The RRS held a launch event at the Mojave Test Area on November 7, 2020. It was a largely overcast day with brief periods of sun. The daytime temperatures reached only 50 Fahrenheit but the winds were no more than 20 miles per hour which meets the criteria for safe launch. Osvaldo Tarditti, our society president, was the pyrotechnic operator in charge for this event.
RECOVERY OF ANOTHER ALPHA FROM JULY
John Krell was able to find the standard length micrograin alpha rocket laumched at the July 2020 event. The rocket was found further north than expected but the downrange distance was about right. This was the rocket with the ceramic coated nozzle which was to have its performance compared to the standard alpha with a plain steel nozzle which is known to erode from the high temperature exhaust.
Unfortunately, the data was absent on the memory chip. It appears there was a malfunction and flight data wasn’t recorded. John is looking into the problem.
The nozzle was removed and inspected from the recovered alpha. Preliminary results show that the nozzle stayed intact. Careful removal of the largely zinc metallic residue firmly adhered to the entrance and nozzle throat must be done to determine how well the ceramic coating survived the 2300 Fahrenheit flame temperature for the quick four-tenths of a second burn time, John Krell is trying a traditional remedy of heated white vinegar (acetic acid) which has been modestly successful in this application.
A FEW ROCKETS IN NOVEMBER
Keith Yoerg and a few others launched five model rockets from the MTA event that day. We’re getting a lot more participants at our launch events which is a trend the society will encourage as we are expanding our organization by supporting a range of projects.
The first was “Star Orbiter” which was prepared and launched by Wilbur Owens, and successfully recovered about 3/4 of a mile from the launch site.
The second launch of the “29mm Rocket” was prepared and launched by Ivan DeDios, and unfortunately was not found after a lengthy search.
The third launch of the day was “Charlie Horse” and featured the largest motor flown that day. The rocket was prepared and launched by Keith Yoerg and resulted in his first flight above Mach 1.0. In addition, it was the first flight of a GPS carabiner used to track the rocket which performed extremely well & was easy to use. Cheaper options of similar technology are being researched for future flights.
The fourth launch was the “Bell X-2” which was prepared and launched by Keith Yoerg, and was a textbook flight with a simple recovery.
The fifth and final launch of the day was “Low and Slow” which was prepared and launched by Alexander Jones. Unfortunately, the parachute failed to deploy at apogee and the rocket came back ballistically & was destroyed along with one of the carabiner GPS units.
The RRS encourages all forms of reaction-based propulsion including commercial solid motor rocket flights. We have our traditional love for the micrograin rockets, but our society is open to all ideas as long as they contact the society and our pyro-op in charge well in advance of our launch events.
BILL INMAN’S SOLAR COLLECTOR
Bill Inman came out to the MTA to test his next prototype of a solar collector. Bill’s latest project is exploring the idea of a solar-based heating system for a steam rocket. This second device had a wider collection area and a longer pipe length. He didn’t have good sun conditions that day and his larger collector structure was a little unstable in the wind, but he was able to get ideas for improvement.
There was a very short period of sun that day and in that time a measurable temperature gain was seen with the new collector.
Testing the same device in the days following the MTA event at another location showed this second design to be a substantial improvement with the larger parabolic mirror area which allowed the water pipe to reach fluid temperatures exceeding 300 Fahrenheit and internal pressures of 90 psig. This is closer to Bill’s goal of reaching above 400 Fahrenheit for his next generation steam rocket from his successful Scalded Cat design almost 20 years ago.
STATIC FIRE OF THE USC RPL ALUMNI SOLID MOTOR
A group of USC RPL alumni static fired an 8-inch solid motor at the MTA. The team worked very hard from the night before and all through the day in getting the motor ready for static fire. The RRS MTA is an excellent location for these operations and conducting safe motor testing.
Unfortunately with some experiments, the results can be disappointing. The hot-fire in the very last hours of daylight ended with a rupture near the bulkhead after roughly one second of the burn.
The details of the event will likely be discussed at the monthly meeting teleconference on November 13th. We’ll be also discussing our next launch event to be held next month in December.
We will likely attempt the nitrous oxide hybrid motor with the modified igniter. The colder temperatures should allow the propellant manifold to operate properly without any modifications.
