BBC Focus Magazine

[M Paul Lloyd]

The possibility of having a single stage to orbit vehicle with fly back capability is a very compelling idea but it does seem a lot to ask of current technology, or indeed anything presently being researched, to achieve this feat economically. Given that such a vehicle would have to have an airframe to payload ratio in the region of 1-5 poses some difficult engineering challenges, and the demise of the American X-33 (prototype for the Venturestar single stage to orbit shuttle) is the most obvious case in point. So let us consider some of the possible alternatives and review the current proposals that might prove viable.

Piggy Back.
Several extremely popular case studies suggest putting a shuttle vehicle on the back of an Antonov An-225 (or similar) Heavy Lift aircraft which could climb to a modest altitude before releasing the second stage which would then reach orbit velocity and altitude under its own rocket power. The idea is proposed as a means of giving the orbiter an economical leg up as part of a budget conscious two stage to orbit concept.
This sounds like an excellent idea in principal but once you start to look carefully into the specifications for this method of launching an orbiter it has to be said that there are some unfortunate shortfalls that are difficult to reconcile. Several variations on this general theme have been put forward and within the limitations of the general specifications most of these designs having many common features. The BAe Hotol single stage to orbit space-plane system has been the subject of a number of ‘refinements’ over the years but the main specifications will be used as a basis for much of the initial comparisons.
The main problem with the Hotol concept is that it seems to have originally been based on specifications for an air launched anti spy satellite system and would appear to be little more than a scaled up version of an air-launched missile rather than an actual space-plane design. The original concept models and drawings produced by BAe show a striking similarity to just such a system and it is thought that the whole Hotol project may have been little more than an attempt to salvage something of a cancelled defence contract. It would seem plausible that if a small missile can achieve speeds in excess of mach 2 and climb 10,000metres then a larger version, scaled up by a factor of 10, could achieve proportionally greater speeds and altitudes, unfortunately it is not as simple as it might seem. The main argument for the lack of development of the Hotol is the lack of a suitable hybrid air-breathing/rocket system that could take the vehicle from ground level to orbit without the need for separate jet and rocket propulsion. Development of this sort of propulsion system is proving rather problematic and seems to be somewhat beyond our current technological ability to produce. The linear Aero-spike engine intended for the X-33 and Venturestar have also failed to produce the expected results. However, these details have not deterred the many enthusiasts who continue to campaign vigorously for the development of such a system.

The Shuttle.
Problems inherent in the current Space Shuttle are worth noting. Each shuttle has a large human crew accommodated in a relatively large two story cabin. This imposes a large weight penalty on the shuttle launch system and given the efficiency of automated flight control it would make sense to dispense with the crew cabin altogether in any future design. Personnel that are required to work in space could be transported to orbit aboard a crew transfer module housed in the cargo bay which could, in theory, be designed to act as an escape system if a launch should fail. The principal of a crew module was planned into the Venturestar single stage to orbit project but this was so plagued with technical difficulties that escalating costs eventually lead to its cancellation. Equally the three main rocket engines of the shuttle are dead weight once the external fuel tank has been expended, whereas the Buran shuttle retains only orbital maneuvering thrusters. These two factors alone could make a considerable saving in all up weight of a redesigned two stage to orbit vehicle with (payload carrying) glide back capability. However the technical difficulties inherent in achieving this goal should not be underestimated.
Maximum Shuttle payload is currently 65,000 lb. 29,478 kg if launched on easterly ascent to low orbit. The current space shuttle spec’s are length 122 ft 2.5in 37.24m span 78 ft 1in 23.79m. Maximum weight on landing, with payload, 188,000lb 84,260kg giving a total airframe weight of 123,000lb 55,900kg.
An alternative An-225/shuttle composite, air-launched system, is said to have the following spec’s.
A vehicle capable of carrying a payload of 11,880lb 5,400kg to orbit and returning to a safe horizontal landing. Length, 124ft 8in 38.0m. Wingspan, 78ft 8in 24.0m. Total weight of orbiter, 125,000lb 275,000kg. This comprising of. Fuel, 550,000lb 250,000kg. Payload, 11,900lb 5,400kg. With an airframe of 20,300lb 44,600kg.
Payload bay is 7.0m long x 4.6m diameter with an estimated maximum payload of 11,880lb 5,400kg which, it is said, could be carried to a 200 km Orbit.
The Antanov An-225 ‘Mriya’ has a span of 290ft 88.4m. length 275ft 7in 84m. max speed 530mph 850kph. Max takeoff weight of 1,323,000lb 600,000kg. capable of lifting a 551,250lb 250,000kg payload to an operational ceiling of 9,500m 32,000ft. This is currently the world’s largest ‘Heavy Lift’ aircraft beating the massive Lockheed Galaxy C-5B’s lifting capacity by over 18,200lb 40,000kg, only one has ever been built specifically for transporting the old Soviet ‘Buran’ Shuttle orbiter.
An air-launching carried out at 900kph from 9,500m altitude is said to impart a velocity gain (compared to vertical launch) of 270m/s which is equivalent to 972kph. This is presumed to be after a shallow dive from 10,000m altitude to aid successful separation. Unfortunately any advantage gained by such an air launch from the An-225 is extremely small when compared with the depth of atmosphere and velocity needed to reach orbit. Indeed most if not all of any advantage will be cancelled out by the air resistance encountered in an inclined trajectory as the second stage climbs out of the atmosphere. It is an often overlooked detail that a vehicle has to accelerate to the incredible speed of 18,000mph 28,800 kph to achieve orbit with all the associated problems of friction heating to contend with making it essential to minimise the time spent in the denser layers of the atmosphere. It is also worth noting that the maximum payload capacity of the An-225 is 250,000kg which, it will be noted, is some 25,000kg less than the total weight of the proposed orbiter complete with payload and fuel.
The only real advantage offered by a horizontal launch system is that it requires less stringent test certification. The combined aircraft/spaceplane could take off in the manner of a normal aircraft yet execute its critical separation and rocket ascent away from any centre of population. This is a desirable feature but it is worth bearing in mind that overall performance gains are minimal and a simpler ‘boosted’ horizontal take off from the ground could be just as effective in achieving the same ends.
Given these factors it would be far better if the launch vehicle could be found that was capable of lifting an orbiter to higher speeds and altitude than the rather ponderous An-225.

