The Reaction Research Society held it’s monthly meeting by teleconference on Friday, October 8, 2021. Some of our members were on travel, but the those in attendance were able to discuss several important issues.
The USC RPL static fire event on 9-26-2021 was safely conducted but ended in a explosion and fire which was ably contained. This was a good example of careful preparations and good management of the people present for the event. A firing report has been posted for this event. Osvaldo Tarditti was the pyrotechnic operator in charge that day.
UCLA had requested the use of the MTA on 10/16/2021 for their next liquid rocket engine test. The MTA was already reserved for Bill Claybaugh’s solid rocket flight that same day and in the days leading up to the event. Dave Nordling was the pyro-op in charge, A firing report for this event will be posted,
UCLA is planning to hold their conceptual design review (CoDR) on 10/22/21 for the next iteration of their liquid rocket. RRS members Dave Nordling and John Krell plan to attend.
Wolfram Blume was on the call and said he was eager to return to the MTA for a second flight attempt of the Gas Guzzler ramjet. With the summer heat gone, he hopes to return at our next launch event which is still being planned. It is hoped that the society can continue their streak of having at least one MTA event per month as we have done since the start of 2021.
The restroom container was purchased and brought to the Compton Airport for interior construction. This 20-foot high cube has a 9.5 foot ceiling and should be able to have two individual rooms with toilet and sink, one of these to have a shower stall. Osvaldo had drafted a floor plan and this was approved by the council. The society will be meeting at the Compton Airport on Saturday, October 23rd, for a late morning barbecue and an in-person discussion of the materials needed to get the restroom interior built. All members are welcome but please notify Keith, Wilbur, Xavier or Dave Nordling if you’re coming as they have access to the airport.
There was some discussion about the septic system and leach field. It is important to maintain an appropriate distance from any nearby water wells, one of which is on Polaris Propulsion property. Sufficient clearance exists based on measurements made and EPA guidelines. The leach field will be positioned to drain away to the north.
The society is considering buying a concrete septic tank but RRS member Wilbur Owens may have a plastic septic tank already available for the society. Some members feel a concrete septic tank will last longer and be less likely to leak. The council is still debating this feature and should render a decision soon.
The society also discussed the water supply to the restroom container and the supporting structure needed to hold a tank on top of the container. There are many important facets to this infrastructure addition which must be weighed carefully.
Nominations for RRS executive council offices will be held at next month’s meeting, November 12th, 2nd Friday of the month at 7:30pm. An election chairman will be selected beforehand and this person must be an active member not holding office nor running for office. A special email address will be set up for the election chairman to gather votes from our active administrative and lifetime members. Results to be announced at the December 10th meeting and new terms to start January 1, 2022.
For any questions, please contact the RRS secretary.
This firing report will be the first in a series of three articles posted on RRS.ORG. This report will cover the launch event and preparations over many days made by RRS member, Bill Claybaugh. As the attending pyrotechnic operator for this firing event, I have summarized this work for the benefit of our readers with the permission and oversight of Bill.
Bill Claybaugh has been planning to build, load and launch a large 6-inch solid motor for many months and the first attempt had finally come to pass at the RRS Mojave Test Area (MTA) over the span of almost a week starting Tuesday, October 12 and culminating in a launch on Saturday, October 16, 2021. He had studied this project very carefully and built a great many new parts and tools from his home in Colorado. The scope of this project is quite extensive and the larger goal was to enable larger solid motor building by other members of the RRS at the MTA. The 6-inch motor was just the first in what will hopefully be a growing series of similar and larger scale solid motors.
The predicted performance of this 6-inch single grain motor was 1350 lbf of thrust for a duration of 8.35 seconds which was expected to exceed 70,000 feet; well above the RRS MTA’s standard 50,000 foot altitude waiver. This “P” sized solid motor in this vehicle required an FAA Certificate of Authorization (COA) for this flight on the prescribed dates during daylight hours. The submission of Monte Carlo simulations of the trajectory (splash analysis) were graciously performed by Chuck Rogers (author of the RASAero II software) and a necessary part of the process to verify no significant concerns for impacting nearby populated areas or structures. Also, the FAA Class 3 rocket waiver that was granted would require the launch team to contact the relevant air traffic control 15 minutes in advance of the intended launch for final permission to proceed. A separate article discussing this subject in more detail will be coming soon.
The rocket had two streamers for a recovery system which were intended to be sufficient for easier spotting of the rocket in descent rather than provide a soft landing.
Many members of the society participated in this project over the several days needed to prepare and conduct the mixing, pouring and casting process. RRS members Dave Crisalli and George Garboden lended their time and expertise in solid motor building which led to a stellar finished product on Thursday. Several of Bill’s family and friends attended and supported the preparations for launch.
