Configuration and Preliminary Design Studies for the “LEO Fracht Transporter” (LFT) Concept

Kurzfassung

For a long time space engineers have been interested in the possibility of enabling single-stage-to-orbit (SSTO) vehicles. Many studies have been performed for such spacecraft, but so far the feasibility has not been proven and accordingly no vehicles have ever been built. The largest bottlenecks are to be found in the high mass of the vehicle, which needs to be accelerated to a high orbital velocity, and the propulsion systems that are available. For a SSTO concept to be successful the mass needs to be lowered significantly, combined with the use of an efficient propulsion system. With this in mind the LEO Fracht Transporter (LFT) study was started; a study to develop a SSTO or at least “quasi” SSTO launcher within the SART department of DLR Bremen. The scope of this study is to implement new technological improvements within an otherwise non-feasible SSTO design, in order to make it feasible. The goal of the internship, which this report summarizes, is to define a starting point for the LFT, from where the improvements with the new techniques can be applied. As with the technology of today a SSTO launcher is not possible yet, the starting point solution of this work is not expected to be feasible as well. To support the definition of the starting point solution several tools were used that are available at DLR. Amongst others, these tools generated a mass estimation for the vehicle, as well as an estimation of the aerodynamic coefficients. Furthermore, the trajectory of the air-breathing and the rocket ascent phase were simulated. A first tool chain was composed to generate a vehicle configuration. With the use of this tool chain a sensitivity analysis was performed to investigate which parameters have a high influence on the performance of the space vehicle, and what values are desired for the parameters under investigation. Three parameters concerning the fuselage geometry, and three concerning the wing geometry were evaluated. It was found that the aspect ratio as well as the wing thickness have the highest impact on the launcher performance, with preferred values of 0.75 and 0.03, respectively. The slenderness of the fuselage and the fuselage slope were found to have a medium sensitivity value. For both of these parameters a balance needs to be found between a longer heavier vehicle with advantageous aerodynamic coefficients, and a shorter and blunter vehicle with higher drag but lower structural mass. The sweep angle was found to have an insignificant influence. The nose radius was also investigated; however it was found that the results for this parameter from the corresponding tool were unreliable. The tool chain was later expanded to include additional tools, which increased the fidelity to which the vehicle geometry could be designed. Furthermore the precision of the aerodynamic coefficient estimation was increased by adding a tool specifically designed to evaluate these coefficients in hypersonic flight. With the improved chain a baseline design for the LFT vehicle was generated. This vehicle is assumed to have 2 SABRE engines as the propulsion system which are both used in the air-breathing and the rocket ascent phase. The engines rely on liquid hydrogen propellant in the air-breathing phase, and a mixture of liquid oxygen and hydrogen in the rocket ascent. The vehicle carries a total propellant mass of 260,000 kg, and has a length of 75 m. As expected the design is not able to achieve orbital velocity and altitude, instead it reaches a final velocity of 4.96 km/s at an altitude of 72.72 km. Several additional investigations were performed to evaluate the impact of selected vehicle parameter variations on the performance, such as the use of different engines, and the effect of a reduction in mass. First the impact of a theoretical reduction of the structural mass due to advanced materials was evaluated. It was found that such a mass reduction does have a significant positive effect on the LFT performance, but this is not sufficient to enable the feasibility of the vehicle. For a decrease in structural mass of 50 % the final velocity reached is increased by only 0.7 km/s. It was also investigated how the vehicle would perform if the propellant and dry mass of the REL/Skylon vehicle are applied. With these mass characteristics the LFT vehicle is able to reach orbital velocity and altitude, which confirms that the selected shape and aerodynamic properties of the baseline vehicle are not compromising its feasibility. The next variation investigated was the theoretic use of kerosene fuel instead of liquid hydrogen in the air-breathing ascent phase. The results in performance of this first order estimation are approximately equal to the performance of the baseline configuration, which uses solely liquid hydrogen in the air-breathing phase. This leads to the conclusion that both the use of liquid hydrogen and kerosene propellant could be interesting options for the LFT vehicle. Finally a variation of the baseline design was investigated which uses a combination of turbo- and ramjet engines in the air-breathing ascent trajectory, and a separate rocket engine in the rocket ascent. As these engines are significantly heavier when compared to the REL mass estimation of the SABRE engines, it was found undesirable to use such a propulsion system for the LFT concept.