Report on timer circuit design

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This is a posting of a report written by our current society vice-president, Frank Miuccio, many years back. It has been reproduced here on our website for preservation. A hard-copy revision of the document will be published in the society archives. The original date shows it was Revision A, dated March 9, 1989, nearly 30 years ago. Some of the figures mentioned in the text are missing (until we find them again) and others have been remade for clarity.

Frank’s timer report, original cover from 1989

In reading the report, you can see that the technology of some aspects of the design are no longer commonly practiced, such as the use of mercury switches and mechanical relays, but the circuit principles are still sound. I have noticed that the model rocketry community, such as our friends at Rocketry Organization of California (ROC), have made great strides in timer designs.

Rocketry Organization of California

There are many commercial suppliers across the country that make a range of simple and complex designs that are reliable and affordable. Some products can be bought ready to use in your rocket application.

Eggtimer Rocketry

Also noteworthy is that lithium polymer battery technology is taking over from the conventional 9-volt. This doesn’t come as a surprise to many, but certainly worth mentioning. Some people still use the old battery types, but there are many smaller and very powerful options in batteries thanks to the growing airborne drone community.

It is the society’s intention to show this report to inspire our members today to expand upon the work done before. Many effective timer circuits are commercially available, but years before, to have such a device required a bit of ingenuity combined with plain trial and error. Enjoy!
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TIMER CIRCUIT REPORT
by Frank Miuccio, RRS

On June 25, 1988 at the Mojave Test Area (MTA), a single stage micrograin (80% zinc, 20% sulfur) rocket was flown with a simple payload that anyone can build. The payload consists of a timer that was set for approximately 18 seconds, a parachute, and an ejection mechanism. The timer was used to eject a parachute 18 seconds after burnout and was designed to incorporate the least amount of components. The timer consists of 4 components, 2 batteries and 2 safety switches and a mercury switch.

The main objectives of the payload were the following:
(1) To verify that the ejection mechanism (shown in Figure 1A and 1B) works properly. The ejection mechanism was designed and built by a member of the RRS.
(2) To verify that the mercury switch activates at burnout and stays on for the time constant.
(3) To verify that the timing circuit (shown in Figure 2) functions properly and can withstand the flight environment.

The flight was a success. The parachute ejected and was spotted by the tracking crew, who were located approximately 1000 feet away from the launcher. All three objectives were met with a positive result. A few shortcomings were noted. The parachute, 24 inches in diameter, drifted the rocket north-east and the rocket was lost. Also, the color of the parachute was white which was Ambien a problem in spotting.

Figure 2: Timer Circuit

The timing circuit has been used three times.

The first time was on December 28, 1986 on a two-stage rocket. It was used as a separation time delay for the second stage. The timer was installed in the uppermost section of the first stage motor prior to fueling. It primary function was to ignite the second stage 2 seconds after burnout of the first stage. During fueling of the first stage, a problem was noted. The timer was being exposed to extreme bouncing due to our fueling technique.

The next time, the timer was used as a stage delay was in December 1988. The circuit was packaged in a separate module which would be installed after fueling of the rockets.

The third attempt wasn’t as successful as the other two. The timer failed to function. A possible culprit could have been one of the safety switches which was installed backwards. The switch was installed with the “ON” in the upward position. This creates a problem since the acceleration (from launch) could force the switch in the “OFF” position (downward).

The timer looks promising that it can cover various time constants. To determine the desired time the values of the capacitor [C1] and resistor [R1] can be varied. One can calculate the values needed as follows in the formula below.

Time delay = [C1] * [R1] * 1.10

The following steps are used to achieve the desired time constant when building the circuit:

(1) Wire and/or solder in the circuitry except the resistor [R1] and capacitor [C1].

(2) Chose a value for the capacitor [C1] and permanently install it in the circuit. Note that the value needs to be in the microfarad (uF) range. In this report, a 22 uF capacitor was used.