The removal and replacement of the bent panel on the vertical thrust stand was deferred because of USC’s static firing of their solid motor. This maintenance activity will remain a high priority since the replacement plates are ready and at the MTA already.
Thanks to the many members that contributed to this report. We will be planning our next launch event for the month of December at the monthly meeting scheduled for Friday, November 13th.
In the absence of our secretary, I took a few notes from the meeting. This is what I recorded. Contact the RRS secretary for updates and corrections.
The Reaction Research Society held its monthly meeting by teleconference on August 14, 2020. Our monthly meetings are always held on the 2nd Friday of every month. We’ve had a lot of success with holding our meetings remotely and we will likely continue for the next coming months to continue our commitment to safety in light of the pandemic. Our membership is in regular contact with our community which has allowed us to promote and hold events including our first launch at the Mojave Test Area (MTA) on July 25, 2020. You can read the details in the firing report posted on this website.
Our members are doing well and thus far no one has reported being infected with COVID-19 which we hope continues to be the case. Frank is in regular contact with the Los Angeles Police Department’s (LAPD) Community Safety Partnership (CSP) but under current circumstances, the next school event may not be until next year. Options are being considered on how to continue our educational programs while maintaining social distancing.
REVIEW OF THE 7/25/2020 LAUNCH EVENT OPERATIONS
The first topic was the recent launch event we held on July 25th at the Mojave Test Area for the first time since the start of the pandemic. We had some difficulties in operating under the summer heat (106 Fahenheit at the peak) but this is nothing unusual for this time of year. Many of us were well prepared for the hot sun with our hats, sunscreen and iced beverages and chilled water. We also did a good job of watching out for each other. Still, the heat was responsible for leaving all but one of the micrograin rockets downrange. It also underscores the importance good planning, coordination and putting safety over all other considerations. We had several mis-fires which we were able to resolve, but maintaining discipline during the event proved to be a larger challenge. The launch protocols will be explained more thoroughly in the next safety briefing. The meeting highlighted that every member and pyro-op attending the event holds a joint responsibilty for the safety of all and it starts with self-discipline and patience by all.
We also discussed proper protocols such as announcing the pyro-op in charge well before the event and the necessity of providing detailed information about the intended operations to the pyro-op in charge in advance. Most of the planned projects were well understood as they were micrograin rockets and the previous hybrid rocket attempted at the last launch event.
DATA REVIEW OF THE STANDARD ALPHA FLIGHT OF 7/25/2020
The only micrograin rocket to be recovered from the launch event of 7/25/2020 was the standard alpha with plain steel nozzle. John Krell has been developing progressively better and more powerful avionics payloads designed to fit the narrow confines of the RRS standard alpha payload tube. John was able to spot and recover one of his payloads and process the flight data captured that day. The avionics payload was intact after being extracted from the desert floor including the solid-state data chip. John was able to recover the data and accurately reveal the huge acceleration of the RRS standard alpha with unprecedented accuracy. A peak acceleration of 114 G’s was recorded at roughly 0.3 seconds just before tail-off and burn-out at 0.4 seconds from launch. I was able to screen capture his plot below.
The second plot shows the velocity derived from the accelerometer readings in the half-second which captures burnout at 0.4 seconds. Burnout velocity was measured at 670 feet/second which is consistent with prior data and trajectory predictions. The alpha is subsonic but travels at substantial speed from the swift acceleration. Given the high air temperature that day, 106 Fahrenheit, the speed of sound was 1165 ft/sec. The altitude of burnout was determined to be 130 feet which is consistent with prior flight data and high speed video footage.
The third plot was made for the whole flight of standard alpha from the 7/25/2020 event from launch to impact at 35 seconds. Given the roets were impacting 2000 to 3000 feet downrange, the sound delay matches with the time to impact witnessed in the observation bunker. The maximum altitude was just over 4,400 feet based on the barometric pressure measurements using the 1976 standard atmosphere model. Base atmospheric pressure reading at the start of the flight shows the elevation of alpha launch rail platform is 2,048 feet.
