The recent rise of commercial rocketry funded by mega-rich technology entrepreneurs holds the promise of finally reducing the cost of placing payloads into orbit, partly through the generation of greater competition in the marketplace and application of more agile business models and project management, but also because we have seen the timely reemergence of the reusable rocket.
Reusability, if it can be mastered, will be fundamental to increasing mankind’s access to space and allowing the construction of larger commercial infrastructures beyond Earth, but such vehicles are not a new idea…
The purest form of the reusable launcher concept is the Single Stage To Orbit (SSTO) vehicle and across the decades the idea of SSTO seems to have gone hand in hand with the spaceplane. Perhaps this is because many programmes have been initiated by the Air Force and the concept of flying to and from orbit just seems to fit. Another concept often associated the SSTO is the air-breathing engine as part of a dual-cycle propulsion system – operate as a jet and use atmospheric oxygen as your oxidiser until reaching higher speeds and altitudes and switching over to Scramjet and eventually rocket propulsion. Such dual-cycle systems often also propose the collection and storage of atmospheric oxygen for use in the later phases of flight.
It’s easy to see why this is such seductive and persuasive idea. One of the huge problems facing SSTOs is the challenge of keeping weight down as no structural weight is shed during flight as in a staged rocket. Providing a way to reduce the oxidiser that must be carried at take-off can achieve this, but this comes at a cost. Combined cycle propulsive systems are extremely complex and have often fallen below their efficiency estimates. Controlled flight through the atmosphere also requires wings or a lifting body configuration. Wings can be a very tricky structure to protect from the ravages of kinetic heating and along with the body require an effective thermal protection system for the hypersonic journey to, as well as from, orbit.
But parallel to the winged aerospace planes a second school of SSTO though has looked at a very different solution. Rather than proposing a vehicle that has to combine the properties of an aeroplane and a rocket, proponents of this second approach pointed to the relative simplicity of vertical takeoff / vertical landing (VTVL) rockets, after all the dynamics and challenges of rocket flight to orbit are well known and have been met.
The first serious attempts to design a reusable vertical takeoff SSTO began in the 1960s. Even at this early stage it was becoming apparent to some within the aerospace community that expendable rockets, although effective at placing payloads in orbit, were inherently wasteful – it’s like building an airliner and throwing it away after every flight was the oft quoted logic. Initial efforts centred on trying to recover the stages from these boosters by flying them back to a controlled landing, but this often required the addition of ungainly control surfaces or recovery devices like the Rogallo wing adding weight and complexity to the system. Rather than try and retrofit a solution to an existing design, maybe it was time to start with a blank sheet of paper and design a new vehicle with reusability in mind from the start. One of the first engineers to really address this idea was Philip Bono from Douglas Aircraft.
OOST, ROOST and a ROMBUS
As the American aerospace industry geared up for the challenges of Apollo during the early 1960s, a few far-sighted designers began to look beyond the immediate requirements of a Moon mission and instead concentrated on the challenges of providing sustainable, economic solutions to provide a wider orbital infrastructure. Philip Bono worked for the space division of the Douglas Aircraft Company who, having proved themselves with earlier projects such as the Thor IRBM had recently won the contract to build the S-IVB stage to be used on the Saturn series of rockets. While work on these more conventional projects continued, Bono began to examine ways of recovering and re-using rocket stages or, even better, entire launch vehicles. His initial concepts were the One stage Orbital Space Truck (OOST) which later gained the prefix ‘Recoverable’ (ROOST). These would use a large, single stage rocket of fairly conventional design with the ability to carry a 160 ton payload to low Earth orbit. The unique feature was the inclusion of a large inflatable drag cone which would be deployed prior reentry to protect the rocket and decelerate it for a sea landing. The rocket could then be recovered and refurbished for re-use.
OOST and ROOST failed to gain any great traction at the time and Bono soon started to work on an improved concept which led to the family of vehicles for which he is best remembered. Figuring that expendable rockets generally work by shedding stages composed of both cheap fuel tanks and expensive engines he came up with the idea of externalising the fuel tanks allowing them to be discarded once their fuel had been exhausted. The tanks would then descend under parachute and be recovered for reuse. This partial staging, when combined with Bono’s innovative plug nozzle propulsion system allowed the vehicle to reach orbital velocities while retaining the engines, some of which would then be used alongside parachutes to slow the vehicles descent after reentry and provide a soft-landing. This concept became known as the Reusable Orbital Module-Booster & Utility Shuttle (ROMBUS) and was patented by NASA in 1964.
As with OOST and ROOST, ROMBUS went no further but Bono continued to build upon the central themes he had established leading to a more sophisticated VTVL SSTO, The Pegasus. Proposed in 1966, The Pegasus was to be a large Saturn class vehicle capable of placing around 90 tons in LEO, but Bono and Douglas also marketed it as a rapid point-to-point sub orbital transporter for either cargo or around 170 passengers. As with ROMBUS, hydrogen fuel would be carried in jettsionable external tanks which would be dropped once exhausted during ascent. Propulsion was provided by 16 aerospike engine modules located around the vehicle’s base. The hydrogen fuel could be circulated through this base to form an actively cooled heat shield during reentry and some of the aerospike engines would be reactivated to act as retro-rockets for final descent to a landing on extendable legs at a specified landing site. The Pegasus was envisioned for service around 1980 and Bono’s concept gained many admirers, especially amongst the US Marine Corps.
