Introduction

The Reusable Launch Vehicle (RLV) Program is a joint NASA and Industry undertaking to develop and demonstrate technologies and approaches for achieving routine, affordable access to space. A demonstrator, the X-33 vehicle, is the focus of this effort. The strategy is to reduce the technical and business risks of a single stage to orbit RLV. The X-33, although subscale, with no crew, passengers or orbital capability, can still demonstrate many of the technologies crucial to eventually building and operating an RLV capable of transporting cargo or passengers to low Earth orbit.

Other programs such as X-34, Bantam (Low Cost Booster Project), Highly Reusable Space Transportation (HRST ) or the Advanced Reusable Transportation Technologies (ARTT) effort are, similarly, about risk reduction through technology identification, development and demonstration.

The example of the Boeing 707 Jet Transport is useful in this scenario. Government spending on jet technology for the KC-135 (a military aerial tanker) was instrumental to concluding, on the part of a commercial entity, that a commercial venture could now be attacked. After all, large amounts of basic R&D funds would not be required, and business risks such as per unit costs and operating costs had begun to be well understood. What was not even a requirement with the KC-135, spurring commercial ventures, becomes, in the NASA case, the primary requirement.

Placing these projects in context is not possible without a review of the only crewed, somewhat reusable launch system in the world - the Shuttle Space Transportation System (STS).

The numbers that follow, even though from 1994, are very useful in that the breakdown is uniquely informative. A similar breakdown is not available for recent years.

Put in another context:

The issue of reusability - true, not in name, goes to the matter of reliability. Reliability in flight is inevitably directly related to the reliability on the ground, during turnaround. This "process reliability" is often referred to as dependability.

Consider that a single stage to orbit concept can eliminate those costs of a space transportation system associated with expendables as shown previously.

Does this automatically result in a truly reusable and hence affordable space transportation system? Not entirely. The reliability or true reuseability of the remaining transportation system element becomes the next concern. As seen previously, the current STS orbiter consumes vast resources in logistics and operations which revolve around parts being tested and many being replaced. How can this problem be resolved?

"Possibilities"

Assuming an all rocket spaceliner single stage to orbit is possible the next concern is how operable can it be made? The basics of gravity, rocket propulsion and attaining orbit dictate a certain "mass fraction" for the vehicle. This is the ratio of vehicle dry weight to total weight. For an all rocket SSTO the numbers dictate a vehicle that is light and mostly propellants, a structure not very likely to be robust. Consider that a commercial jet is quite the reverse, mostly vehicle, not propellant. Admittedly the demands of reaching orbit, high velocity and systems unique to orbit and return, apply to space transportation and not today's commercial aircraft. However, the possibilities of reduced required mass fraction, and relations to operability through greater margins or robust designs (one example is thermal protection), apply in both cases.

The X-33 will demonstrate the use of composite materials for structures and cryogenic tanks, new avionics and metallic thermal protection systems as well as hosts of other new technologies. However, beyond X-33 lies new propulsion. Airbreathing is about robustness and reduced required mass fractions allowing operability.

Consider an aircraft analogy again. For a turbojet all of the thrust is from the exhaust gases of combustion.

For a high bypass turbofan the majority of the thrust is from the bypass, the air in which the engine travels.

Consider the rocket evolution along these lines. For a rocket all thrust comes from the reaction mass that is entirely carried by the vehicle, both liquid oxygen and liquid hydrogen for example.

For an airbreather the air is used to add to reaction mass and or thrust. Hence not all oxidizer is carried aboard. The airbreather uses the sea of air in which it flies, adding to thrust and specific impulse. Mass fractions much lower than Shuttle or any Rocket SSTO become possible.

Specifically:

• All that the X-33 will demonstrate is also applicable to an airbreather
• Aircraft like operations require something that is more like an aircraft, such as in reduced required mass fraction

Admittedly, the functions of commercial aircraft are as distinct from an orbital capable spaceliner as the early Wright Flyer is from the DC-3 of old (many are still flying). Also, mass fraction is only one operability aspect among many, albeit a very important one. Multiple complexity issues must be resolved focused on the lessons of current systems such as Shuttle. This means eliminating toxic fluids, reducing different fluids, integrating propulsion, reducing interfaces, increased automation and much greater reliability from components up to systems levels is required to achieve truly aircraft like operations. Further technology, tied to these prior characteristics, are required to be demonstrated.

Kennedy Space Center has been actively involved in activities to define the quantifiable features that should guide future systems development so as to achieve low cost in the long run.

Ultimately this comes down to:

• Accepting the challenges before us in technology and approaches
• Joining with industry partners to overcome these
• The strategy is not to operate or institutionalize a space transportation system, but rather to spur growth on the space frontier by enabling private sector initiatives that otherwise would not have been commercially viable

Opening the Space Frontier - putting out the technology that will allow entrepreneurs to one day say "This can be done, the risk is manageable, a business can be formed, the high risk R&D has been completed, the concept has been demonstrated and the business venture awaits". Competition and expansion of markets then drives costs further and further down bringing the benefits of space to multitudes in ways never before imagined.

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Edgar Zapata, NASA Kennedy Space Center

Shuttle Process Engineering Directorate, Fluid Systems Division