John Krell has really accomplished something with these custom avionics packages. He has been mentoring some of our other RRS members and the society encourages other members to build and fly their own payloads to spread the knowledge.
The society hopes to recover the other two alphas and the beta for further data analysis. Both of the unrecovered alphas from this last launch event had ceramic coated nozzles which should not erode. This should result in a more ideal performance as the throat area will not open up. The actual effect of this design improvement can best be assessed with recorded flight data. Also, we hope to compare the trajectory of the four-foot propellant tube with the standard length. Lastly. if the beta is recovered with recorded flight data, we may be able to assess its performance in unprecedented detail. The society hopes to report this flight data soon.
IMPROVEMENTS TO THE NITROUS OXIDE FILL/DRAIN MANIFOLD
The failure to launch the second build of the hybrid rocket was discussed at the August 2020 meeting. After discussing the launch procedures and corrective actions followed during the attempt to launch the nitrous oxide hybrid at the MTA with Osvaldo (the Level 1 pyro-op in charge) and racing experts at Nitrous Supply Inc., Huntington Beach, California, the cause of the fill valve’s failure to open became clear.
In the racing industry, these normally-closed direct-acting solenoid valves are commonly used to open the flow of stored nitrous oxide bottles against the full supply pressure in the storage bottle. These are called “purge solenoid valves” among racers because it is this solenoid valve that opens the flow of nitrous oxide which displaces or purges out the air in the engine lines during the race. Buying these 12-volt DC high pressure solenoid valves from racing suppliers is much cheaper given they are made in greater numbers for the racing industry. (~$120 each versus $400+ each from reputable solenoid valve manufacturers).
In researching common designs for normally closed (NC) solenoid valves, the excessive heat of that day simply created too much inlet pressure against the internal valve seat for the electromagnetic solenoid coil to overcome and open the flow path. 1000 psig is likely the limit to reliably open these valves according to advice given by Nitrous Supply Inc. who has decades of practical experience at racing tracks around the country using purge solenoid valves for an application nearly identical to the needs of hybrid rocketry fill and drain operations. The ambient temperature at the MTA on launch day was creating a bottle temperature of 1400 psig accordling to the bottle pressure gauge and the separate pressure gauge in the manifold when the bottle was opened. This is well above the 900 psi recommended pressure range seen by marking on the gauge. The bottle, valve body and fittings are rated for these higher pressures, but opening mechanism of the solenoid valve was not.
A color-coded example of direct-acting normally closed solenoid valve is below. Blue shows the high pressure fluid path which is holding the seat down along with some assistance from an internal spring only for low inlet pressure conditions. With current applied to the electromagnetic solenoid (Orange), it pulls up on the moving armature (in red) which then allows the fluid to slip past the seal and through the flow control orifice when commanded open. Only a slight amount of movement is necessary to lift open the valve. However, if the fluid inlet pressure is too great, the solenoid can not provide enough force to lift and open the seal, therefore the valve stays shut.
To understand the relationship between pressure and temperature of the nitrous oxide you must consult the vapor pressure curve for nitrous oxide. This set of data points spans between the triple point and critical point of any pure fluid. NIST provides accurate data to generate such a curve.
The critical point of any pure fluid is where the distinction between gas and liquid phases disappears. This is not necessarily hazardous but it does mark a fundamental change in fluid behavior. The critical point of nitrous oxide (N2O) is 1053.3 psia and 97.6 degrees Fahrenheit according to Air Products company literature. This means the nitrous oxide conditions in the bottle at the launch (1400 psig as read on the gauges with an fluid temperature of 106 Fahrenheit or more) was well in the supercritical range, but again, this is only hazardous if the pressure vessels and plumbing connections aren’t able to safely contain the pressure. If the solenoid valve could have been opened, the pressure drop would have returned the supercritical fluid back to normal conitions and would flow dense liquid into the rocket when the fluid naturally chills down from the expansion.
Keeping the bottle pressure below 1000 psia means controlling the external temperature of the bottle to a lower temperature. Below is a tabulation of state points along the vapor pressure curve for nitrous oxide (N2O) for common ambient temperatures. You can see that small shifts in ambient temperature can greatly affect the vapor pressure of the pressurized liquid. Keeping nitrous oxide under pressure is the key to retaining its denser liquid state. As long as the tank pressure is above the vapor pressure at that fluid temperature, you will have a liquid phase in the tank. If the pressure on the fluid drops below the vapor pressure, the liquid will begin to boil away.
