The BPM2 engine is the smallest and simplest motor in the BPM series. It is specifically intended for investigation into the relationship between fuel mixture and heat transfer to the engine.
The engine is designed to be as simple as possible to minimize production time. Thus it consists of a LOX-manifold, an injector and an engine tube. The latter is essentially just a 60 mm diameter tube welded to a flange that attaches to the LOX/fuel-manifold. The engine tube does not have a throat and is not regeneratively cooled as the BPM5. The missing throat and expanding nozzle gives a significant reduction in thrust but this engine is purely designed to test heating rates for various fuel mixtures.
The LOX-manifold is machined in aluminum and so is the injector. The pipe and flange is made in steel however.
The injector is also a significant simplification if compared to the BPM5 engine. It consists of 14 pairs of like-on-like 2.1 mm fuel injection holes and 14 pairs of 2.2 mm LOX injection holes. All parts are made in house at the CS machine shop.
Take a closer look at the BPM2 engine in the video below
Water flow test
Prior to static test the fuel and LOX injection holes are characterized through a water flow test. In this test the flow rates are characterized as a function of feed pressure and based on this we can fine tune the feed pressure to give a certain oxidizer to fuel (O/F) ratio.
Get a closer look at the BPM2/BPM5 teststand used in this test in the video below
The BPM2 is used for two main purposes. First, it is used to test out the new test stand which has never been used before. Second, it is used to characterize the thermal response to various fuel mixtures.
We have for a number of years used a fuel consisting of 75% ethanol and 25% water. The water is added to lower the combustion temperature and thereby minimize the thermal load on the combustion chamber. This gives a significant penalty in performance however, this penalty comes in several parts. First, it reduces the optimal O/F ratio and hence it reduces the average propellant density which means a rocket needs bigger tanks. Second, it increases the average molar mass of combustion products which in turn result in a decrease in thrust.
Thus, the time has come to look into this and optimize the fuel performance with a series of tests. In particular the goal is to compare the combustion chamber heating rate and maximum temperature for fuel mixtures with varying water content. In addition to this experiments are also made with adding TEOS (tetraethyl orthosilicate) to the fuel mixture. When burning TEOS a layer of silicon oxide will form on the chamber wall and this will cause a significant decrease in heat transfer to the chamber wall. The only way to quantify this effect is through static tests!
Check the video with Jonas below for more details on the TEOS additive experiment
On April 12th and May 2nd we conducted a total of 12 hot fires of BPM2 engines.
The BPM2 is pressure fed with a feed pressure of 19-20 bar and at this operating point it delivers a thrust of about 3 kN as seen in the graphs below which represents a typical burn.
The most interesting aspect is however the thermal response from the engine tube. This is monitored with five thermocouples TIG welded directly to the engine. So far only data from the first three tests have been analyzed and it is shown below.
In the pure ethanol test we reach about 480C after 5 seconds and this appears to be thermal equilibrium. Adding 25% water to the fuel decreases this temperature to 330C at the 5 second mark, the temperature is however still increasing slightly at this point. Switching to a 1% TEOS, 24% water and 75% ethanol mixture decreases the temperature at 5 seconds to just 240C. Thus we see great promise in the use of TEOS as an additive in our fuel, this is however still pending analysis of data from the latest BPM2 test series.
Watch the nine BPM2 hot fires conducted on May 2nd below.