(3) Calculate the value needed for the resistor [R1] by using the time delay formula. Note that this will only give you an approximation of the actual time delay. The resistance will be in the kilo-Ohm to low mega-Ohm range.
https://fmcquaker.org/getting-prednisone/
(4) Adjust a potentiometer (also called a “pot” or a “trim-pot”) to the calculated value (Pin 1 to the wiper) and temporarily connect the pot in place of the resistor [R1] (pin 1 to the wiper).

Potentiometer (adjustable resistor) next to a fixed value resistor

(5) Test the timer to find out if you need to adjust the pot by increasing or decreasing its resistance. Note that if the timer delay is longer than the desired time constant, decrease the pot resistance. Conversely, if the timer delay is too short, increase the pot resistance.

(6) Adjust the pot as needed and repeat Step 5 to get the timer delay correct.

(7) Measure the resistance value of the pot (Pin 1 to wiper) with a voltmeter then find and permanently install a fixed resistor of that value in its place [R1]. In this report, a value of 732 kilo-Ohms was measured when the circuit met the desired time period. A more common size of resistor is 750 kilo-Ohms which is close enough.

(8) Test the timer to verify the accuracy of the time constant.

(9) Once the circuit is tested and complete, surround and enclose the timer circuit with RTV. This is needed due to the G’s experienced during flight.

GOOD LUCK!

—————–

Editor’s notes:

I have found that in modern times (circa 2018) electronic component stores are not as common as they once were. RadioShack is still in business, but they are not the big company that they used to be. I have had good luck in getting what I need from a local store in my neighborhood in Westminster, CA (Orange County). They have nearly everything an electronic hobbyist could want including lithium polymer batteries of all sizes.

JK Electronics in Westminster, CA

JK Electronics – Westminster, CA

Ordering from online suppliers (DigiKey) is always an option, but the catalog information posted by the mainstream suppliers can be difficult to interpret if you are not an electronics expert. Also getting small quantities (less than 100 units) can also make ordering excessively expensive when the shipping costs far more than the handful of parts you are ordering. Amazon and Ebay can be a helpful resource, but the buyer must be aware of the specifics of exactly what you need. Always do your homework, consult the advice of experts and you will be more sure to get the components you want.

Also of historical note, the K1 RELAY element of Frank’s timer circuit used a W107DIP-5 (5-volt) mechanical relay made by Magnecraft Electrical Company of Northbrook, Illinois. Frank had the actual catalog from Magnecraft in his report so I took a photo of the relay he had selected.

Magnecraft Electric Company, original print catalog

In this photo to the left, you can see the circuit diagram of how this Dual In-Line Packaged (DIP) reed relay is connected. This 107 model is a normally open (NO) single-pole, single-throw (SPST) type of device and contact rated for 10 VA.

Magnecraft W107DIP-5 catalog specs and circuit diagram

I’m not sure if Magnecraft Electric Company is still around, but a modern update to the timer circuit design would likely use a solid-state NPN type of bipolar junction transistor (BJT) instead of the mechanical relay. This exercise is left to the individual to pursue, but not in this article.

NPN type of Bipolar Junction Transistor (BJT)

example of an NPN-type of BJT, rated for 1-watt, connections are labelled

It is important to make sure you know which pin or connection is which. The polarity of the circuit element you are using can be critical. For example, the longer lead on a capacitor is often the positive (+) one. The case on a capacitor should also have a negative sign (-) or a dash symbol to indicate which pin is the negative one. The circuit diagram that Frank included in his report has been careful to show these important details on polarity.

Spark Fun website on electrolytic capacitors and proper polarity

One should also note that very often the pins on a chip are numbered in specific sequential pattern, but the circuit diagrams often don’t follow these and simply call out the pin location by number only. I have put the pin diagram for the common 555 timer chip below to illustrate this important distinction between a physical layout and the schematic which doesn’t always match the physical locations.

555 timer chip with the actual pin locations, notice the notch at the top to show where “1” starts

Just to give a little more detail on the mechanical relay that Frank used, I have re-created the pin diagram from the Magnecraft catalog picture showing the layout of the 14 pin connections. You’ll only need four of these connections (2, 6, 8, 14) as seen in Frank’s circuit.