30 F, 440.05 psia
40 F, 506.63 psia
50 F, 580.33 psia
60 F, 661.71 psia
70 F, 751.46 psia; liquid density 48.21 lbm/ft3, vapor density 0.1145 lbm/ft3
80 F, 850.46 psia
90 F, 960.09 psia
97.6 F, 1053.3 psia; density 28.22 lbm/ft3, CRITICAL POINT
Molecular weight = 44.01 lbm/lb-mol
At first, it was thought that there wasn’t sufficient current from the lawnmower lead-acid battery we use. The summer heat can cause batteries to fail, but even after switching to a car battery, the failure to open was the same. Having a 12-volt solenoid requires greater current to actuate the solenoid valve, but it is a common standard for automotive grade parts which can be less expensive yet reliable. A current draw of 15 Amps over the long cable runs of a few hundred feet can be taxing to the firing circuit battery. This was not the cause of the problem, but it is a regular concern making sure sufficient voltage and current is available to both ignition and valve control.
To exclude outright failure of the solenoid valve, Osvaldo brought the unit home, allowed it to cool to room temperature then dry-cycled the valve from a battery to see if it still actuated. This simple test was successful and the filling valve in our nitrous oxide manifold continues to operate. At the next launch attempt, we will be prepared to chill the nitrous oxide supply bottle with an ice bath if necessary as was originally suggested at the prior launch event. Keeping the bottle pressure in an appropriate pressure range for fill operations is dependent on controlling the fluid temperature (60 to 90 F) under extreme heat or cold environments.
In researching purge solenoid valves, a second 12 VDC normally-closed valve was found and purchased. Nitrous Supply Inc., was out of purge solenoid valves but offered many alternative suppliers in the Los Angeles area. After some searching, I selected a high flow purge solenoid valve sold by Motorcycle Performance Specialties (MPS) Racing in Casselbury, Florida, for the purge solenoid valve used for venting our nitrous oxide manifold. The control panel is already equipped with the second command channel to open the vent from the blockhouse should it be necessary in launch operations. A schematic illustration is provided in this article.
The previous drain solenoid valve equipped with the nitrous manifold I bought was not deisgned for the full bottle pressure in the manifold so it quickly failed during initial checkouts. A manual valve was used in its place to carefully bleed out the remaining pressure in the line after the main bottle valve was tightly closed. This second solenoid valve will be used for draining the nitrous in the event of a launch scrub. Although the Contrails hybrid motor already has a small orifice and vent tube at the head end of the nitrous tank to provide slow release of pressure buildup, it is better to have a remote option to quickly depressurize the vehicle if the need arises.
With some re-plumbing of the nitrous oxide manifold to include the new vent solenoid, a soap-bubble leak check would be needed to prove the system before use. Given the significant overhanging weight of two solenoid valves, it may be wise to mount both valves on a separate plate structure to avoid excessive bending loads on the bottle connection. Design changes like this will be considered in preparation for the next launch event.
PYROTECHNIC OPERATOR TRAINING SESSION BY FRIENDS OF AMATEUR ROCKETRY
Mark Holthaus of the Friends of Amateur Rocketry (FAR) organization is offering an online training session for those interested in becoming licensed pyrotechnic operators in the state of California. The event requires registration on the FAR website and a fee paid to FAR ($10) to attend this two-hour introduction to the licensing and application process to be held on August 26th.
Amateur rocketry in California is controlled by the same laws governing fireworks which require licensing by a state exam. The application forms and guidelines are available through the Office of the State Fire Marshal in the state of California (CALFIRE).
This training course for pyro-op applicants is another example of FAR and the RRS partnering to help the cause of amateur rocketry. The RRS, FAR and Rocketry Organization of California (ROC) last year met to create a joint set of recommendations to help CALFIRE improve the definitions used to govern amateur rocketry when CALFIRE they were seeking input from rocketry organizations. It is to the mutual benefit of the whole rocketry community and the public that there be more licensed pyro-op’s in amateur rocketry to both increase awareness of state laws and improve the culture of safety in our hobby and professions.
