Author - Colin Wagstaff, February 1997
In Spacesignl 85, I expressed my surprise that NASA's proposed new shuttle was to be a single stage-to-orbit vehicle. I remarked that my understanding was that we had not yet developed the fuels with a sufficiently strong punch to do this. It seems from information from the ESA that the Americans do not yet have this capability, despite the impression created by the narrator of the TV item.
The power density-specific impulse of present day fuels is not sufficient, even with the lightest materials, to achieve orbital insertion in one leap. According to H.Pfeffer of the ESA Future Launchers Office, Directorate of Launchers, Paris, the best existing propellant is still hydrogen burned with oxygen. This combination gives the highest practical rocket exhaust ejection velocity.
The ESA is searching for "...ways to reduce the cost of access to space and thereby open up new markets." The ultimate design goal is for a single-stage to orbit vehicle able to take off from many possible bases the way aircraft can, accelerate to orbital velocity, release it's payload and land at the takeoff site, ready for the next flight after a quick turn around.
The best fuel available today (cryogenic propulsion) requires that about 89.5% of take off mass is fuel. Assuming the payload mass is 1%, the vehicle itself can only be 9.5% of take off mass. Today's materials are insufficiently strong to allow this thus fuels and/or materials need to improve before genuine single-stage to orbit can be achieved.
The HOTOL designer Richard Bond was, I think, the first to consider an air breathing engine to reduce fuel weight at take off. SKYLON, another British effort would combine air breathing with rocket propulsion. Taking a space vehicle to high altitude on the back of an aircraft has also been studied. ESA's conclusion being that it's advantages are outweighed by limitations on take off mass imposed by the aircraft's lift capability and by the "...dangerous separation in the presence of aerodynamic air flows".
Another of ESA's ideas is to replace the aircraft in the previous example by a rocket capable of returning to the launch site, the second stage being reusable or expendable. With today's technology, this idea would marginally reduce flight costs but would offer higher reliability and safety. By giving the first stage rocket a once round the globe sub-orbital capability, a further reduction in fuel needed for the orbit insertion stage would be achieved.
Further studies have been concerned with improving the two stage method by adding aerodynamic lifting body and waverider shapes. Because the first stage would need to have good aerodynamics for the return flight, the next step would be to use these features for horizontal take off and lifting on rocket power only. The lifting surfaces acting to compensate for a vertical take off vehicles shorter time in the drag of the atmosphere. It is believed that such a vehicle could lead to worthwhile improvements in that it could lead to a horizontal take off/glide to land vehicle for global travel.
The advantages of boost-glide travel are:-
The boost phase is a pure acceleration phase with little energy loss. Rocket exhausts contain no nitrogen oxides as no air is consumed.
The ballistic phase is outside the atmosphere and has no effect on the planet.
The hypersonic glide is at such high altitude that the sonic boom does not reach the ground thus allowing hypersonic flight over land masses.
The vehicles high speed allows it to benefit from centrifugal force in order to reduce the required lift, in turn reducing drag and providing a greater operating range for the same initial energy.
Global travel times are much reduced compared to supersonic travel.
With today's emerging technologies, launcher reusability offers the opportunity to serve both the space launcher and global travel markets with one type of vehicle. The vehicles themselves would differ in detail, depending on their exact roles, but would rely on a common technological base.
Such synergy would be to the benefit of both communities: it would enlarge the technological base available for space activities, it would spread the resulting development costs, it would amortise more rapidly all investments and it would stimulate a new approach to travelling around our planet which in turn could provide a strong motivator for the younger generation to pursue a scientific/technical career.
Space has the potential to develop into a very large industrial park stimulated by cheap, routine access to space in the same way global markets on Earth have been facilitated by the growth and falling cost of transport. Assessment of the possibilities for making this happen deserves commensurate funding for exploratory work. ESA's FESTIP programme will hopefully contribute to this goal.
With thanks to the European Space Agency Bulletin for permission to use their material in this article.