Given the size of the 6-inch rocket, Bill designed and built a T-slot type of launch rail with a 24-foot length on an aluminum truss structure. The system was designed to be deployed in a green-field site and easily assembled by a small team of people. There were some challenges in getting the design to work but through the combined efforts of those at the site during the afternoon and early evening on Friday, the erecting and loading process was safely completed. Susan and Ed Wranoski both had a lot of great suggestions about getting the right placement of the come-alongs to bring the launcher up to a sufficient angle to secure it by the chains and strap anchors around the pad.
The new launch rail system will be the subject of a separate article coming later on RRS.ORG. Design improvements and substantial changes are being planned such that the next launch event will have an easier time in raising and lowering this important asset for the launching of larger rockets from the MTA.
During the first launch operations of the rocket, the wireless telemetry wasn’t receiving signals. After restarting the computer and replacing the nosecone, the pyrotechnic charges in the recovery system accidentally fired due to a short. The payload system was removed, inspected and replacement pyrotechnic charges installed. After protecting the terminals from a similar short during final installation of the payload and nosecone, the telemetry system was working and the launch could proceed.
The launch event coincided with the launch operations of our neighbors’ (FAR). We were in constant communication to assure everyone was under cover at the proper times. The weiather was nearly ideal with very low winds the whole day. After road and air checks were completed, we prepared for launch.
The initial launch was swift and powerful as the motor ignited and came to full thrust leaving the launch rail. The rocket canted to the northeast opposite the intended direction of the launch rail and the vehicle appeared to corkscrew as the motor burned to its full duration before going out of sight. The recovery system appears to have fired early as one of the streamers and the entire payload module fell back to the northern side of the MTA. The spent rocket motor casing has not yet been recovered. Bill was able to bring back the payload segment for inspection at the MTA while others continued the search for the rocket.
Based on review of video footage, it appears the sudden turn uprange occurred at around 100 feet and took less than 1/4 second. The current thinking is that the separation system depressurized, producing the side-thrust that caused the sharp turn after leaving the rail. It is assumed the telemetry loss of signal (LOS) was a result of the antenna snapping off during this sudden turn. LOS occurred at 119 feet and 425 ft/sec. About 0.25 seconds later, the payload can be seen starting to fall away from the rocket which can only occur if the system is depressurized. The payload was recovered about 300 feet from the launch tower and on the ‘new’ azimuth.
After the initiators fire–and both were fired–it would be expected that applying pressure to the quick-disconnect (QD) fitting would:
(1.) NOT result in the four retention pins extending, and,
(2.) would cause venting through the diffusers.
That is, the burst disk is supposed to be punctured due to the piston driving the hammer through it when the initiators fired and any gas generated in the system is vented past the burst disk and through the diffusers.
The recovered flight hardware instead extended all four pins, did not vent through the diffuser, and did vent through the outlet reserved for the hot initiator gases. This means that the burst disk was not opened and pressurizing gas was somehow leaking into the hot gas circuit. The image below of the burst disk shows its condition as found upon opening.
Further disassembly showed that the O-ring seal separating the hot and cold gas circuits around the hammer that penetrates the burst disk appeared damaged from heat. That seal damage was allowing the cold gas to escape into the hot gas circuit and then vent. Further, the O-ring prevented hot gas from getting to the subject O-ring around the piston that drives the hammer through the burst disk was in two pieces and showed clear evidence for melting at the edges. Thus, when the dual-redundant initiators fired, the piston O-ring failed (or had previously failed, although it was undamaged when installed) which allowed hot gas to leak past the piston (which nonetheless hit the burst disk hard enough to dent it but not tear it) and to damage the O-ring separating the hot-gas and cold-gas circuits in the valve. These two damaged O-rings then allowed cold gas to vent via the hot gas circuit, resulting in the payload seperating from the rocket.
Naturally, none of these failures ever occured in previous ground testing.
Wind shear was considered as a cause for the sudden change in vehicle direction witnessed during launch right after clearing the rail. Even in calm wind conditions on the ground, there have been past launch events at the MTA which have had sharp unseen discontinuities in the wind profile causing serious perturbation of the flight path in a rocket flight. This potential cause can not be fully excluded, but it is thought to be unlikely..
The venting of the hot and cold gas _may_ have caused the sudden pitch over as seen in video footage. As of now, this is being carried as a working hypothesis. However, none of this explains why the initiators apparently fired a few fractions of a second after lift-off.
The telemetry data will soon be downloaded from the ground station to see if there was any indication of the beginning of this sequence of events. Because the ground station showed loss of signal (LOS) at 119 feet, and that LOS appears to have been the result of the antenna snapping off in the course of the sudden pitch change. There might not be any recorded data of the relevant accelerations or rates from the ground station.
