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.

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Lots of progress with RRS archiving and history

We’ve had a lot of progress recently with archiving RRS newsletters and other publications. I’ve put up six more newsletters. I’ve posted three more newsletters I received from Osvaldo a while back and had backlogged to post. This includes issue Vol_54_4_Dec_1997.pdf which is the second half of the RRS’s summary of the history of liquid propellant work at our society.

RRS liquid rockets from the 1990’s

Three more editions of the early Astrojet newsletter where also posted in the newsletters section with the help of Dave Nordling who got Caltech’s JPL library to send us what digital copies they had of Astrojet. I suspect they may have more in their archives because it seems odd to have only three non-sequential issues, but we haven’t been able to locate them through their catalog. The RRS is thankful to the Caltech JPL library for retaining and helping us obtain a copy of these early works of the Glendale Rocket Society (former name of the RRS). We hope to work with them more in finding all we can on the RRS history.

RRS founder, George James, with his “Slim Jim” rocket

Dave Crisalli shared with us ten more articles and two videos. These reports and videos have been posted on the “Other RRS articles” tab of the RRS website.

Six of these articles are from High Power Rocketry Magazine and were authored by Dave. Also included was the most recent versions of David Elliot’s and Lee Rosenthal’s hydrogen peroxide (H2O2) monopropellant rocket report from the 1950’s.

Dave Crisalli’s package included a Scientific American magazine article from 1957 which includes mention of the RRS. This is on the “Articles about the RRS” tab of our website.

Also we have an improved version of Dave Crisalli’s 1976 liquid rocket report that has been updated with more pictures and drawings.

Dave also shared two fantastic videos. One is the video report on the hydrogen peroxide (H2O2) monopropellant rocket that was filmed at the time of construction and test in the 1950’s with audio narration by David Elliot being added later by Dave Crisalli.

The second is a video report on Dave’s sounding rocket, a written report on the Rocket was also shared. This video also includes some liquid propellant work done at the time by other RRS members.

Dave Crisalli’s regeneratively cooled LOX-kerosene rocket firing

Both of these new videos can be seen on the RRS YouTube channel linked below. Please subscribe to our channel so you can be informed of updates as we make them. There will certainly be more to come!
RRS YouTube channel – ReactionResearchSoc

Frank Miuccio, our society VP, contributed a small collection that I’ve also posted to the “articles about the RRS” tab of the RRS website. This included six magazine articles and some news clippings.

Image from Popular Science article

Another item added to the other RRS articles section, brought to us by John Mariano and scanned by RRS secretary, Dave Nordling, is a short book on early micrograin rockets jointly from the Reaction Research Society (RRS) and the Pacific Rocket Society (PRS). The title is simply “Micrograin Rockets” and was written by B.J. Humphreys Jr with excellent illustrations and descriptions.

Micrograin rocket and an early hybrid design (B.J. Humphreys Jr.)

Dave Nordling is also working on scanning a second book from John Mariano’s personal library which should be available soon.

A lot more will be coming soon. Frank Miuccio managed to obtain a collection of RRS newsletters. They are issue numbers 59 through 100 and he has lent them to me to scan. This will double the number of newsletters we have archived!

In addition to that, Dave Nordling has gotten for us the RRS material that happened to be scanned and collected by the Google Books Library Project. This was five documents some with several Astrojet editions included. The collection totals 934 pages in all. Although some of the Astrojet newsletters are duplicates. The collection includes reprints of the first eight RRS newsletters from our founding in 1943! A truly impressive find which we hope to share soon.

All this new material will have to be scanned and parsed before being posted, which I hope to accomplish one at a time over the next few months. I’ll write a follow-up post once it’s all up on the RRS website.

More RRS newsletters

We strongly encourage our current and former membership to consider donating or just letting the RRS borrow articles, photos and reports for scanning and inclusion into the ever-expanding RRS archives. We hope to build a great story for the RRS history project coinciding with our upcoming 75th anniversary next year.

If there are any errors, clarifications, questions, suggestions or other things you’d like to add or donate to the RRS archives, contact me at research@rrs.org.

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