Floats as Tanks

This is a picture of hydrotesting I did on a 6" 316 Stainless steel float from McMaster-Carr (2762K49). It failed at 2150 psi. Interestingly, the failure was a tiny pinhole that opened right at the weld termination. My grease gun was not able to pump enough water fast enough to get the pressure higher higher than 2150 once the hole opened up.

I proof tested a second of these floats to 1900 psi without failure. And 3 more were tested to 1500 psi.

Valves (again!)

I have developed a direct drive version of the valve I've been using. It was used in my last testing which was described in the previous post.


The packaging comes out quite nicely. They have several advantages to the linkage actuated version - this will be my go forward design.

Along those lines, I have been wondering if there are people out there that might like to buy a pre-made valve like this one. It was actually more of a pain to build than I thought it would be and personally, I prefer to spend my time building rockets rather than various peripheral devices.

Here are some of the relevant details:

• CV of 1.6 and is rated for 1.6 gpm of flow at 1 psi delta and 5.1 gpm at 10 psi
• rated to 3000 psi
• can throttle fairly well and has fast response (as can be seen on this blog)
• o-ring sealed and can be compatible with most non-cryogenic propellants, including hydrocarbons and hydrogen peroxide
• the servo would be a Hitec digital servo with titanium gears (6.0V or 7.2V input)

If you're interested in buying a valve like this, drop me an email.

Fits and Starts

Long time, no post. I wish I could say the delay was due to my making lots of progress and not having time. The truth is much closer to not having much to share.

I was able to get out and test some modifications this weekend. The good news is that the urethane o-rings seem to be working. I did a long term pressure test with N2O to see if they would remain sealed for several days. Also, after the testing this weekend they showed no signs of damage.

The main point of this weekend's testing was to test a new configuration with the propane/spark ignition with the goal of eliminating the initial pressure spikes. The new configuration seems to have reduced the tendency towards hard-starts. Unfortunately, the ignition sequence which was quite reliable before is no longer. I spent quite a bit of time just getting it to light at all and stopped testing without arriving at a reliably repeatable sequence.

Valves

Actuated valves are a tricky thing for rocket motors. Their requirements usually exceed what is available from the McMaster catalog. Of course, if you are willing/able to pay big aerospace money, you can have pretty much what ever you like. Unfortunately, most amateur and "new-space" organizations don't find themselves in that situation.

A common solution is to use ball valves. They have several advantages including reliability, availability, and some throttle-ability. However, the packing around the ball generates considerable amounts of friction, especially when the valve hasn't been moved for some time (sticktion). As a result, ball valves require rather significantly sized actuators which are often heavier and larger than the valve itself.

While browsing through the Swagelok catalog some time ago, I discovered a valve they call a "plug valve". It is similar to a ball valve but with a cylindrical rotating plug that is sealed with o-rings. The result is a valve that has very low friction and thus can be actuated with correspondingly smaller actuators. The primary downside is that the o-rings become a consumable item and prohibit their use with cryogenic fluids such as lox.

Here is a picture of a plug valve actuated with a Hitec servo.

This is a movie of some early testing I did with a garden hose and a standard Futaba servo.

If you look closely in the video in this post, you can see the valve moving in the lower right-hand corner.

As I alluded to in the previous post, I've had several o-rings tear when being closed after use at part throttle with N2O. The valves come from Swagelok with PTFE coated FKM o-rings. I assumed they were not rated for lower temperatures so I ordered some PTFE and urethane o-rings. Below is a disassembled valve with a urethane o-ring.

Last night I did some simple testing with the urethane o-rings and to see how cold the N2O is getting downstream of the valve. The picture of the setup is below showing the valve, thermocouple, and injector orifice.

As you can see, it gets pretty cold. At minimum throttle, the temperature reached a steady state value of ~ -84F after 20 seconds or so. The urethane o-rings are rated to -65F and the PTFE rings are rated to -100F. I'm going to try the urethane rings first because the PTFE rings are very hard.

Last up is a neat picture showing the formation of solid N2O at the orifice exit. Visible are both ice-sickles and snow.

In summary, with the right o-ring material, these valves should work for all storeable propellants such as ethanol, kerosene, and peroxide. They should also work with saturated liquids stored under their own vapor pressure, like ethane, propane, and N2O. The odds of working with LOX are slim to none.

10 Points for Damien

Recently, I've been working on integrating my test setup onto a trailer so that I'm not constrained by working in the garage. I haven't had any complaints but I felt that doing 60-90 second burns might be pushing my luck in the future.

Last night was the first attempt at using the trailer to test and I was very pleased with how it all turned out. It took almost exactly 1 hour from the time I arrived home from work to the first test. During that time, we loaded the van, drove to the test location (about 10 minutes), and setup the electronics on-site. There were no issues with the test stand and everything worked correctly the first time.

If only the rocket motor were so easy ...



The light sequence is very reliable. I actually had two lights that are not shown in the video - for the day, the motor was 5 for 5 on light attempts. Unfortunately, there is still a fairly high pressure spike at ignition that was triggering a redline on chamber pressure. I had lowered the chamber pressure redline to just a tiny margin above the design maximum - the startup spike is around ~25% higher than that. On the first two tests of the day (not shown in the video), the software redlined at engine start and aborted the test. For the the first two tests in the video, I raised the redline limit and the motor started fine and executed a scripted throttle profile. On the last test, I knew that there was probably a damaged seal in the throttle valve but I decided to initiate a test anyway (the venting between the second and third light is a result of the damaged seal). The end result of that test is obvious - another blown gasket.

I'm not sure yet what the exact cause of the blowout was - the data did not show a pressure spike that was larger than any of the previous tests that day. I have not yet reviewed the data in detail nor have I disassembled the motor to inspect it.

So I have three problems to solve with this motor right now:

1) Pressure spikes at start up - I think I have a plumbing solution to this
2) Blown gaskets at start up - I think solving 1) should help solve this one. Maybe a new gasket material is in order.
3) Repeated failures of valve seals - More on this later

PS - Damien gets 10 points. The propane bottle can be seen in the video at the center of the screen. However, I would not call it a "torch".