Despite over 80 attempts taking place over seventy years, a true spaceplane has never been flown. No clear answer has emerged thus far why this concept is so elusive in its realization. Buffo characterized “True Spaceplanes” as:
- Horizontal take-off and landing systems
- Using conventional or slightly modified aircraft runways
- Being Single-Stage-To-Orbit (SSTO) or Two-Stage-To-Orbit (TSTO) concepts with air-breathing propulsion
- Using advanced materials
- Re-entering the atmosphere and landing with or without propulsion power
- Being either manned or unmanned
Spaceplanes are visualized as taking off horizontally from a conventional runway, ascending directly into space, and re-entering to loiter and land on a runway in an easy, seemingly effortless manner like a conventional transport aircraft.
Undoubtedly the best and briefest answer to this perplexing question is, in my mind, Don Pettit, the very articulate NASA astronaut and veteran of the International Space Station who called it the “Tyranny of the Rocket Equation”!.
This paper does precisely what Petit calls for: new operating paradigms and new technology for reusable spaceplanes designed for a giant leap for mankind to attain low earth orbit.
After NASA published Don Petit’s insights, there has been many a clarion call from spaceplane advocates for a fresh look at hypersonic air-breathing propulsion (HAP) as the key to low cost access to space. The applications of HAP surpass its use for launching satellites. It extends far into the future for space industrialization, including harnessing solar energy in space for planet earth and other planets being explored; space exploration and space security missions.
The need for HAP and aerospaceplanes was first felt by the USAF in 1950’s and 1960’s; but their voices were dimmed and almost extinguished by the impact of Space Shuttle.
By mid-1980 the Shuttles had aged. 1986 saw the tragic accident to the “Challenger”. Cold War rivalry prompted President Reagan to prematurely announce the advent of the National Aero-Space Plane (NASP) as a vehicle to sustain US domination in space through military presence and enhanced civil space missions as well. Other spacefaring nations saw the 1980s era as an opportunity to look at new space futures beyond conventional multi-stage space rockets.
A full thirty years have passed since the Challenger disaster; and nearly 70 years since the advent of interest in hypersonic airbreathing propulsion. Yet today, spaceplanes are still not flying; their systems design concepts and technologies are still in various stages of early development; and the road from mid-1980’s to today is strewn with the wreckage of a substantial number of failed, or failing spaceplane systems concepts and sub-optimal approaches (like two-stage to orbit reusable launch vehicles and reusable booster stages of conventional rockets).
But quite apart from HAP, there is another, deeper reason why spaceplanes are not yet flying anywhere in the world even if HAP were to be available right now off-the-shelf.
Confluence of flight science cultures
In the concept and design of spaceplanes lies a hidden confluence of flight science cultures, wherein lie the proper domain of spaceplane system design choices. An attempt towards a scientific understanding of the profound importance, proper place and role of hypersonic airbreathing propulsion in an analysis of direct ascent of an aerospacevehicle from earth to orbit in a single stage, was well brought out in the August 1987 issue of the Journal ‘Aerospace America’ in a 3-page article by R.A.Jones, Chief of High Speed Aerodynamics of NASA Langley, and C.dP. Donaldson, Chief Scientist of Titan Corporation. This insightful piece of scientific work brought out for the first time the fundamental design principles underlying SSTO aerospace vehicles and a direction. In the great rush of 1980’s to get into advanced aerospace flight technologies, these fundamentals were completely overlooked, indeed forgotten by 1990’s, as they were in 1970’s with the advent of the Space Shuttle.
Forgotten also was the creative work in 1964 by W.H. Avery and D.L. Dugger of the Applied Physics Laboratory, John Hopkins University who highlighted that the type of spaceplane that that had received most development effort (although very little was openly published) was a spaceplane flying in the hypersonic (Mach 5-8 range) region. To achieve this, the spaceplane could carry liquid hydrogen fuel on ramjet power, and use air collection and fractionation to obtain oxygen-enriched liquid. This could place in orbit payloads in the neighborhoods of 30% of take-off weight. Subsequently after 1960’s development work on in-flight lox collection ceased.