Bigger, Faster.
Piggyback airlaunch systems have a long history with perhaps the most famous of the early versions being the Short-Mayo Transatlantic aircraft from the 1930’s which was designed to carry Air-mail across the Atlantic.
However this is a far cry from the technical requirements of air launching a spaceplane, and although the Shuttle was tested in this way, being launched from the back of a converted Boeing 747, this was achieved at a relatively low altitudes and speeds for glide tests only.
Experiments have been conducted with air launched ‘drones’ carried piggy back fashion by the Lockheed Blackbird high altitude spy plane designated M-21. This system was designed for air launch at speeds in excess of 2,000mph 3200kph (mach 3.3) and altitudes of 80,000ft 24,380m. Unfortunately the relatively dense atmosphere required by the ‘mother-ships’ air breathing propulsion system made the process of releasing the drone too unpredictable and the programme was cancelled after the loss of the first launch aircraft.
It is possible that a specially designed launch vehicle could be constructed that uses a combination of jet and rocket propulsion to fly from an ordinary airfield and beyond the limits of the atmosphere where high speed separation would be less risky. This vehicle would have to be capable of carrying a second stage fly-back shuttle to a suitable altitude and speed compatible with a successful separation and orbit insertion. Taking the current space shuttle performance as a guide, and basing the figures on NASA estimates, it would be acceptable for the upper stage separation to take place at an altitude of around 50miles 80km and a speed (at separation) of mach 10. This would be satisfactory for a second stage with a dry weight of 198,000lb 90,000kg carrying a fuel load in the region of 550,000lb 250,000kg and a payload in the region of 60,000lb 27,000kg to achieve orbit. This would in turn require a first stage with a dry weight of approximately 814,000lb 370,000kg carrying a fuel load of 3,000,000lb 1,364,000kg, along with the second stage and payload.
Unfortunately this would result in not only a first stage vehicle of prodigious proportions but once the second stage and payload are added it becomes at least three times more massive than a fully loaded An-225 which in turn places some very difficult engineering hurdles before us. First of all the average ratio between weight of airframe and payload for Heavy Lift aircraft is usually around, 40% airframe 60% payload/fuel, in the case of the AN-225 the ratio is actually 60% airframe 40% payload/fuel. However the figures for the two stage to orbit system suggest that something nearer to 25% airframe 75% payload/fuel would be required. This would be extremely difficult to achieve even with the most advanced engineering techniques currently available. Even factoring in such weight saving expedients as fully automated control systems that dispense with a human crew and a front mounted fin we still have to consider the additional weight of a heat shield and the orbital manoeuvring control rockets for the de-orbit stage and atmospheric re-entry. The figures given for the proposed air-launched Hotol are actually in the region of 14% airframe to 86% payload/fuel, which does seem rather ambitious to say the least and are more in line with figures for an expendable rocket system rather than something that would be a long term and reliable workhorse. Attempts to construct such a highly loaded airframe proved to be the major problem in designing the X-33/Venturestar single stage to orbit vehicle.

Ideal Trajectory
It has to be said that the optimum launch trajectory still remains Oberth’s ‘parabolic’ synergy curve which is obviously most efficient when starting from a vertical launch position. We need to look at a redesigned two stage system utilizing a fully automated, fly back, liquid fuelled first stage launcher, rather than the strap on solid rocket boosters and external tank currently being employed. This system should still have the capacity to propel a suitable payload to 3,500mph 5,600kph (mach 5) and an altitude of 250,000ft, 76,000m for separation of the upper, orbiter, stage. It should then be possible to achieve orbit velocity (28,800 kph 18,000mph) with a payload similar to or even larger than that carried by the current shuttle.

Buran.
The Soviet response to the US Space Shuttle, the Buran shuttle, flew only once -November 15th 1988. This was an unmanned flight returning to Earth on autopilot. The combined Buran, and Energia booster, weighs more than 2000tonnes, 4.4 million lbs at launch. It has a payload capacity of 30 tonnes, 66 000 lbs carried in the cargo bay similar to the US Space Shuttle. The Buran is 16.45 metres, 53 feet high. 36.4 metres, 118 feet long with a wingspan of 23.9 metres, 78 feet. The US Shuttle is 17.25 metres, 56 feet high. 37.24 metres, 121 feet long, and has a 23.79 metre 77 foot wingspan. The Buran and Energia are not currently in use but could potentially be used in the future. The Energia booster has flown twice.

In Conclusion.
Sadly it would seem that, at least for the foreseeable future we are somewhat limited to ‘staged’ orbital vehicles and taking all costs into consideration a certain amount of expandability is going to remain part of the equation for some time.

M Paul Lloyd © 2004