This FAR training course only serves to provide applicants with basic guidance on how to begin the application process and prepare to take the examination. Members of FAR, the RRS, ROC and any other amateur or model rocketry organization are welcome to apply. Several members of the RRS have already applied as the society continues its campaign to grow our ranks of licensed pyro-op’s at all three levels.
Completion of this training course does not substitute for any part of the pyro-op application process set by CALFIRE. As each applicant is required to pay their own fees including fingerprinting, they must also provide five letters of recommendation from licensed pyro-ops at or above the level of license being sought. After this class, each applicant must formally request these letters from state licensed pyro-ops in writing. For a licensed pyro-op to offer a letter of recommendation to an applicant, they must be willing to endorse their skills, knowledge and character to the state of California based on their personal experience with that individual. This is done through active participation at launch events through rocketry organizations having licensed pyro-ops leading their operations. Apprenticing, studying and attentiveness are all ways that a pyro-op can get to know an applicant personally and thus build confidence that the applicant is ready to have the responsibility of being licensed in rocketry. A letter of recommendation is given solely at the discretion of the licensed pyro-op which means their standards and expectations may vary significantly from others. It is important to establish a working relationship with both the society and the specific pyrotechnic operator over several projects to demonstrate skills and learn best practices through active participation.
As the RRS has more licensed pyro-ops than FAR at this time, this training course will be successful if both organizations support it. Some of the RRS pyro-ops have already offered their support as this means more people will need to become active with the RRS and conduct their projects at the MTA.
ROCKET LABORATORY AT THE COMPTON AIRPORT
Keith Yoerg announced that there is a tentative plan to create a rocket laboratory in a hangar at the Compton Airport, Although, the hangar will be used from time to time to store or service light aircraft, there is a great deal of working space which will help the RRS continue their liquid rocket project already underway. Several members of the RRS are also active with civil aviation and are members of Chapter 96 of the Experimental Aircraft Association (EAA 96). The EAA has generously supported the RRS over the last two years and we hope to continue and expand this partnership.
NEXT EVENT AT THE MOJAVE TEST AREA
The RRS has been planning the next event at the Mojave Test Area which will be dedicated to repairing some of our facilities including the adjustable rail launcher damaged in solid rocket launch explosion in August 2019. The consensus at the meeting was that we should not to return to the MTA for a formal launch event until the seasonal temperatures decrease from the excruciating desert summer. October 3rd was selected for this work event, Our hope is the weather will be cooler and we can accomplish more on that day. We may also take some time to search for more rockets planted downrange from past launch events.
The RRS may also conduct a few static firings or even a launch if member projects are ready. All such proposed hot-fire and launch activities must be proposed to the RRS president and the selected pyro-op in charge for that day. Some of our member projects such as Wolfram Blume’s Gas Guzzler two-stage ramjet and my second-build of the high-powered hybrid rocket are both still works in progress and may be ready for the early October launch date. Larry Hoffing has been working on an improved solid motor chemistry which he may want to test at the MTA.
The RRS is available for private events before that time, but one must make their request to the RRS president as usual. Some have indicated interest in returning to the site for just a few hours to recover more rockets downrange. Its our policy that at least two members be present for any excursions to the MTA and the RRS president must be notified in advance.
Some topics were not able to be covered including the overview of the new RRS Constitution as it gets ready for administrative membership review. Also, facility improvement plans at the RRS MTA including new restroom facilities and blockhouse should be discussed.
The next RRS meeting will be held by teleconference on September 11, 2020 as it is unlikely we will be permitted to return to the Ken Nakaoka Community Center by then. We hope everyone continues to stay safe during these days of the pandemic and try to stay in touch as we are planning another event at the MTA for October 3, 2020.
If there are any questions, please contact the RRS secretary.
On July 25, 2020, the Reaction Research Society held its first launch event at the RRS MTA since the start of the pandemic. Our pyrotechnic operator in charge that day was our society president, Osvaldo Tarditti. I was his backup. We also had Jim Gross come out for the event who has been our pyro-op in charge at many of these events.