Forgotten also was the work of Vandenkerckhove and Czysz who ardently advocated that the US must rekindle development work on oxygen-enriched liquid obtained by the development of lightweight air collection and fractionation equipment because (among other reasons) “even India” had recognized its merits.
After these stumblings, a multitude of aerospacevehicle designers and technologists brought out a variety of systems concepts, some quite bizarre, none of which have thus far demonstrated after 70 years even a single flight of an aircraft from earth to orbit with runway take-off and landing. One should not stumble again. As Karl Hoose brings out, HAP is the key, and many nations have this to different extents.
To begin with, Jones and Donaldson brought out that an orbiting vehicle has both kinetic and potential energy; and 95% of the total energy in orbit is the kinetic component. Since kinetic energy is proportional to square of the Mach number, practically all the work is done at very high speeds. Over 75% of the kinetic energy is added after Mach 8; and more than 50% after Mach 15!
Thus from take-off to Mach 8 only 25% of kinetic energy is required to be added in airbreathing atmospheric flight while the hypersonic propulsion system propels the vehicle to well above the sensible atmosphere to around 30 to 40 km altitude. The rocket propulsion system still delivers over 75% of the total energy for flight to orbit from runway take-off and also needs fuel efficiencies a little (~7%) beyond the performance of today’s rocket engines. The energy to accelerate the vehicle and the energy left behind in the atmosphere as heat due to aerodynamic drag, all come from the fuel; for it is in this atmospheric region where most of the aerodynamic drag exists, and it is in this region where fuel efficiencies need to be and indeed are the highest, at least 8 times more.
Jones-Donaldson Design Space
Jones and Donaldson also brought out a parametric study that is shown graphically in their 1987 paper. They found that just three parameters namely, hydrogen fuel fraction at take-off, propulsive efficiency and thrust-to-drag ratio dominate the trade-offs in SSTO spaceplane design. These three design parameters form a spaceplane design space containing possible design solutions for flight from earth to orbit in a single stage. A SSTO spaceplane needs to fall into a design space with boundaries of hydrogen fuel fraction from 0.25 to 0.60; propulsive efficiency from 0.30 to greater than 0.90; and thrust-to-drag ratio between 2.0 and 4.0.
The Second Law of Thermodynamics precludes propulsive efficiencies that approach unity and a specific averaged propulsive efficiency of around 0.4 may be the highest realizable value. Their work showed that thrust-to-drag (T/D) ratios dependent on vehicle configuration design above 3.5 were of little use to attain orbital speed. Above all they brought out an important spaceplane design requirement for direct ascent to earth orbit: more than 56% of the take-off weight must be hydrogen fuel to achieve orbit. It would be impossible, they said, to design an aircraft with a hydrogen fuel fraction of 56%. For example, even if a Being 747 were completely emptied, passenger seats and all, and then filled with liquid oxygen its hydrogen fuel fraction would not exceed 33%. Here the authors did not think beyond aircraft design practice and include the principles of rocket design practice when considering flight from earth to orbit in a single stage in an aerospacecraft. Notwithstanding these observations by the authors, two creative spaceplane design approaches have emerged within Jones-Donaldson Design Space and these are discussed here later.
Confluence of Flight Science Cultures The flight science of an aerospacecraft was similar to an aircraft only while flying in the sensible atmosphere where the focus of designers was to maximize fuel fraction (as oxygen was freely available outside); maximize lift and minimize drag (i.e. maximize Lift-to-Drag ratio). Beyond the sensible atmosphere where there was no airflow to sustain lift, the aerospacecraft behaved like a rocket and followed Tsiolkovsky’s rocket equation to determine flight to orbit.
Transcending the Rocket Equation The Tsiolkovsky rocket equation’s focus was not fuel fraction, but propellant mass fraction (that included fuel as well as oxidizer); and to minimize drag. These rocket scientists however forget that a rocket flying like an aircraft in the sensible atmosphere would follow the laws and practices of aircraft design where fuel fraction and maximizing lift-to-drag ration was the design focus. A synthesis of flight science cultures was needed and was provided by this author in 2010 in a paper “The Spaceplane Equation” brought out by The Online Journal of Space Communication, Issue No. 16: Solar Power Satellites, Winter 2010; and again in 2010 his “Critical Factors in Conceptual design and Techno-economics of Reusable Spaceplanes.
