November 2017 meeting

The RRS held its November monthly meeting at the Los Angeles Cleantech Incubator in downtown Los Angeles. Our regular location at the Ken Nakaoka Community Center in Gardena was closed for Veteran’s Day on Friday, November 10th.

Los Angeles Cleantech Incubator (LACI)

Meeting topics included the upcoming launch event with the kids from the Imperial Courts neighborhood. This event was postponed to November 18th and was greatly anticipated as the kids worked hard and learned a lot in the five-week program put on by the RRS events team with the help of the LAPD CSP program.

Imperial Courts students prepare their paper rockets for launch

The program included a tour of Los Angeles Air Force Base at the SMC Heritage Center with Lt. Col. Alec Porter where the kids got to build and launch paper rockets.

Paper rockets at LA AFB with the Imperial Courts RRS class

USC Impact News has been documenting the whole Imperial Courts class program including the launch event at the MTA. The RRS looks forward to seeing their finished program at the end of the semester this December 2017.

The programs with LAPD CSP have been very successful and another program with Florence Griffith Joyner Elementary School is in the works for early next year.

The RRS symposium is in the planning stages for the upcoming 75th anniversary symposium on April 14, 2018. Several invitations to presenters and exhibitors have been issued and more are in the works. Frank Miuccio is leading the symposium again and we hope to have an even grander event for the public on this special occasion.

The Astrojet newsletter still needs submissions! We have had several interested parties, but no submittals have been received.

Please! Submit your drafts to the RRS secretary right away.
secretary@rrs.org

The Astrojet newsletter was the original publication started by the RRS when it was founded by George James in 1943. We thought it fitting to issue this one-time edition of the Astrojet for our 75th anniversary on January 6, 2018. It will be available in print only and the funds collected will support the RRS in our anniversary year events. Issues will be available by mail to our membership and other interested parties for a cost of $10 per issue.

Nominations of next year’s officers for the RRS took place at the meeting which is our normal custom. Voting by the administrative membership is underway and the results of the elections will be announced at the December 8th meeting. The meeting will be at the Ken Nakaoka Community Center in Gardena, as usual at 7:30pm.

Please let me know if I have missed or misstated anything in this post. See you at the next meeting.
secretary@rrs.org

RRS standard alpha rocket

Some time ago, I was asked to explain in more detail about the RRS standard alpha rocket. Although it has been frequently referenced, some of our general audience may not be familiar with the many aspects of the alpha. Therefore, I have decided to devote an entire article to this subject.

Alpha rocket iso view

This standard design at the RRS has been a common beginner’s rocket in our amateur rocketry society. We use it in our build events with schools, offer it as an experimental testbed for universities and also for our members to conduct their own experiments. It has a long history with the RRS and we still continue the tradition of building these rockets as it is a nice platform for experimentation and introducing newcomers to amateur rocketry.

RRS president, Osvaldo Tarditti, holds a pair of alphas

A similar “Ft. Sill alpha” rocket design was mentioned in the 1960 book, Rocket Manual for Amateurs, by Bertrand Brinley. Over the years, there have been changes made to the alpha design, but this article describes what has become the RRS standard in the alpha rocket design. I have been told that the 1-inch alpha design was created as a smaller and cheaper-to-fly design from the 2-inch beta design.

The alpha is a single-stage rocket consisting of a 3-foot length of 1.25″ outer diameter (OD) drawn-over-mandrel (DOM) steel tubing to hold the propellant. It is often erroneously referred to as a 1-inch rocket, which is more of a relative size measurement. The propellant tube has four trapezoidal sheet steel fins welded at their edges near the bottom such that the rocket fits with the launcher rail design at the Mojave Test Area (MTA).

the RRS launcher rails for four-finned rockets,
beta launcher is shown

Once ready, the alpha rockets are top-loaded into the rails and the pyrotechnic operator (pyro-op) in charge hooks up the igniter wires once we go into a launch mode.