This report will be updated as new information becomes available.
In conclusion of that day’s launch event, with the recovered parts from the rocket payload examined and packed for shipment back to Bill’s home, the remaining team worked to carefully lower the launch rail back to horizontal using the reversed process used to successfully and safely raise it. The launch rail support legs were left at the MTA as Bill and Mike Pohlmiller were going to consider a new design approach using the same T-slot backbone. Although there was no evidence of the rocket hanging up on any discontinuity, some repairs of the interconnections between the three segments should allow the combined rail path to be more straight.
The RRS is grateful to the many members and participants we had over those several few days. It was a big success despite some significant challenges and disappointment in the results. The project was designed to be a pathfinder to subsequent large solid motor projects and we expect the next motor build and improved payload system design in the new calendar year, 2022.
originally published 9/30/2013, reprinted with permission
It seems that at least one 4-20 milliamp (mA) measurement is required by our typical customer, and the way to do it is a constant source of confusion for many. So I thought I’d zero in on the various 4-20 mA current loop configurations and elaborate on the specifics you need to know to make a successful measurement. The following is ordered from the most to least common configuration, and I hope to cover all those that I encountered in customer applications. If yours isn’t included, please contact DATAQ technical support.
4-20 mA Current Loop Basics
Sensors or other devices with a 4-20 mA current loop output are extremely common in industrial measurement and control applications. They are easy to deploy, have wide power supply requirements, generate a low noise output, and can be transmitted without loss over great distances. We encounter them all the time in both process control and basic measurement data logger and data acquisition applications.
The idea behind 4-20 mA current loop operation is that the sensor draws current from its power source in direct proportion to the mechanical property it measures. Take the example of a 100 psi sensor with a current loop output. With 0 psi applied, the sensor draws 4 mA from its power source. With 100 psi applied the sensor draws 20 mA. At 50 psi the sensor draws 12 mA and so on. The relationship of mechanical property measurement to current output is almost always linear, allowing the resulting current loop data to be scaled with a simple mx+b formula to reveal more useful measurements scaled into engineering units.
How you actually measure the 4-20 mA current loop signal is a function of the sensor’s architecture and the capabilities of the instrument you’ll use for the measurement.
So that my discussion translates well across the various kinds of 4-20 mA current loop configurations, I’ve opted to standardize the terminology I use to describe each. Here’s an overview:
“E” (DC excitation)
Most configurations that follow will show a DC voltage excitation source that I denote as “E”. Many who use current loop sensors for the first time are surprised to learn that they need to supply this excitation source. Nonetheless, unless the sensor is self-powered (i.e. AC line powered) an external dc source is required. The good news is that this can sometimes be supplied by the instrument, and the range of acceptable supplied voltage values is usually very wide, typically 10 to 24 VDC.
“R” (shunt resistor)
Here’s a bit of trivia for you:
No instruments measure current directly.
They all do it indirectly by measuring the voltage dropped across a resistor of known value, and then they use Ohm’s Law to calculate actual current. The resistor is referred to as a “shunt”, is absolutely required to make a current measurement, and is either supplied externally to, or built into the measuring instrument. For clarity, I assume that it’s supplied externally.
“i” (current loop value ranging from 4 to 20 mA)
This is the 4 to 20 mA current signal generated by the sensor. Note that some sensors may draw 0 to 20 mA and even other values, but the vast majority of them use the 4 to 20 mA convention.
“v” (shunt voltage that’s proportional to current)
This is the voltage drop across the shunt that is actually measured by the instrument. Since our industry has standardized on a shunt value of 250 Ohms, “v” will range between 1 and 5 volts for a 4-20 mA current loop signal.
Note that shunt resistor value is arbitrary as long as it’s a known (fixed) value. You also need to ensure that it doesn’t burden the loop, so lower values are better than higher.
Yes, I mean lower.
Remember that we’re working with current, not voltage, so the rules are inverted. Just as infinitely-high resistor loads work well for a voltage source, you can take the load all the way to zero Ohms for a current source without consequence.
Self-Powered Sensors (Most Common)
I promised to order these configurations from most to least common, and the self-powered sensor just noses out the first runner up. Self-powered sensors are those that, well, power themselves. The sensor may have an integral ac power supply, thereby negating the need for an external DC power source.
Or it may not be a sensor at all. It could be an output from a Programmable Logic Controller (PLC) or other source that is internally powered.
2-wire Sensors (Low-side Shunt)
Okay, this can get confusing for first-time 4-20 mA current loop users.
Yes, it is possible to both power the sensor and measure the current it draws over the same two wires. In the 2-wire examples shown here, only two wires connect the sensor to its power supply, and the sensor draws current from it in direct proportion to the mechanical property that it measures. As current changes, the voltage developed across resistor “R” will change, thus providing a signal that’s suitable to connect to a measuring instrument like a data logger or data acquisition system.