We observed social distancing as best as we could and everyone was wearing a mask. Protective equipment is normally required for loading operations and keeping our people spread apart only makes good sense. The heat (107 F) was significant but everyone was largely prepared to endure the exhausting environment. We had a few glitches in the launch process which can happen at any event. It is times like these that make patience and planning very valuable.
We held a short safety briefing before beginning launch operations. I reviewed the natural and man-made hazards at the MTA, underscored the importance of hydration, the buddy system and montioring each other and ourselves for hest exhaustion. We had a lower turnout as this was a private society event and with the heat we sought to run through the micrograin launches in one straight series holding the hybrid rocket flight for last. After the safety briefing, Larry performed a propellant burn demonstrstion then we adjourned to the observation bunker while the pyro-op’s began to ready the micrograin rockets in the rack. John Krell assisted me with the rack loading and arming process.
We had four micrograin rockets and the hybrid rocket for this launch event. There were three alpha rockets with slight differences in their design. John Krell had built three avionics payloads, one for each, to capture the trajectory data (acceleration and barometric pressure) so that an apt comparison could be made. We also had an avionics package and recovery sytem (parachute) built into the beta by Jerremy Hoffing, son of Larry Hoffing. The hybrid rocket would be last in the series,
Bill Inman came to the launch event to both spectate the launch of the micrograin and hybrid rockets and also examine portions of his launch rail unit from his Scalded Cat steam rocket project. He has already begun planning a newer steam rocket design.
This segment talks about the three alphas we built and flew to compare two design changes. The three designs were:
standard alpha with three-foot propellant tube, plain carbon steel nozzle
standard alpha with three-foot propellant tube. ceramic coated nozzle
longer alpha with four-foot propellant tube, ceramic coated nozzle
Among these three designs, we were examining the effect of the ceramic coated nozzles which used a proprietary coating process used on automotive engine pistons and exhaust pipe interiors in the racing industry. Specialized Coatings was the company providing the service which we have used before. The coating was proven in a prior alpha flight in 2017, but the nozzle was misplaced and lost after photos were taken at the event. A repeat test was warranted to not only provide photographic evidence but also to cut-up a nozzle to see how the coating survived. It is likely that a ceramic coated nozzle can survive multiple firings before erosion sets in.
The other variable explored was to change the length of the propellant tube and thus increasing the propellant available. Past projects have explored using longer propellant tubes, but this project would bring flight data for direct comparison. To achieve maximum altitude, a second ceramic coated nozzle was used. Just based on the time of flight observed from the observation bunker, the four foot alpha remained aloft for at least four more seconds. John Krell took some video like a few others did. We may be able to estimate the trajectories if we fail to recover the data from one or all of the alphas.
BETA WITH RECOVERY SYSTEM
The beta rocket used at the launch event had a recovered nozzle which had some minor erosion. This was sufficient for this flight. The two features were the parachute recovery system and the avionics package to record altitude data.
The beta was the first micrograin rocket ready for flight and thus it was loaded into the box rails built for the beta. This beta design differed from the standard design by having a straight coupler meaning that the aluminum payload tube was the 2.0-inch diameter as the 2.0-inch DOM steel propellant tube. Because of cost, betas are produced in smaller and less frequent batches. This sometimes leads to more variations in the design. With a little more part production, we can achieve greater consistency between betas.
The typical aluminum coupler design flares out to a 2.5-inch aluminum payload tube. The standard design better fits the box rail launcher which was made with a 2.5-inch bore. The standard payload tube size would have offered more room for packaging the recovery system. Nonetheless, Jeremy was able to fit everything together and the beta propellant tube was filled and made ready.
The 2.0-inch rocket did lay properly inside of the quad-rail launcher, but the sloppy fit was a little concerning. We had considered using a sabot to fill in the gap, but no practical solution could be made. The solid steel rails would contain the rocket but the concern was whether the avionics switch would get bumped into the off-position. To avoid this, a small block of wood was used to lift the beta high enough to clear the switch near the top of the payload.
The first launch attempts resulted in no firing. After re-checking the cabling and my hookups, no error was found. Second attempt also had the same negative result. To expedite the launch process we proceeded with the alpha launches.
