Design and operational aspect of LOX/LH2 propulsion system of reusable vehicle testing (RVT) (2001)

For achieving the high performance required for future vehicles, liquid hydrogen has the greatest promise as SSTO (single stage to orbit) rocket fuel. However, for ground operations of the vehicle, cryogenic fuel imposes a number of constraints which are essential for operational and safety reasons. For the flight test campaign of a reusable rocket vehicle, a small liquid oxygen (LOX) / liquid hydrogen (LH2) engine was built. This is a regeneratively cooled engine with a continuous throttling capability to make vertical landing possible, and the vehicle's fueling systems are designed to facilitate repeated flights and quick turnaround. This paper summarizes the engine design and operational lessons learned in the flight test campaign. INTRODUCTION For the growth of future space development, extremely high transportation cost is the most serious problem to be solved. Space transportation costs must be reduced by two orders-of-magnitude in order to be able to utilize space as a means for solving present social problems. Reusable launch vehicles offer the promise to solve this cost problem by replacing existing expendable vehicles. However, in order to realize the above cost target of an order-of-magnitude or more cost reduction, not only reuse of vehicle hardware but also a simplified launch system avoiding complex operations is essential; this includes simple deactivation of a vehicle after landing and quick checkout for its next flight, which will enable quick turnaround so as to realize daily flights like today's aircraft. In order to meet the requirements, in the United States, several development test programs of reusable vehicles are underway in the Integrated Space Transportation Program (ISTP). The European Space Agency has studied reusable launch system concepts as well as some of their required technologies in the FESTIP program. In Japan, the Japan Rocket Society conducted the first ever systematic study on commercial space travel in the world, and proposed the concept of * Research Associate, Space Applications Division. Member AIAA. f Research Associate, Space Propulsion Division. Member AIAA ^Senior Engineer, Space Development Division. § Assistant General Manager, Space Development Division. Copyright© 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. aircraft-type operations of space vehicles. A committee led by the Ministry of Education, Science and Technology is currently studying concepts for a future space transportation system. In these activities, many technical problems needing to be solved have been identified, and design concepts for realizing the required space transportation system have been proposed. From the viewpoint of propulsion system development, LOX/ LH2 seems the most promising combination because it will enable SSTO rocket flights and is environmentally friendly. With regard to the ground operation of the vehicle, the use of cryogenic fuel imposes constraints which are essential for operational and safety reasons. To achieve quick vehicle turnaround, therefore, it will be necessary to develop and refine repeatable operations techniques for fuelhandling. However, there have been very few experimental efforts aimed at this objective. To date, the DC-X program is the only one which has made operations data available. In view of this situation, the Institute of Space and Astronautical Science (ISAS) has built a small test vehicle, with the objective of learning about the operation of reusable rocket vehicles and demonstrating repeated flights. This campaign is named Reusable Vehicle Testing (RVT), and has been conducted since 1998. A conventional 10 kN class LOX/LH2 rocket engine system is used for RVT because of its mature technology and existing components for using cryogenic fuel. As the vehicle for RVT, a vertical-takeoff-andlanding (VTOL) type has been selected not because it is known to be the most promising but because it is convenient for performing repeated flight. In particular, a VTOL system does not require as wide an area for flight demonstrations as alternative systems. Using this test vehicle, vertical landing flights and one-day turnaround were successfully demonstrated in 1999. The vehicle has been partly modified since the first flight test campaign, and ground firing tests were completed in 2000 for the succeeding flight tests. This paper describes the design and operation of the reusable rocket propulsion system used for the RVT program, and lessons learned. ROCKET PROPULSION SYSTEM REQUIRED FOR FULLY REUSABLE VEHICLE The authors consider that the propulsion system (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. required for a fully reusable vehicle must have the following features: • LOX/LH2 as propellants because these offer promise for rocket SSTO, and are also environmentally friendly. • Sufficient performance, namely high specific impulse, wide operational region, and others. • Durability and long life. • Ease of maintenance and simple turnaround operations. • Sufficient redundancy to provide continuous abort capability. • System improvement to the level at which the probability of loss of the vehicle is as low as that of today's aircraft. At the start of the RVT program, we targeted repeated use of the propulsion system utilizing conventional technologies. This RVT objective has given priority to practicing re-use and turnaround procedures on a LOX/LH2 system rather than to achieving performance improvement in order to reach orbit. We therefore applied the following factors to the RVT vehicle: • Use of LOX/LH2 system as the propellant. • Use of rocket engine for the main propulsion system. • Vertical take-off and landing type. • 50 reuses in order to achieve repetitive operations. The next sections describe the design and operational aspects of the RVT propulsion system in detail. DESIGN AND FLIGHT-TEST RESULTS OF RVT RVT Vehicle Overview The main objectives of the RVT program are as follows: 1) To learn the technique of vertical landing using a rocket engine. 2) To acquire the techniques needed for the construction and operation of a propulsion system capable of quick turnaround. We planned to build a vehicle system fulfilling the above four factors (namely LOX/LH2 system, rocket engine, vertical-take-off-and-landing, and repetitive flights) with minimum components. The reusability of each component was ensured basically by derating the system. Then we equipped the RVT vehicle with a rocket engine with the following features: • The engine combustion chamber was a refurbished one which had previously been used for development tests. • A gas pressure feed system was used to simplify the propulsion system, namely so as to minimize the number and the kind of pressure regulators and control valves. A schematic view of the RVT vehicle is shown in Fig.l. The vehicle is 3.1 m in height, 317 kg in dry weight, and carries about 50 kg of propellant. It is capable of vertical ascent, horizontal movement and RCS GN2 tank Fig.l RVT test vehicle. vertical descent, lasting about 15 seconds in total. It was not equipped with an aeroshell because of its low flight speed. Its navigation and guidance control system is based on an inertial measurement unit (IMU) and a laser altimeter. Attitude control is conducted by the reaction control system (RCS) using a cold gas jet at the top of the vehicle. A support structure such as a launch service tower is not necessary for flight operations since the vehicle can stand on its own four legs. Many details about the sub-systems and control-system of the RVT vehicle are summarized in References 2 and 4,