite thick and was also the connecting strut with the carrier aircraft.ας στήριξης με το SR-3.

XOV used very advanced rocket engines, not only they had aerospike nozzles, but they used a boron based gel as fuel. This gel is much denser and has a lot more specific energy when burned than conventional fuels. The XOV used four aerospike rocket engines with 2D nozzles which were developed before the XRS-2200 that the X-33 would use if evolved to be operational. Additionally there was also a small rocket engine in the base of the vertical stabilizer (right above the 4 main engines), which probably was used for orbital and re-entry maneuvers.

XOV's landing gear was a combination of wheels and skis.

There are very few information or even speculations about the flight profile of the Blackstar system. XB-70 has a maximum operational altitude of 75.000ft so it is possible that SR-3 reached this altitude with a maximum airspeed of 2 mach before releasing the XOV. Then the SR-3 was heading for landing while the XOV reached orbit and performing its task before re-enter the atmosphere and land also.

The orbiter was transported from the landing site with the use of modified C-5B military cargo aircraft. These heavily modified C-5s had a wider fuselage (+8ft), internal changes in the cargo bay and a stronger landing gear.

The Blackstar system had a lot of novelties but it was based in tried and tested technology. It probably performed several missions in total secrecy (there are rumours for the existence of quite a few XOVs) and its technology while not readily avaliable exist as a know how for future designs.

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The first rockets where simple tubes with a hole from which the detonation of gases produced the thrust. This primitive design exploits only the reaction and a large amount of energy from the plume expansion is wasted. The purpose of a rocket nozzle is to convert the largest possible amount of energy from the fuel burn to thrust.

The bell shaped nozzle is maybe the most popular. It has a parabolic (or similar) intersection and its shape converts the pressure front from the gas expansion to a reactive force (thrust).

There is an optimum nozzle shape for every external atmospheric pressure (which is mostly a function of altitude). In higher altitude (lower atmospheric pressure) the nozzle is too small for the plume and in lower altitude (higher atmospheric pressure) the exhaust plume is distorted from the higher-pressure ambient air that enters inside the nozzle. The nozzle dimensions and shape for a vehicle that will travel in various pressure conditions is a compromise.

One solution to this problem is the aerospike nozzle with its inherit altitude compensation. You can trace its design in the following graph. One derivative of the conventional bell shape nozzle is the annular nozzle. An assymetrical derivative eliminates the outer wall and retains a long central spike that emulates a very long nozzle.

The long spike can be replaced with an abrupt cut and produce a compact setup. This may have a small penalty performance-wise in comparison to a full spike but it is much more compact and more efficient than a conventional bell shaped nozzle of the same size.

It operates by exploiting a secondary subsonic recirculating flow at the base of the central 'spike'. This flow acts like a conventional spike and it increases substantially the nozzles performance. A small bleed at the base can also increase the performance almost at a full-length spike's level.

A big advantage of the aerospike nozzle is that by using the atmospheric pressure as a virtual outer wall it has inherent altitude compensation capabilities. The flow is adjusted naturally to the ambient conditions and the overall performance of the nozzle stays close to the optimum.

Its main disadvantage is the much higher complexity and a small weight penalty. However it can be easily integrated to the vehicle and it can be modular in a linear arrangement.

The most advanced aerospike engine is the P&W XRS-2200, destined for the X-33 and later for Venture Star spacecraft, both cancelled. The XOV vehicle mentioned in the article was supposedly designed around a linear aerospike engine.

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