From this author’s early estimations made from available published literature, the only spaceplane systems concepts that satisfy the Jones and Donaldson design matrix of 3 key parameters for flight to orbit in a single stage from runway take off are:
- Approach “A”: Shaped like a Winged Aircraft India’s Hyperplane shaped as a winged hypersonic aircraft to deliver a payload fraction of about 10-11% in low earth orbit. Hyperplane has a Hydrogen Fuel fraction at take-off of 58%; mission average propulsive efficiency about 0.4; and mission average thrust-to-drag ratio of about 3.0.
- Approach “B”: Shaped like a Rocket (Cylinder body) The other one is the British “Skylon” that delivers a payload fraction of about 4% but from a different design point in the Jones and Donaldson design matrix. It is shaped like a conventional (cylinder body) rocket, which attains orbit with 28% hydrogen fraction at take off; mission average propulsive efficiency probably > 0.9; and a mission average thrust-to-drag ratio of probably about 4.0.
In-Flight Aerocryogenic Technology
In all these designs, the hypersonic airbreathing system plays a key role. The important observation here is that the results of 1987 Approach “A” Study corroborates the 1964 Study by the Hypersonic Propulsion Division of John Hopkins University: spaceplanes with a phenomenally high payload fraction are obtainable not just by hypersonic propulsion technologies but by their integration with aerocryogenic technologies that use on-board liquid hydrogen flow through a heat exchanger on its way to the combustion chamber of the hypersonic engine. The cryogenic heat exchanger cools down and liquefies incoming high temperature air in stages.
The liquid air is then fractionated in various ways again by liquid hydrogen emerging from the hydrogen fuel tank to generate liquid oxygen. The resultant cryogenic nitrogen is then circulated back into a separate front-end heat exchanger resulting in high thermodynamic system efficiency where the in-flight oxygen production system, which delivers up to 18 Kg of liquid oxygen to every Kilogram of liquid hydrogen, is circulated from the propellant tank to the hypersonic engine. Some elements of aerocryogenic technologies are said to have been developed and demonstrated in the US, Japan, France, Russia and India.
A typical 100-tonne SSTO spaceplane designed to fall in the Jones-Donaldson Spaceplane Design Space by Design Approach “A”:
- Would have to carry 56% or 56 tonnes of liquid hydrogen.
- It would not carry any liquid oxygen on board at take-off hence its take-off weight as an aircraft weight would be (100-56) = 44 tonnes dry weight that would contain 8-10 tonnes or 8-10% of useful payload.
- This spaceplane would ascend with a Turbojet/Turboramjet engine to 26 kilometers altitude and speed of Mach 3 or 4; thereafter its scramjet engine and in flight liquid oxygen collection system (FLOX system) would swing into operation.
- The spaceplane would expend 75% of its 56 tonnes of hydrogen fuel from take-off up to the end of a half-hour lox collection phase when its speed is Mach 8 and altitude 30 kilometers, leaving behind 25% or about 14 tonnes of liquid hydrogen at the end of collection phase of ascending hypersonic flight (up to Mach 8).
- In this half hour the FLOX systems would be programmed to collect about 70 tonnes of LOX for a mixture ratio = 5 for combustion in the hydrogen-oxygen rocket engine.
- Hence in flight at Mach 8, the spaceplane would weigh 44 tonnes (dry weight) + 14 tonnes (hydrogen left after flight to Mach 8) + 70 tonnes (lox collection) = 128 tonnes. At take off there is no lox and hydrogen fuel fraction is 56%; in flight it reaches a speed of Mach 8 when its rocket propellant fraction will be 66% which is that of a conventional space rocket.
The cost of access to space is dependent on the initial size of the launcher, its payload fraction and structure fraction. Carrying no lox onboard at takeoff would cut down launch weight by nearly 60%, while enhancing payload fraction to very high values reported in the US to be as high as 30% and in India up to 15%. Thus Approach “A” would lead to a very low cost access to space.