Pin layout for reed switch type of mechanical relay, Magnecraft WR107 DIP-5

Also, a word about the timer delay formula is that it is based on the basic RC circuit type that has an exponential rise relationship once the circuit closes and starts. For simplicity, this formula just assumes a fixed 1.1 ratio to relate the product of the capacitance and resistance value into the predicted time delay in seconds.

Sample calculation of the approximated time delay with capacitor and resistor values converted to seconds

It is important to understand that this is only an approximation and actual experiments are required to be more precise. Each of the lines and connections adds a little bit of variance to the actual delay time you will see and it’s hard to know exactly what this is without testing. Frank’s instructions go into how to do this by first using an adjustable resistor (potentiometer / pot / trim-pot) to measure what resistance you need, then you go get a fixed resistor to install at the end. This way, you can adjust for some of the real-world effects of your connections and verify that your timer will give you the fixed time delay you want. Also remember when buying capacitors and resistors, you will have to buy them in the sizes that are common. Even with the modest precision of these devices (+/-10% on capacitors; +/-5% on resistors), you can still get very close to the time delay you want and these parts aren’t very expensive.

Some people will put in an adjustable resistor in their circuit designs which is fine if the pot can stay tuned on the exact setting you want and you have the access to make adjustments if needed. Typically, you don’t have good access once the payload is installed on the rocket, so this is why this design has chosen to permanently attach a fixed value resistor after some testing to validate the operation.

Lastly, I should make a note on the use of RTV for “potting” or encasing the circuit. Room temperature vulcanizing (RTV) silicone rubber is a liquid compound that usually comes in small tubes and bought in automotive shops (e.g. Autozone) that after it dries will make a rubbery solid. This final step of encasing your circuit in a flexible but firm solid is considered by some to be necessary to secure the timer circuit from deflecting and possibly malfunctioning under the high acceleration experienced in the rocket flight.

Others feel that this step is not necessary. It has also been said that RTV is corrosive to electrical contacts and should not be used. In any case, you must make a structurally robust circuit that stays put, doesn’t break and will protect your connections from accidentally shorting against the interior metallic walls of your rocket parts (if you have them). The high G-loads from a micrograin rocket’s acceleration are not trivial. Proper packaging your payloads is a very important consideration in rocketry.

The method of successfully potting a circuit in RTV is probably worthy of a separate discussion. It is wise to have all of your leads sticking out of the drying potting compound you’re using better it sets otherwise you can’t connect the right parts when the mess is dry. It sounds obvious, but wait until you screw it up?! 😉

Thanks for reading. Look to the RRS.ORG for more articles on different rocketry subjects past, present and future.

June 2018 meeting

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The RRS held its monthly meeting for June 2018 on Friday the 8th at the Ken Nakaoka Community Center in Gardena. We were well-attended but got a late start on the agenda items. Wilbur Owens came back to see us again and has decided to become a member. Mohammed Daya who has joined the RRS also was able to stop by before the meeting started just to say “hello”. We were also glad to have Sam Austin back in town. Sam is a student at MIT interning at SpaceX this summer. He also paid us a visit both at the MTA on June 2nd and at the meeting tonight to discuss the liquid motor he built. There was lots to talk about at this June meeting between our outreach events and the hot fire valtrex testing.

RRS discussing things before the June 2018 meeting starts

A lot to talk about at the June 2018 meeting

[1] The RRS event at the Two-Bit STEAM Circus in Hawthorne was quite a success. The RRS was a bit short-handed, but we have great volunteers that stayed busy the whole time. The air rocket launcher was one of the big hits at the Circus event and we have been invited back for the next event in September. Frank made some improvements to the paper rocket template that is cut, folded and secured with tape to make the narrow tubes with attaching fins that comprise a paper rocket for the pneumatic launchers we have. The build process can be challenging for some, but it always is satisfying to see the finished product fly. The RRS is glad to have these events and we will surely do more.

Here’s some photos that Osvaldo took at the event.

RRS at the Two-Bit Circus STEAM Carnival in Hawthorne, CA. Frank works with kids to make the paper rockets for the RRS air launcher.

The air launcher is made ready to fire a paper rocket.

On a side note to this topic, USC is having a CRASH STEAM Carnival in 2019. We were invited to attend this year’s carnival, but the RRS is shorthanded and couldn’t support. We hope to expand our society to be able to come out for more events across the great city of Los Angeles. We’ll have more information on this event later.

[2] UCLA held their launch event at the MTA on June 2nd. The event was two-fold, it was the final project for the UCLA rocket propulsion class taught by Dr. Spearrin. It was also an opportunity for the UCLA Rocket Project team to static fire their hybrid motor. The event was a success for the class with the winds being nearly calm for most of the day. The small hobby rockets with F-sized motors reached good heights without being carried too far by the winds. Each had an altimeter and a hard-boiled egg as a payload that had to be safely recovered. Most were successful, but others not so much. A full write-up was done on an earlier RRS posting.

RRS MTA launch event – 2018/06/02

As a further note, UCLA was using a Jolly Logic Altimeter 3 model in their hobby rockets. These devices used by UCLA have proven to be very reliable and easy to use. The RRS will acquire some of these to fly in the payload tube of an alpha rocket to see what heights we reach in this micrograin mainstay rocket of our society.

Jolly Logic Altimeter 3 – manufacturer’s site

After UCLA’s static firings of their hybrid motor, the RRS flew an alpha rocket with a parachute system. This is a first in a long time. Osvaldo’s design had a safety switch to engage the battery only when the rocket is loaded to prevent it from getting depleted in waiting for launch. The parachute system also had a pull pin to start the timer circuit when the rocket lifted out of the rails. Osvaldo did bring another prototype of the alpha parachute system to discuss its features at the meeting, but we didn’t have enough time.

Osvaldo’s parachute deployment circuit that fits in a standard alpha rocket

After the June 7 launch event at the MTA, Osvaldo managed to find an RRS standard beta launched by UCLA last year. Although the payload segment sheared off in the extraction process, the nozzle is the precious part that can be http://pted.org/Propecia.php cleaned up and reused.

RRS standard beta recovered from the RRS MTA; payload segment was not recovered

Osvaldo was also kind enough to make the adapter piece necessary for testing the RRS standard alpha second-stage solid motor I designed in the horizontal thrust stand at the next event. With this simple doubled-ended adapter that goes in place of the nose cone, the second stage motors once finished can fit into the load cell adapter and the RRS can get thrust measurements. Chris Lujan is working on a sucrose-KN solid grain and Larry Hoffing is working on an AP/HTPB/Aluminum motor grain. I have done the preliminary calculations for both and pressures should be appropriate for the 1.75″ aluminum payload tube. More discussion on this topic in future posts.

RRS alpha second-stage load cell adapter piece for the horizontal thrust stand. It goes in place of the nose cone.

[3] The next RRS build event with the LAPD CSP officers will be with another group of kids in the Jordan Downs housing projects of Watts. We’ll get started next week, 6/15/18, and run six educational sessions on Friday’s and Saturday’s until the launch event at the end of the program at RRS MTA. This will take place on July 21, 2018. This had to be re-scheduled due to the extreme heat predicted for the original date of July 7th.

The students will paint and assemble a set of RRS standard alpha rockets. More alpha rockets means more fun for our guests and also more opportunities for our RRS members to try payloads. It’s my hope we can demonstrate another (https://openoralhealth.org/ativan-lorazepam-without-a-prescription/) one of Osvaldo’s parachute systems and fly an altimeter chip if we can secure one in time.

Richard Garcia said that he already has an Eggtimer Quark chip which has an altimeter. I had the chance to meet Cris Erving of Eggtimer Rocketry at the last Rocketry Organization of California (ROC) launch event in Lucerne Valley on June 9, 2018. I hope we can get an altimeter payload ready to fly in a standard alpha payload tube by the July 7th launch.

Eggtimer Rocketry

Rocketry Organization of California

[4] The new RRS membership card design has been finished. Many thanks to Bill Janczewski for pulling this together. We have had a few requests for membership cards from members and the RRS has agreed to produce these only on demand.

