Amateur Rocket Motor Design

 The 2.375" rocket motor is ready to test but I haven't had the opportunity to get out the test site.  I have been feeling the urge to be doing some rocketry though and with that in mind I decided to build something.  I chose something small.  While the SStS project was immensely satisfying and a great educational experience it did lack a big part of what I enjoyed from my early days in rocketry.  The thing i really miss is taking something from concept all the way through to completion, being involved with every step, and then actually getting out to the desert and launching the vehicle.  It turns out that I had a rocket that I had promised to rebuild after it was lost over 8 years ago.  That rocket was powered but Richard Nakka's C-400 motor, slightly modified to utilize KNSB, a Sorbitol based propellant versus Sucrose which the C-400 was originally intended to be fueled with.  I came across this VCP design printout a while back.  At the time it wasn't the largest rocket I had built or flown but it was definitely the highest performance one with a lot of attention to drag, weight, and construction details.

Here's a photo of its only launch.  My brief blog entry of the launch can be found by clicking here.



 As you can see by the photo, my daughters paint scheme won out over my own preference.  I hope to document this build to a much greater extent than the first time around.  With that in mind here are my initial design thoughts.  The hope is to create a motor/rocket that is simple to construct, durable, and easy to prep for launch.  I'm initially designing towards a Dextrose based propellant but am hoping to be able to use several of the other sugars available.  Actually my notes indicate that the above launch used a propellant that was comprised of 50% Sucrose and 50% Sorbitol. 

Casing design:

SRM output: 

Here is my initial CAD:

2.375" Motor Updated

I changed the design again. I decided to optimize the it for KNER propellant by adding two additional segments. The additional propellant brings the motor up to a solid L-class motor and the chamber pressure sims. at right about 1000psi. I'm hoping to have an opportunity to test fire it for the first time in early December

Update: I updated the SRM output to correct a mistake. I had been using the wrong grain OD.

Old Photos

This is a rocket I flew back in March of 2004. My friend Peter and I got a lot of ribbing from the guys as the vehicle was constructed almost entirely from sewer pipe. The motor utilized an unrestricted burning grain, which basically means that it lept off the ground like a bat out of hell. That's me holding a propellant segment, Peter with the rocket, leaving the ground with a bit of an arc, straightening out, and the landing site. Fun Day!

Photos from November

Last November my friend James and I participated in a propellant class at the FAR site. James took some great pictures of the days events. The pictures above are of my KNO3/sugar motor and of my AP/AL/HTPB motor. The photo of the composite test was taken during the day but was "stopped down" in an effort to reveal the shock diamonds. Video of both of these firings can be seen here. More of James' photos of the days events are online here.

Static Test

This past Saturday I participated in a sugar propellant class at the Friends of Amateur Rocketry (FAR) facility. Brent Dougherty did a nice job presenting a general overview of how to safely work with and cast sugar based propellants. Thanks to the FAR guys Kevin, Mark, Ted and the others for letting us use the site. They have done an amazing amount of work on the site, including a massive vertical test stand (I don't remember what it's rated for).



As part of the class I fired an L-class KNO3/Sorbitol demonstration motor. The firing went well and good data was collected. This is a simplified set-up from what I've used in the past and it worked well. I'm planning on making some adjustable steel brackets that mount with bolts rather than the cargo straps. Here is the set-up:




In the picture taken just after the firing you can see the nozzle glowing red hot, once I got the motor torn apart and cleaned, everything appeared to have survived the firing just fine. The gauge measuring thrust began to register data about a tenth of a second or so before chamber pressure began to be recorded. I suspect that the pressure port was briefly clogged by the igniter initially. After tearing the motor apart I found a glob of melted plastic that used to be the soda straw that held the BP.

New Motor

Here is a 3D model of the new motor. The insulation is not shown so that the grain assembly can be clearly seen. This motor uses a ring of screws to retain the nozzle and bulkhead that are not yet shown.

Here is a view inside the casing showing a propellant segment and the insulation.

Early Dual Burn Motor Test

This is a video of a static test that I conducted back in 2005 when I first got involved with the Sugar Shot project. The dual burn concept is not new, but I haven't heard of any amateurs pursuing it. At this size, I don't think you'd realize much benefit. However, in a slightly larger motor, such as the one that we are currently constructing, you begin to see some real benefits. The concept is similar to staging, but in some ways simpler to execute. One of the major difficulties is that the first half of the casing and the nozzle are subjected to twice the heating that they would have to endure in a two stage design. The motor that we are currently working on has the potential to reach nearly 50,000 feet, which is considerably higher than you would expect to go with a "standard" motor that expends all of it propellant close to the ground in denser air. Despite some minor problems, I considered this early test to be successful. The o-ring on the upper bulkhead was damaged, and I didn't have a replacement. That resulted in a small leak during the 2nd burn. However, there were several key things that did seem to work. Most notably:
The first propellant grain burned properly and ignited the epoxy plug .

The epoxy plug burned through without igniting the second propellant grain.
The burnt remains of the epoxy plug did not obstruct the nozzle.
The mid-bulkhead did not seem to have a significant effect on the 2nd phase thrust.

Four Inch Diameter SRM

This is a four inch diameter motor that I've been working on for a while now. This motor is very similar to the motor that I flew last month. Both motors used 6061-t6 aluminum for the casing and 12L14 steel for the nozzle. The grain geometry was also very similar, with the major difference being that this motor uses six segment instead of four. This motor has been fired twice now. Both firings occurred at the RRS's Mojave Test Area. I normally go out to the site the night before to do propellant/motor prep, then sleep in a tent on site that night. The first firing experienced a burn through in the casing. The motor fired well for most of the burn and damage was limited to the casing, so I was very pleased with the initial firing. I made several small changes to the design; first was reducing the diameter of the casting tubes/inhibitor slightly to accommodate an increase in the casting tube/inhibitor thickness of 15% , lastly the casing insulation thickness was increased by 25%. I consider the second firing to be successful. The ignition was a bit slow, due to much of the igniter material being expelled from the nozzle. The use of a pyrogen canister should solve this problem. I am pleased with the resulting ISP of 127 (ISP being a measure of propellant performance). The important thing for me was that the casing temperature didn't begin to rise until several seconds after the test indicating that the insulation was effective. Post test inspection revealed that the insulation had been breached in several places, but the video and temperature data do seem to support the idea that these breaches occurred post firing. I may still add a bit more insulation for added safety though.

Motor Stats

I thought I'd post some quick stats on the motor that I flew on the 28th of April. The motor was designed to produce approximately 425 lbs of thrust for 2.2 seconds for a total impulse of nearly 950 lbs. The motor (nozzle, casing, bulkhead, snap rings, inhibitor, and casing liner) weighted in at 5.96 lbs. The propellant grain, consisting of 4 segments in a bates grain configuration, weighted in at 7.31 lbs, with a port/throat ratio of 1.5625. The propellant was mixed in a ratio of 65/17.5/17.5 (KnO3/Sorbitol/Sucrose) and cast into 3" mailing tubes that have an ID of 3" and an OD of 3.125". The casing insulation consisted of four layers of rosin paper, purchased at Home Depot. The casing and bulkhead are fabricated of 6061-t6 aluminum. Using Richard Nakka's casing.xls the design safety factor was determined to be 2.2 with a burst saftey factor of 2.94. The nozzle is fabricated from 12L14 "leaded steel" and had a throat diameter of 0.64" and an expansion ratio of 12. There was no measurable throat erosion post firing and the nozzle appears to ready for another firing. When I took the motor apart post flight the insulation was nearly intact, having been breached near the bulkhead. The casing appears to be fine and there is no measurable deformation.