In most situations, care should be taken to place the resistor in the low-side of the loop as shown here, as opposed to the high-side. Doing so will allow non-isolated instruments to make the measurement. In the next section, I’ll deal with a high-side shunt placement and discuss these cautions in more detail.
2-wire Sensors (High-side Shunt)
This configuration is almost exactly like the low-side, 2-wire approach, but it places the shunt resistor in the high-side of the loop. Note that while the voltage across the resistor is proportional to the current drawn by the sensor (just like the low-side approach), there is also a common mode voltage (CMV) present on either side to ground. On one side to ground the CMV is equal to the supply voltage. On the other side to ground it’s equal to the supply voltage, less the voltage dropped by the resistor (v).
The presence of the CMVs places conditions on the instrument that you use to measure v. Specially, the instrument needs to have an isolated front end so it can float to the level of the CMV and still successfully make the measurement. Try this with a non-isolated, single-ended instrument and you will short-circuit the sensor to ground. A non-isolated differential instrument will either saturate or provide erroneous results.
Three-wire sensors with a process current output have a separate wire for ground, signal (4-20 mA), and the power supply. This configuration is the easiest for current loop beginners to grasp, one input for power and a second for the current loop with a common ground. The primary advantage of a 3-wire sensor over its 2-wire counterpart is its ability to drive higher resistive loads. Resistors drop voltage for any given current in direct proportion to their resistance value. Holding current constant, higher resistances drop more voltage. Turning back to the 2-wire sensor and holding current constant, as the shunt resistance increases the voltage drop across it also increases. You might reach a point where the voltage dropped by the shunt lowers the voltage drop across the sensor below the minimum required for it to operate properly.
We had a customer whose 2-wire current loop measurements functioned beautifully until loop current reached about 18 mA, at which point everything went haywire. Upon close examination, we determined that the supply voltage she used was too low by at least 0.56 volts. She needed 2 mA more measurement to reach full scale, which translates to 0.56 V with her 250-Ohm resistor. The solution was to use a higher voltage power supply to ensure that the voltage drop across the sensor stayed above the minimum level. She could have also used a 3-wire sensor, which ensures that the voltage applied to the sensor is independent of shunt resistor voltage drop.
Watch Your Grounds (or use an isolated instrument)
Contrary to what many believe (and have been erroneously taught in school), grounds are almost never the same in industrial settings, exactly where most 4-20 mA current loop sensors are used.
Two or more grounds that are the same means that they are at the same potential. If so, a measurement between the grounds of the various field sensors and the instrument using a digital volt meter (DVM) on both its DC and AC settings will show zero volts, or very close to it.
In reality, you’ll measure at least several volts, and I’ve seen as much as 75 Volts. When grounds that are not at the same potential are tied together (which you need to do to make a measurement), current flows through them, creating several possible measurement outcomes for non-isolated instruments:
The measurement is noisy.
The measurement is inaccurate.
You irreparably damage the instrument.
You saturate the instrument (it’s not damaged, but you can’t make a successful measurement, either.)
To remedy these problems requires the following:
Use an isolated instrument for your 4-20 mA current loop measurements. This single decision allows you to ignore all other grounding issues in exchange for successful measurements in any situation. If you don’t have an isolated instrument, read on…
Ensure that the loop power source is isolated. This means that its output ground (the one connected to the sensor) is not tied to its input ground (the one that connects to AC line power.) An isolated power source means that the output ground can be tied to another ground (like a non-isolated instrument) without consequence.
If using self-powered sensors, ensure that the low-side of the loop is isolated from its power source.
If using sensors that require an external dc power source, ensure that the shunt resistor is placed in the low side of the loop (see “2-wire Sensors (Low-side Shunt)” above.)
If you lack control over the power sources and determine that they are not isolated, then your only option is to power ALL devices (power supplies, self-powered sensors, the instrument, and its connected PC) from exactly the same power outlet. Don’t make the mistake of using outlets that are close to each other. If you run out of receptacles on a single outlet, then use a power strip.
Again, it’s worth repeating that all of the cautions associated with proper grounding disappear if an isolated instrument is used to make the measurement.
Sensors with 4-20 mA outputs are encountered in all disciplines and in many configurations.
Contact DATAQ with any questions that arise in your unique situation.
This article has been reprinted with permission from DATAQ Instruments, a manufacturer of quality data acquisition and data logger products used by many professionals and amateur rocketry hobbyists.
The RRS is thankful to DATAQ for their assistance.
Also, you can watch DATAQ YouTube instructional videos on this and other subjects.
For information on DATAQ products, go to their website: