Design and optimization of two stage launch vehicles with the same liquid propellant rocket engines in both stages

Abstract

Space exploration is an important technological catalyst for humanity. While researching space and its practical uses, it accelerates the development of new technologies. Reaching orbit is a difficult and complex problem. To get the speed required to stay in orbit, launch vehicles need to have very high propellant mass fraction ratios and high performing propulsion systems. Reaching the performance limits to reach orbit needs high technology and expensive materials to be used. Because of this it is very expensive to put payload into orbit. In the recent years private space companies are entering to the launch vehicle market. These privately funded companies try to drop the prices to be able to compete with existing launch service companies to insert payloads into orbit. To do so they try to reuse the same liquid rocket engines in all stages to drop the development and manufacturing costs. Most of the private launch vehicle companies are designing only one rocket engine and are using them in their 1st and 2nd stages. While the 1st stage engines are bundled together using engines that have sea level optimized nozzle. The same engine is used in the 2nd stage with a vacuum optimized nozzle. Doing so, they reduce the development costs, complexity and manufacturing costs of their launch vehicle. Also the new trend is to design the launch vehicle as reusable as possible. This allows for cost reductions that make the launch vehicle more competitive in the market. Some companies that use this approach are SpaceX, RocketLab USA and Relativity Space. In this thesis, a launch vehicle optimization tool is developed specifically for two stage to orbit vehicles that use the same liquid propellant rocket engines for all stages with only minor modifications. In the 1st stage many sea level optimized engines are bundled together and in the 2nd stage a single vacuum optimized engine is used. It can design launch vehicles for different propellant combinations and liquid rocket engine cycles. Most launch vehicle design methods estimate the stage properties and try to distribute the mass of the stages based on estimations. After finding a viable solution it is designed in detail and the assumed performances of the stages cannot be achieved. This causes an iterative design loop that is resource draining. To solve this problem in this thesis the liquid propellant engines and stages are designed in detail. Firstly, the liquid propellant rocket engine is designed in detail and after that the stage is created by adding tanks and pressurization system. The stage design tool is connected and implemented such that it can design stages with bundled engines for the 1st stage and modifies the same engine as vacuum optimized for the 2nd stage to create the desired launch vehicle. The stage design tool is connected to an optimization algorithm and launch vehicle design tool to create the specified launch vehicle design tool necessary for this thesis. One of the most important design parameters for a launch vehicle is the required delta V for the selected mission. But without simulating the launch trajectory making a good estimate for required delta V is difficult. Therefore, to validate the designed launch vehicles, an orbital trajectory simulation code is developed based on MATLAB. Using this simulator, the designed launch vehicles are trajectory simulated and if successful they are validated or if they are unsuccessful the design parameters are updated accordingly in launch vehicle design code and the process is repeated to find good performing launch vehicles. Designing a launch vehicle is a complex multi-disciplinary and multi objective problem. To rapidly design the launch vehicle the most important parameters are selected as payload capacity, vehicle delta V capacity and T/W ratio at liftoff. The payload capacity and delta V capacity mostly influence the mass of the launch vehicle. Whereas the T/W ratio at liftoff determines the engine thrust and orbital launch performance of the launch vehicle. The optimization algorithm is developed such that it searches for the launch vehicle with minimum liftoff mass while ensuring the design input parameters are met with minimal error.