Soon Bono was working on a larger Pegasus derived vehicle named Ithacus which in it’s largest variant would have been able to carry a battalion of 1,200 troops to any conflict zone within hours. Proposals included launching these vehicles from converted aircraft carriers, but the entire concept did seem to have a few unaddressed flaws. The noise levels generated on takeoff and landing by the aerospike engines would have been huge to the point of causing structural damage to the vehicle not to mention its human cargo. The practicalities of Ithacus in a battlefield situation also seem highly dubious. Firstly, the descending craft would have provided an easy target for surface to air missiles and the loss of 1,200 troops in a single crash would seem untenable for any army. Secondly, once landed the vehicle would have insufficient fuel to takeoff again, so would have to remain in-situ until at least partial re-fuelling could take place – again a very tempting target for any enemy. The idea soon fell from favour, but didn’t mark the end for Bono’s SSTO plans.
During his work for Douglas, Bono was fully aware of the development and capabilities of the Saturn S-IVB stage. He calculated that given its performance versus weight this stage was practically capable of placing a modest payload in orbit on it’s own. This realisation led to a 1967 study for a Saturn Application Single Stage To Orbit (SASSTO) vehicle. By modifying the stage to use his favoured aerospike design – in this case a single large nozzle – as opposed to the J-2 used in the Saturn launchers, and revising the tank structure he predicted performance increases that would allow for orbital flight. The vehicle would perform a controlled reentry in the same manner as an Apollo capsule before retiring the engine to allow a short hover and final descent to a soft landing on extendable legs. The payload was to be either a Gemini capsule or a similar mass of cargo carried initially under an aerodynamic fairing. Proposed missions included service and potentially rescue flights in support of the proposed Apollo Application Program space stations.
As with Bono’s other SSTO proposals, SASSTO never went beyond the planning stages. In this case the dramatic reduction in the Apollo Applications Program in the aftermath of the Apollo 1 enquiry curtailed the need for such a vehicle. Bono’s designs also struggled to gain any solid support within NASA who saw many of the technologies involved as unproven and risky when compared to either an expendable rocket or a winged reusable vehicle.
While Philip Bono was responsible for a great many VTVL SSTO proposals during the 1960s, any discussion of such vehicles would be incomplete without mention of the General Dynamics NEXUS proposal of the early sixties. This was a massive vehicle with a proposed payload of 900 tonnes to LEO. The design was devised by Kraft Ehricke, one of the German team who had worked at Peenemünde during the Second World War. Having travelled to America under Operation Paperclip he worked for Bell Aircraft before moving to Convair where he designed the cryogenically fuelled D-1 Centaur upper stage – one of the great advances in American rocketry during the early years of the space race.
Like his associate von Braun, Ehricke was a major advocate for interplanetary travel and NEXUS with its colossal payload allowed him to envision large planetary missions.
As with Bono’s designs, Ehricke’s NEXUS relied on large size to overcome any penalties from structural weight versus payload. In this respect these concepts were broadly comparable Bob Truax’s Sea Dragon and suffered from the same major drawback – there simply wasn’t a big enough demand to place huge payloads in orbit on anything like a regular enough basis to make such vehicles economically practical.
With the political winds in the late 1960s turning against spaceflight as the culmination of the Apollo programme approached planning was already underway within NASA for a new generation of spacecraft that would offer cheap routine access to orbit with reusability as a fundamental feature. Informally this began to be known as the ‘shuttle’.
SERV: A Shuttle without wings
While interest in Bono and Ehricke’s respective VTVL SSTO proposals had faded by the end of the sixties, the opportunity afforded by NASA’s quest for a reusable shuttle afforded an opportunity for a craft clearly influenced by these earlier concepts – the Single-stage Earth-Orbital Reusable Vehicle (SERV). While NASA had made it known that their preferred approach for the shuttle was a winged vehicle (as championed by legendary NASA designer Max Faget) the requirements circulated to industry did not forbid other concepts from being submitted.
The Chrysler Space Corporation had been engaged in producing the first stages for the Saturn 1 and 1B rockets at their plant in Louisiana and with the end of Apollo looming were keen to find a new programme to utilise its and workforce facilities. Under engineer Charles Tharratt a team began to work on the problem of providing a fully reusable space truck to fulfil NASAs requirements. Working under a Phase A contract awarded by NASA in 1968, they produced an initial design study outlining the SERV and an optional crew carrying spaceplane which could sit atop the main vehicle called the Manned Upper Reusable Payload (MURP). Shaped like a huge ‘gumdrop’ capsule SERV was unlike its competitors in almost every respect. It featured a huge central payload bay at its core surrounded by an arrangement of propellant tanks and an aerospike engine with twelve nozzles arranged around the base of the craft along with forty jet engines to be used during landing. To allow for different operating modes for this complex propulsion arrangement a system of sliding doors and metal shields would be used during different flight phases.