Jim Gross will be the first recipient of this new style of RRS membership card. This year’s design has the 75th anniversary watermark on it.

The new 75th anniversary RRS membership card

There was some general discussion about the payment of dues. Even as we are growing in membership in our society, the RRS has not been collecting dues on a regular basis. We’re content to primarily use the honor system and gentle reminders to our membership to pay their annual dues of $40 per year or student memberships at $20 per year. It is this small revenue that helps the RRS stay on top of our bills. Student memberships are good as many university projects can require multiple tests at the RRS MTA which is covered with signing the RRS indemnification form and paying membership dues to the RRS.

All membership applications must be sent to the RRS president and approved by the RRS executive council.
president@rrs.org

Payment of RRS dues ($40) and the added cost of a membership card ($5) can be done by check and through the Paypal donation button we have on the RRS.ORG website. It’s important to make a note on Paypal that you’re paying your Membership Dues. The extra price of $5 for membership cards is pretty small and compensate for the cost of low-volume production as most members may not opt to get one. To those desiring a membership card, please contact the RRS secretary.

secretary@rrs.org

For all of our regular membership, I had proposed that the RRS return to using membership cards which were used in the past in the society. Membership cards were issued to all members upon payment and re-payment of their annual dues. This provides a physical mechanism to verify that each member is in good standing with dues paid. The membership cards would have their name and an expiration date that says when annual dues must be paid again.

Although some felt the idea had merit, others felt that we should continue to have the council take the initiative to track payments and remind members to pay their dues as we have been doing. Since members join at different times in the year, this can get complicated but we will rely on members to stay on top of this.

It was a good discussion that also raised issues about what constitutes “active status” in RRS membership and our broader membership policies including corresponding membership for those who live outside of the Los Angeles area but want to remain a part of the RRS in some capacity. It was agreed to revisit this broader topic in the July 2018 meeting as some of our newer members may not be familiar with the past and current membership policies at the RRS.

[5] Sam Austin gave his presentation the Hercules Rocket Engine project at MIT. His liquid rocket propellants are LOX and kerosene. Sam was kind enough to bring his liquid motor that he is finishing. It’s a 500 lbf, 600 psi LOX-kerosene engine with an unlike impinging injector. His stainless steel chamber with a graphite nozzle insert ought to hold up to short burn durations. Everyone was able to inspect the injector, chamber and nozzle parts that Sam made at the MIT machine shop. The delicate work to get a clean injector pattern was impressive. He’ll be water flow testing the injector soon to verify that everything looks right.

The RRS recommended Specialized Coatings, a ceramic coating vendor in Huntington Beach, that we have used with success in the past on alpha and beta nozzles.

Specialized Coatings – Huntington Beach, CA

Sam Austin’s liquid motor nozzle with graphite throat

Sam Austin’s injector assembly for his liquid rocket

Sam is still working on the propellant feed system. He already has a pair of liquid carbon dioxide vessels that are of the right size. After safely removing the original valves and getting the rest of his control plumbing, he will hopefully have what he needs to conduct testing at the RRS MTA or at FAR next month in July 2018.

There were a few questions about different features of Sam’s liquid motor, but overall it looks like it should work. Sam is getting prepared to finish the propellant supply system for a static fire of this rocket motor. With luck, he should be able to get into hot-fire at the RRS MTA or FAR site next month and hopefully before he returns to MIT in the fall. We are glad that Sam has decided to join the RRS as a student member.

The RRS membership had a few suggestions for improvement and a few recommendation for low cost regulators, ball valves and relief valves that have been used in other amateur and professional projects.

[+1] We managed to talk about one bonus topic by showing the video from the vertical static fire of the vehicle-sized solid motor by Jack Oswald and his team at the RRS on Thursday, June 7th. The video clearly shows a nozzle failure after two seconds from start, but it seemed that there may have been grain fracture leading to a partial blockage of the nozzle and then the resulting pressure surge shattered the nozzle. We may upload the video to our YouTube channel once we ask Jack and his team. Hopefully, Osvaldo can extract a few still photos from his footage. I think some of those stills will show an impressive start followed by a change in the flow pattern and abrupt failure with ejecting fireballs of propellant that followed. The RRS works safely and are glad to have our own remote testing site like the MTA to do these larger projects.