RRS alpha sitting in the rails

launch rails for the alpha as viewed from above

The propellant tube has a bolted bulkhead at the forward end sealed with an O-ring. With good tolerancing, we’ve had no leakage from this joint and the four 1/4″ fasteners have sufficient retention under the brief ~1000 psi chamber pressure surge during combustion. This solid aluminum 6061-T6 bulkhead is installed first into the top of the propellant tube to begin loading the powdered propellant from the aft end.

coupler and bulkhead piece for the alpha

alpha bulkhead loaded and bolted in

The powdered propellant is loaded using a metal funnel a little at a time and gently and periodically bouncing the tube against a wood block to help settle out any air gaps. Many different improvements to increasing the packing density have been tried by the society over the years, but the society uses no special method for increasing the packing density of the micrograin propellant in most of our launches today.

Alpha tube loaded with micrograin propellant

Next the nozzle is loaded with a thin plastic burst disk (or diaphragm) with two tiny through holes to thread in an electric match (e-match).

electric match and burst disk

An e-match is a common pyrotechnic device used to initiate larger reactions with propellants. An e-match is two thin-gauge wires with a segment of nichrome heating wire bridging them. Covering the nichrome wire is a small amount of pyrogel compound that creates a brief high temperature flame once the match is given sufficient current. The e-match is single-use as the tiny wire is destroyed after ignition.

an Estes rocket igniter or e-match, shown as an example

With the burst disk sitting on top of the nozzle facing inward to the propellant, the e-match is packed into the propellant with the thin wire leads running to the outside. The burst disk sits inside the propellant tube held behind the nozzle closing off the propellant powder in the rocket. Although the zinc/sulfur micrograin propellant is fairly insensitive and stable, the e-match has sufficient energy to ignite the micrograin propellant behind the burst disk.

loaded propellant tube with nozzle and burst disk ready for attachment

The use of a linen-filled Micarta burst disk is not only for practical reasons of holding the propellant inside the tube after the tube is turned right-side up, but it helps build up the chamber pressure after the first few moments after ignition. The burst disk is designed to sacrificially break under the elevated pressure created from initial ignition from the e-match. The thickness of the burst disk is carefully chosen to not over-constrain the initial pressure rise in the propellant tube on ignition. The burst disk fragments then quickly exit the nozzle as the rocket takes off leaving the lead wires behind.

alpha nozzle bolting into the bottom of propellant tube

nozzle loaded on to propellant tube with e-match wires sticking out

Above the coupler is the payload tube. The standard alpha design uses a 1.75″ OD, 0.065″ wall, aluminum 6061-T6 tubing. The standard design calls for an 18-inch payload tube length, but shorter versions have been flown with 12-inch lengths being common in some of our school launches.

Nose cones have been made from wood, Delrin plastic and from solid aluminum. The RRS standard alpha design uses a tangent ogive shape which has been more of a traditional choice. Nose cones sometimes have hollow space inside for more payload capacity, although solid nose cones have also been used. The aluminum nose cones are fairly light and are very damage resistant compared to the plastic nose cones that mash from impact or the wooden ones that shatter. Aluminum nose cones have been re-used in subsequent builds after some turning and polishing.

12-inch payload tube with aluminum nose cone

Instruments are flown in the payload section and although space is very limited in these small rockets, smaller chips have increased the number of measurements possible (altimeters, cameras, barometric pressure sensors…). Smoke tracers have been used in recent events with increasing success. This helps in spotting the direction of flight and where to start looking to recover the rockets after impact. In these flights, we have a second set of ignition wires running to the rocket to first light the smoker before lighting the motor.

vented payload tube with smoke grenade inside, wooden nosecone

The alpha is a solid fueled rocket by what is called a micrograin propellant. The zinc and sulfur fine powders are one of the earliest solid propellants used in amateur rocketry and was invented by RRS founder, George James. The RRS standard mixture is 80% zinc and 20% sulfur by weight. Different ratios have been tried in the society, but this is our standard. Although a low performer among today’s solid propellants, it is inexpensive, simple to find, comparatively stable and quite fast once ignited.

zinc powder

sulfur powder

micrograin combustion demonstration at MTA

The zinc and sulfur powder constituents are separately measured and weighed then added to the 30-pound capacity metallic mixing drum. The mixing drum has internal metal baffles to speed up mixing as it is rotated on an electric motor driven rolling carriage.

metal baffled mixing drum with the zinc and sulfur, before mixing

electric motor driven mixing rolling carriage used for micrograin propellants

alpha launch 03-25-2017

The empty weight of the alpha is 3.65 pounds. Measured after propellant loading, the alpha fully loaded is 6.55 pounds. The calculated propellant load would be 2.90 pounds.