With its capsule-like shape SERV would have re-entered the atmosphere like an Apollo capsule, benefitting from the partial protection of the shockwave formed ahead of the blunt base. Ablative coatings would also be applied to the aluminium structure to remove much of the remaining heat. The SERV would then engage the jet engines for a brief hover before setting down on legs. One of the many unique features of SERV amongst the Phase A submissions was that it could operate uncrewed. If it was needed to ferry passengers to orbit, then the MURP could be used with the spaceplane separating once in orbit.
The whole SERV concept won few supporters within NASA. Many saw it as too complex and unconventional, especially given how far it diverged from Faget’s vision of a winged craft. SERVs ability to operate robotically also won few friends in the astronaut office. Many of the technologies were unproven at the scale required – aerospike engines had not been attempted at this scale and an extension of better understood engine technologies seemed a safer bet for the shuttle. The heat shield arrangement was also questionable given that NASAs only practical experience with resprayable ablative heat shields at the time had been the abortive use of a similar system on the X-15A-2.
While SERV never looked like progressing in the process, the final factor which counted heavily against it was the lack of operational cross-range. NASA had hoped that any orbiter would allow a certain amount of cross range to allow for different recovery options and a greater margin of safety under abort scenarios. As NASA were also reliant on Air Force support to progress the Shuttle, the final vehicle would also have to suit their requirements including flights to polar or high inclination orbits and SERV could never have offered them the degree of flexibility they required. It’s also worth noting that at the time of the Phase A shuttle studies, the vehicle was mainly envisioned as a simple logistics truck to support large orbital space stations and their attendant nuclear space tugs. While SERV could possibly have performed this limited role NASAs needs quickly changed as it became obvious that the shuttle would be the only element of these grand plans that the Nixon Administration would fund.
New politics, new opportunities
The 1970s were lean times for NASA as they fought to retain the Shuttle and the Grand Tour (later known as Voyager) and Viking programmes. The final moon landings played out to public indifference and the troubled start to Skylab – the sole remaining element of the Apollo Applications Program – hardly helped matters. Public opinions were set against space exploration given the background of more earthly social and economic problems and when astronauts and cosmonauts shook hands in orbit during the Apollo-Soyuz Test Project in 1975 it was clear the imperative for space superiority so prevalent during the sixties had now faded from the national consciousness. But as the 1980s dawned and the Shuttle finally shook off its teething troubles and approached flight status, spaceflight was about to receive a new champion in the guise of the 40th President of the United States, Ronald Reagan.
As Reagan came to office in 1980, it was against the background of a tense geopolitical situation. Much of the ground gained through the detente of the Nixon years seemed to have been lost through actions such as the Soviet invasion of Afghanistan. Reagan had stressed his presidency would be marked by a stronger American response to this perceived aggression and he wasted no time once elected in ramping up defence spending. Conservative voices both within and surrounding his administration questioned the perceived balance of power created under Mutual Assured Destruction and looked for ways in which the American lead in technology could tip the situation in their favour. An idea that quickly gained traction was an active space-based missile defence system, able to provide a shield against enemy attacks.
Advisors including Edward Teller assured the President that such a system was within the realms of technical possibility, but would require the launch of large constellations of detection and kill vehicles. The Shuttle flew for the first time in 1981 and while hopes of a high flight rate remained initially high it was quickly realised that other launch systems may be needed to service the newly named Strategic Defence Initiative.
Coupled to this new defensive need for launchers was Reagan’s encouragement of the commercial exploitation of space. Through new policies he hoped to stimulate a new generation of satellites and manufacturing in space. Much of this was to be serviced by competitively priced flights on the Shuttle – a policy culminating in the construction of Space Station Freedom, announced in the 1984 State of the Union Address, where private enterprise could construct a foothold for their industrial operations in LEO. But by 1984 it was already becoming clear that the Shuttle would be unable to deliver on its promise of high flight rates and aircraft-like operation. The position of the Space Transportation System as the sole national launcher began to come under question and beyond NASA a number of enterprising engineers and designers began to plan and test the first generation of private launch vehicles. As these concepts developed some familiar themes began to appear and, inspired by the work of Bono, Ehricke and Tharratt, it looked like the VTVL SSTO might yet have its day.
Single Stage to Orbit: Politics, Space Technology, and the Quest for Reusable Rocketry – Andrew J. Butrica
Space Shuttle: The history of the Space Transportation System – Dennis R. Jenkins
Hypersonic: The story of the North American X-15 – Dennis R. Jenkins & Tony R. Landis
Secret Projects: Military Space Technology – Bill Rose
The Space Shuttle Decision: NASA’s Search for a Reusable Space Vehicle – T.A. Heppenheimer
False Steps: The space race as it might have been – Paul Drye
United States Patent Office: Recoverable Single Stage Spacecraft Booster (3,295,790)
Facing the Heat Barrier: A History of Hypersonics – T.A. Heppenheimer