Studies in India also revealed that Approach “A” design is down-scalable to around 25-35 tonnes take-off weight to deliver about 1 tonne to orbit. Their simulations show that a 30-tonne Shuttle-class payload fraction can be launched by a spaceplane weighing less than 300 tonnes at take-off compared to the Shuttle’s 2100 tonne launch weight. It can be expected that early stages of spaceplane research, design and development would use a set of different engines based on upgraded gas turbine (turbojet) engine technology for speeds up to Mach 3 or 4; followed by a scramjet engine up to Mach 6 to 8. This is the current approach in China as well.
While Japan is said to have designed a gas turbine engine that operates up to Mach 5 or 6 with air cooling in hydrogen heat exchangers, others in USA and Russia prefer an integrated turbo-ramjet engine in the low hypersonic speed region. The UK Skylon has a unique helium cycle turborocket engine called the “Sabre” engine for flights up to Mach 5 / 6. While some components of such engines were demonstrated on a small scale, none of them have so far been demonstrated as a complete engine even as scaled down models. Conceptual designers of HAP visualize that ultimately a single Liquid Air Collection Turborocket Engine (LACE) would power the spaceplane directly from runway take-off to low earth orbit.
In conclusion the need to rekindle work on hypersonic airbreathing engines and aerocryogenic technologies for in-flight air-liquefaction with light-weight, high temperature hydrogen cooled heat exchangers; and lox separation are crucial for expansion of space applications into space industrialization, space exploration and space security. This is unlikely to happen unless spaceplane designers see the need to transcend the Tsiolkovsky’s rocket equation of 1903 and transcend the boundaries, approaches and cultures of flight sciences in the design of aircraft and space rockets.
 M. Buffo “Technical Comparison of Seven Nations SpaceplanePrograms”, AIAA Space Programs and Technologies Conference, September, 1990. AIAA-90-3674-CP.
 Don Petit “The Tyranny of the Rocket Equation “ https://www.nasa.gov/ mission_pages/station/expeditions/ expedition30/tryanny.html” 05 January 2012
 R.A. Jones and C du P Donaldson, “From Earth to Orbit in a Single Stage”, J. Aerospace America, 25, pp.32-34, 1987.
 W. H. Avery and D. L. Dugger, Applied Physics Laboratory, John Hopkins University, “Hypersonic Airbreathing Propulsion”, J.Astronautics & Aeronautics, pp.42-47, 1964.
 J. Vandenkerckhove and P. Czysz, “SSTO Performance Assessment with In-Flight LOX Collection”, Acta Astronautica, 37, pp.167-178, 1995.
 Gopalaswami R “The Spaceplane Equation” Journal of Space Communications, Winter 2010, https:// spacejournal. ohio.edu/ issue16/ gopal2.html
 Gopalaswami R and Srinivasan K, “Systems Preliminary Design of an In-flight Air Liquefaction and Oxygen Separation System for a Hypersonic Flight Test Vehicle” International Conference on High Speed Transatmospheric Air and Space Transportation”, Hyderabad, India, 29-30 June 2007.
 Ibid (4)
 R. Gopalaswami, S. Gollakota and P. Venugopalan “A Single-Stage to-Orbit Vehicle Hyperplane”, J. Aeronautical Society of India, 40, pp.1-14, 1988.
 Gopalaswami .R , Prahlada, Satish Kumar & Satyanarayana Y – “Spaceplanes with Aerobic Propulsion – Key to Low Cost access to Space “ AIAA 3699-2001 08-11 July 2001 and International Aerospace Abstracts (IAA) 2001
 Takayuki Kojima, Hiroaki Kobayashi, Daisaku Masaki and Hideyuki Taguchi “Development of Air Pre-Cooler for Hypersonic Turbojet Engines For Realization of Spaceplane” Aeroengine Technology Center, Future Space Transportation Research Center, http://www.aero.jaxa.jp/eng/publication/magazine/sora/2006_no10/ss2006no10_01.html