Specific impulse of the zinc/sulfur micrograin is quite low, 32.6 seconds. With an ideal combustion temperature of 2,600 degrees Fahrenheit, despite best efforts in packing, a significant part of the powdered propellant falls unburned out of the nozzle from the rapid acceleration even as the propellant is combusting. The rocket is supposed to operate as an end-burner with a 90 inch per second burn rate measured in many tests. Although most rocket groups no longer use the micrograin, the RRS maintains the tradition and it is hard to beat for simplicity.

The burnout time is about 0.8 seconds and burnout velocity is subsonic (roughly 600 ft/sec). Apogee for the alphas have been estimated at 5,500 feet based on the flight times (35 to 38 seconds) from launch to impact. Despite the long history of launching the alpha, some of these performance figures haven’t had many recorded measurements. The RRS is working on making systems to take better measurements, not only for the alpha, but for any of the rockets we build and test at the MTA.

If there are any questions about anything in this article or there is anything more you’d like to know about the RRS standard alpha, feel free to post a comment on our forum.

***

MTA event, 2017-05-27

The RRS was glad to support UCLA in a series of cold flow tests of their liquid rocket system at the Mojave Test Area (MTA), Saturday, May 27, 2017. The flow tests provided useful data as the UCLA liquid rocket group proceeds with their build for the upcoming FAR-MARS competition put on by the Mars Society.

Mars Society – liquid rocket competition at FAR

UCLA conducted three tests in the series to gather pressure data in the blowdown tests for what will be their gasoline and nitrous oxide liquid rocket.

Richard Garcia, Osvaldo Tarditti and I supported the event with new member, Angel Perales.

Richard had spotted an alpha embedded in the earth just north of the MTA and slightly east of the alpha launch rails. It was an odd place to find one of the alphas and from the wooden nose cone it would seem that it was one launched from last year. Leakage around the nozzle and one of the nozzle bolts being missing probably contributed to the rogue trajectory.

Angel Perales stands next to the alpha found in the the north

rogue alpha found in the north

alpha with wooden nosecone from last year

UCLA seemed to get useful data and the results indicated the performance was close to predictions. Several parts including the thrust chamber are still under construction, but the valves, propellant tanks and venting systems were all tested. UCLA should be ready for a hot-fire test next week at next launch event, Saturday, June 3rd at the MTA.

UCLA sets up their liquid rocket breadboard for flow testing

UCLA makes final checks, prepares for flow test, 05-27-2017

UCLA conducting remote testing of their liquid system, 05-27-2017

Between tests, Osvaldo and I took a walk downrange in the more southern areas on the BLM land looking for more of the alphas and betas we’ve launched but haven’t yet recovered. It is not yet known where the betas have been coming down, but we think the distance may be greater than previously thought. I found two more alphas much further downrange than expected (~3100 feet).

Alphas found far downrange

Google Earth has a ground distance measurement tool I used based on my relative recollection of the spot we found those two alphas. My cell phone doesn’t have a strong enough signal to get a specific location marker.

With the hot temperatures, Osvaldo and I only had the strength to dig up one of those found which was an alpha (#10) from the GALA event in March. The aluminum nose cone was very much intact despite the ballistic re-entry into the earth.

GALA alpha #10 recovered, 05-27-2017

recovered alpha with aluminum nose cone, scratched but straight

Recovered alpha from the GALA event

The tight clearances and good craftsmanship in this new batch of alphas was very evident in the lack of leakage seen around the nozzle and the considerable downrange distance the alpha traveled.

Next launch event will be June 3rd. It is planned to have UCLA hot fire their liquid rocket, but this will depend on key parts being ready in time. Dr. Mitchell Spearrin of UCLA will have his scheduled launch event with us for the undergraduate laboratory class (MAE 157A) with ten F-motor model rockets, two alphas and two betas. It’s my hope that UCLA will have time to mount some instrumentation in the alphas and betas. It should be a really big event.

If there are any questions or corrections to be made, please contact me:
secretary@rrs.org

For other groups interested in working with the RRS at the MTA, please also contact our president:
president@rrs.org