Sam’s presentation was very engaging, but we ran out of time before the Community Center closed at 9:00PM. We did not address all of our main agenda items or some of those added at the last minute. We will roll these topics to the July 2018 meeting.

* Osvaldo’s alpha parachute system and the video of its launch on 6/2/2018 at the MTA
* Getting a sign at our first metal gate as you reach the MTA
* Saturday morning seminars for members and how to get those started
+ Richard’s progress with the RRS standard liquid rocket
+ Discussion about the 2019 symposium

+ We did agree to discuss the topic of RRS membership policy and what constitutes being an active member.
+ Also, on the meeting agenda for July 2018 is the quarterly update on the SuperDosa project. I hope to have something ready to present by July 13th.

If there is anything I have missed or misstated, please let me know:
secretary@rrs.org

Again, we will have another launch event at the MTA on July 21th with the LAPD CSP program and member projects to be discussed later.

The next monthly meeting will be July 13th at the same place and time (7:30PM).

MTA launch event, 2018-06-07

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The RRS held another smaller launch event at our private testing facility (MTA) in the Mojave desert on Thursday, June 7, 2018. This was a special event for the RRS members from the former Chaminade High School Rocketry group led by Jack Oswald. They have been working hard on their solid motor design. After a successful test series on a single 6-inch Bates grain, they moved up to a vehicle sized test with multiple six-inch grain modules installed. Dave Crisalli and Osvaldo Tarditti were available to assist in the loading and installation process making ready for testing. The vehicle static fire test was oriented nozzle up with a load cell at the bottom of the frame secured to the vertical test stand at the RRS MTA.

The static fire testing was for a 10,000 lbf-sec flight motor. The motor was expected to perform at 1500 psi, produce over 2+ tons (>4000 lbf) of peak thrust, and burn its 43 lbs of composite BATES grains in approximately 3 seconds.

Osvaldo had video of the firing where we got the still photos below. Osvaldo will bring the video footage to the meeting tomorrow on Friday, 6/8/2018, where everyone can see what may have happened.

Vertical static fire of a solid rocket motor at the RRS MTA, 06-07-2018

The solid motor started okay. The nozzle plume looked good for the first few moments.

Jack’s motor starts off just fine. A nice nozzle plume is evident.

Unfortunately, shortly after start, about 2 seconds into the burn, the pressure climbed substantially to 2300 psi causing a nozzle failure and subsequent burnout.

It looked like something disturbed the flow in the frame just before the huge fireball and the disintegration of the nozzle

After the nozzle failure, the solid motor spews chunks of fiery propellant until it fully burned out.

In the coming months, Jack’s team will make another motor reducing the Kn factor and significantly reinforcing the nozzle design to carry through with their plans of launching a boosted dart to an altitude of 150,000 feet sometime this year. This is still an impressive accomplishment and with some perseverance, success will come. More details to come as things proceed.

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During the June 7th event, Osvaldo took some time to search down-range for rockets with his extractor tool. As luck would have it, he found one of the RRS standard beta rockets launched by UCLA in 2017. It was found about 3000 feet downrange which isn’t terribly far away. The winds must have been very favorable to allow the beta to plant itself much closer to the launch site. From the photo, you can see that this RRS beta had a fin-can type of fixture at the tail which is easier to manufacture.

Beta planted 3000 feet downrange from the launch rails, straight west more or less

The convenience of pulling the rocket straight from the ground with the manual winch is tremendous, but the method often shears off the payload tube in the hole. Shoveling does have the advantage of removing most if not all of the parts if one is inclined to spend the hours necessary to dig four feet below the surface. The payload tube from the beta unfortunately was not extracted with the propellant tube. Osvaldo will bring the beta to show everyone at the meeting tomorrow.

Osvaldo lifts the beta rocket from out of the desert floor

We’ll have our monthly meeting (every 2nd Friday of the month) on June 8th at 7:30PM sharp. Please stop in!