Development and Tests of the SpaceForest (SF) Rocket Engines
A rocket engine produces thrust by expelling hot gas at a very high velocity. Calculating thrust and specific impulse are fundamental to the design of any rocket engine. Achieving the optimal oxidizer/fuel (O/F) ratio and understanding how the chamber pressure and the combustion process affect specific impulse is the key to achieving an efficient system.
In the early stages of our rocketry activities we were focused on simple solid rocket engines. While hybrid propulsion systems offer certain advantages, they are not as simple as solid rocket motors based on ANCP (Ammonium Nitrate Composite Propellants) developed by SpaceForest engineers even before SF was founded. However, hybrid propulsion technology offers a number of advantages over the solid rocket fuel systems.
Among all types of rocket propulsion systems, hybrid motors offer unique properties and parameters. Solid rocket fuel systems allow to apply simple design motors, characterized by great energy density (due to high propellant density). Simple design, high thrust and low cost of the motor are major advantages of SRB’s (solid rocket booster). On the other hand, SRB’s cannot be shut down, cannot be throttled, operate for a relatively short time and present a potential risk of explosion. What is more, these systems require solid rocket fuel, which is actually an explosive and requires special manufacturing, handling and using conditions (both technical and legal). This causes an increased cost and many restrictions for the launch campaign.
Recently, more and more attention is paid to hybrid rocket propulsion. These motors fill the gap between solid and liquid systems, having advantages of both of them. They can be shut down, throttled, restart and operate for prolonged times thank to possibility of regenerative cooling just as liquid motors, but they are much simpler, thus cheaper in design and manufacture than liquid fuel motors. What is extremely important, hybrids present a negligible risk of explosion. Chemicals used in combustion process are usually non-toxic, cheap and easily accessible. A vast choice of different compounds can be used as fuel. Among them, a substance of especial interest seems to be paraffin wax. It’s a cheap, non-toxic, easily accessible fuel, offering high specific impulse and high regression rate. It can also be easily modified by a variety of additions.
SpaceForest Ltd. is located in the Pomeranian Science and Technology Park (PSTP) in Gdynia, Poland. As a member of the PSTP our company has access to a common technical area of the Science and Technology Park, which includes various measurement and manufacturing devices. Apart from this, thanks to Polish Rocketry Society, SpaceForest has access to a special industry area outside the PSTP. This place is safeguarded from unauthorized entrance and all tests of hybrid rocket motors are performed there. Our recent test bench is fully mobile and equipped with measuring instruments up to 5 [kN] of thrust.
The SF1 hybrid rocket engine for SF small research rockets is the largest of its kind ever made in Poland. In order to launch the 100 mm diameter research rocket to the altitude above 10 km it will provide an average thrust of 3,0 [kN] for 6 seconds. The SF1 hybrid rocket engine uses nitrous oxide as an oxidiser and a modified paraffin wax as the fuel.
Our SF1 motor carries 10 kg of N2O in a lightweight high pressure tank. At 51 [bar] it is supplied to the chamber through a high speed main ball valve equipped operated by an electric actuator.
The hybrid combustion chamber is closed with a specially designed injector and a pyrotechnic preheater. At temperature of about 600°C the N2O is decomposed to nitrogen and oxygen. The reaction is given as:
N2O→ N2 + 1/2O2 + HEAT
Oxygen thus generated reacts with fuel, forming gases at a temperature of about 3000°C. The combustion products then enter the post combustion chamber for better mixing and reaction and to the nozzle which converts the high pressure, low velocity gas into high velocity low pressure gas generating up to 3,3 kN of thrust.
The chamber contains 2,0 kg of fuel which can burn for up to 6 seconds, consuming nitrous oxide in liquid phase plus several seconds in gaseous phase.
Twenty firings have been conducted to test various aspects of combustion chamber, fuel grain and injector design. Most of the rocket development work so far, has been conducted on a lightweight aluminum alloy chamber (EN-AW 2007) with 100 mm outer diameter. The safety coefficient is 2. This approach provides a possibility of using the same motor in the actual rocket.
The development of the engine brought some unexpected technical problems due to unstable combustion process. Next ten ground tests were needed to find a solution of this problem, however avoiding the combustion instability of the SF1 engine is still a large technical challenge.
The motor test with fully filled tank was successfully conducted in the middle of April 2015.
However one of the challenges is to maximize the specific impulse of the SF1 rocket engine while reducing the mass of its key components (specially the post-combustion chamber, nozzle and throat) – this is still the “proof of concept” project. Attempts of calculating the heat transfer to rocket motor components as well as many variables in a combustion chamber dynamics require extremely complex mathematic models (including CFD analysis) and was not taken into account in those early stages of the project.
Currently, the work on the 100 [mm] chamber includes a research to create a partial CFD models to enable an understanding of how the mixing, flow, heat transfer and chamber arrangement affect combustion efficiency. The combustion modeling is made in cooperation with Technical University of Gdansk.
Our target specific impulse for SF1 hybrid engine is 2000 [Ns/kg] at the sea level. Currently this goal is almost achieved, and we are expecting better efficiency in following tests. Next tests will be performed to improve a thermal insulation of the chamber, obtain better thrust stability and combustion efficiency.
In the end of April 2015 the engine was used to launch the Candle 2 rocket.
The system performed well during the 3 second firing but some thrust instabilities were observed.
By the third quarter of 2015 we are planning to have completed further development firings with the fuel mixture modifications. By the end of 2015, the beginning of a parallel up-scaling internal SpaceForest programme to obtain half a tonne of trust is expected.