Design of propelled projectiles to provide escape velocity for Nano-Satellites

Kurzfassung

Since the eighties, the potential to launch small scientific payloads by means of an electromagnetic railgun is being investigated in many countries. In Europe, EADS Space Transportation is leading a study aimed to investigate the possibility to launch pico- and nano-satellites into LEO, based on a combination of an electromagnetic railgun and a one stage small rocket launcher. In the frame of that investigation, DLR is responsible for the design of a small rocket launcher, here called propelled projectile. The paper deals with the main design aspects to be considered for such propelled projectiles, particularly with emphasis in the flight mechanics, aero/thermodynamics and propulsion properties to be fulfilled. For the present study it is assumed that an electromagnetic railgun is able to launch a 120mm diameter projectile with a speed of 6km/s. In order to achieve orbital motion of the payload and to compensate the velocity losses due to gravity, a rocket engine with simple pressure feeding technique, ablation cooled combustion chamber and plugged (aerospike) nozzle is integrated into the projectile. Three concurrent propulsions systems are here considered for the rocket engine: solid, liquid and hybrid type. It is shown that in spite of the more potential development of liquid rockets, hybrid rocket engines are today the most promising, i.e. they have lower total mass, are simpler, cheaper and more reliable. Furthermore, the aerodynamic form of the projectile is optimized to achieve minimum drag force, which also leads to lower heat loads. Launch angles and trajectory analysis are carried out by means of 3 degree of freedom simulations (3DOF). The calculations have been done accounting for three different technology levels on propellants (present, near and long term), launch altitudes (geographical elevation) and elevation angles of the railgun muzzles. The study considered also the technology aspects related to the large acceleration force at which the projectile is exposed during the start (10,000g) together with the projectile mass constraints imposed by the railgun. Due to the high start acceleration, the projectile experiences large mechanical and thermal loads in particular at the nose and at the junction between body and fins. Based on the results obtained with the 3DOF code of DLR, critical trajectory points are identified. At those points, the thermal and mechanical loads are estimated using the DLR computer code TAU, a finite volume Reynolds average Navier-Stokes. The structural mass is mainly analyzed on the basis of carbon fiber reinforced materials. It is shown that launching today small payloads into LEO by means of a railgun system is a realistic goal.