After the alphas flew, we re-tried the beta rocket with a dual-igniter for redundancy, the first electric match was found to be defective. This time after some initial trouble with the battery, on the third attempt we got ignition.
SECOND FLIGHT OF THE HYBRID ROCKET
A new rocket body was built to hold the same Contrails H222 nitrous oxide hybrid motor flown earlier. this year. Larry Hoffing did a lot of work building a new rocket body from scratch. It’s boat tail was fitted to accept the 16-inch long, 38mm casing of the Contrails H222 model. Osvaldo built in the parachute recovery system and all parts of the rocket fit well together at the RRS MTA. I changed the location of the vent tube and routed the line to the outside trimming the excess away once the rocket was vertical and captured in the 1010 rail. A lot of this preparation was documented on the RRS Instagram page.
The Contrails H222 motor is a very simple design made for reloading and re-use. The designs are built to common metric standards used in model rocketry. Using the smallest size, 38mm, for a first hybrid project made sense as we would learn the practical things necessary for a successful launch. It also was a size very close to the micrograin rockets that the RRS commonly uses.
The Contrails design is very simple and easy to assemble with the right tools and lubricants. The interior of the 16-inch long motor is divided into two parts, one for filling with nitrous oxide liquid supplied under pressure and the other holds the inert plastic reloadable fuel propellant grains and a graphite nozzle. The two volumes are separated by a dual O-ring sealed piston called the floating injector.
The motor uses a snap-ring retention method for securing the graphite nozzle plug in the aft and another snap ring is used to keep the vented top plug in place. The internal pressure of the nitrous oxide liquid holds the floating injector down against the fuel grain. The injector consists of a stainless steel Parker push-to-connect plastic tube fitting. The ignier is designed to break the filling line inside of the motor releasing the flow of nitrous oxide and providing ignition nergy to start the combustion of the plastic fuel grain in the presence of newly streaming oxidizer flow. It is a very simple and impressive system. Contrails also sells kits and replacement parts to replace those that wear out.
Last launch attempt successfully demonstrated the motor assembly, motor integration into our first rocket body and loading process. The remote actuation of the nitrous filling line and separate electric ignition circuit required a two-channel firing rig which operated well as expected. The flaw in the first aunch was failing to quickly and cleanly sever the thick-walled nylon fill line.
The nitrous bottle was recharged with liquid and secured to an I-beam. The valve manifold was attached and after a quick tightening was free of leaks. We secured the electrical and fluid connections to the rocket and ran our control lines back to the old blockhouse with all of our observers in the safety of the observation bunker. Osvaldo and I conducted all operations with care. Then the first problem struck.
We couldn’t get the fill solenoid to open. This was first thought to be the battery. In past summer events the heat can degrade the battery. We had several no-fire conditions which led us to suspect the battery health. For the beta, the fault was a broken lead on the electric match. Running a voltmeter showed a little weakness of the battery but 12-volts was showing on the needle. We moved one of the cars closer to the blockhouse to use its battery but the solenoid still wouldn’t open. Given, the late hour in the peak of the afternoon, we scrubbed the launch attempt and safed and disconnected the fluid and electrical system.
The bottle pressure was reading very high that day and although the vessel and plumbing is amply rated for the 1400 psi reading on the gauge. By weight, the bottle wasn’t overfilled, but the heat of the day certainly brought the pressure up. The solenoid valve was bought as part of an assembly sold by a different supplier. With no labelling or marking on the solenoid, there is nothing to identify the manufacturer or model number. A couple emails were sent to the seller but no information on the valve make and model has been given. The internal design and operating limitations of this 12 VDC normally closed solenoid valva is unknown but it is possible that the high pressure against the seat was too much for the solenoid to overcome. Chilling the bottle or simply venting the bottle to lower the pressure might have helped. More tests of the solenoid valve will be done to verify its functions and perhaps some careful disaasembly of the valve may reveal markings to identify it. We are also considering building our own simple solenoid valve fill and drain assembly once the right parts can be specified.
It was a long day but very worthwhile. We hope to have another launch event soon. The results of the day’s events will be discussed at the August 14, 2020, monthly meeting which will be held by teleconference.