My motor choice went up in smoke
Recap: A plan that hang-fired
I briefly covered my initial motor choice in the ‘Rocket Design’ blog post. For my rocket, GLYPO-001, I required a G-class motor that I could fly at my ‘local’ club FOG, and a more powerful H-class that I would fly elsewhere in the UK with a UKRA (UK Rocketry Association) certified RSO (Range Safety Officer).
I was not looking for altitude, I was looking for a nice dramatic slow launch. The Cesaroni AP (Ammonium Perchlorate) motors seemed ideal. They have some long-burn grains that I hoped would deliver the right stuff. By way of comparison, the Cesaroni Pro29 G-class motors (80–160 N·s) with three-grain refills have an average burn duration of 1.8 seconds across the range. They offer high-velocity grains that burn as quickly as 0.4 seconds, all the way to very slow-burn motors that last 4.4 seconds (Mellow).
I had selected, for my G-class, the ‘Red Lightning’ that burns for 3.0 seconds and the H-class ‘White Longburn’ that burns for 3.1 seconds. The Mellow was a little too mellow and would not give my rocket enough speed off the launch rail to ensure aerodynamic stability.
Video: A much larger, 98 mm, Red Lightning test fire from Cesaroni’s YouTube.Falling short: What I did not take into account
That all seemed pretty reasonable at the time of my conceptual design in OpenRocket. Except a fundamental oversight on my part, it is bloody difficult to get Cesaroni motors here in the UK. In my defence, I thought it would be easier. Rockets and Things appears to be the go-to place for Cesaroni, but they did not have what I needed in stock. I had used ModelRockets.co.uk for several purchases of Estes kits and black powder motors, but they hold a far smaller stock of Cesaroni parts, and a prompt helpful message from them suggested that may not change any time soon.
The only place I could find with stock was a Dutch company, Euro Space Technology, for which I felt less confident relying on overseas transport. This left me with a problem. I had built a majority of my rocket at this point. Here it stands in its half-painted form:
The fin assembly, which is built around a 29 mm diameter phenolic motor tube, is firmly glued into the body tube. This provides a clear constraint for any replacement motor. It must be 29 mm. I was left with a dilemma: do I try to order some Cesaroni motors from mainland Europe (and risk misunderstanding the ramifications of importing ammonium perchlorate into the UK), or do I change course?
Change in direction: Now pointing upwards
This was a pretty easy decision. I do not feel comfortable importing anything propellant-related into the UK, therefore I explored my options and there seemed to be a clear winner. You may have read in my BOM post that I used Wizard Rockets to source the nose cone. My experience was really positive, and they just happen to import AeroTech motors, which I understand to be the main competition to Cesaroni.
Images: AeroTech website – washers, O-rings, and a lot of parts to squeeze into the casing (bottom-right).
It is fair to say that the AeroTech motors are not as intuitive as the Cesaroni. As I write this, I am still not certain I have fully grasped their system. However, Wizard’s owners, Stuart and Naomi, are excellent communicators and I am certainly much clearer. The fact they have stock, are so helpful, offer guidance in assembly of the motors, and attend most model rocketry events in the UK all inspires a lot of confidence. AeroTech is now the clear motor choice because of Wizard Rockets’ great support.
Performance: Not yet having a blast
Having browsed through the Wizard Rockets website, I identified three motors:
- G75J-10A 29/180 RMS-PLUS
- H128W-14A 29/180 RMS-PLUS
- H238T-14A 29/180 RMS-PLUS
These would all fit inside the RMS-29/180 full motor casing, straddling both G-class and H-class impulse. From the AeroTech Master Motor Matrix (updated 2 April 2023), I was able to establish the following information:
| Designation | Total Impulse (N·s) | Average Thrust (N) | Peak Thrust (N) | Thrust Duration (s) | Propellant |
|---|---|---|---|---|---|
| G75J-10A | 135.6 | 75.0 | 70.3 | 2.40 | Black Jack |
| H128W-14A | 172.9 | 128.0 | 168.7 | 1.30 | White Lightning |
| H238T-14A | 165.5 | 238.0 | 263.4 | 0.70 | Blue Thunder |
Straight away I could discount the H238 on personal preference. A 0.7 second motor burn would surely allow for a fast launch, but I feel the slower burns are more dramatic, and thus to my taste. At 1.3 seconds, the H128 is still shorter than I would prefer, but it is much closer to my ideal. It is not clear how the G75 could possibly have a higher peak thrust than average thrust. Surely an error on their part.
Straight away I could discount the H238 on personal preference. A 0.7 second motor burn would surely allow for a fast launch, but I feel the slower burns are more dramatic, and thus to my taste. At 1.3 seconds, the H128 is still shorter than I would prefer, but it is much closer to my ideal. It is not clear how the G75 could possibly have a higher peak thrust than average thrust. Surely an error on their part.
Now I was down to two motors. I could compare the above motor matrix data with the data given on the product pages, and there were yet more inconsistencies. From the AeroTech product pages of the G75J-10A and H128W-14A, the following can be seen:
| Designation | Total Impulse (N·s) | Average Thrust (N) | Peak Thrust (N) | Thrust Duration (s) | Propellant |
|---|---|---|---|---|---|
| G75J-10A | 155 | (Not given) | 76 | 2.0 | Black Jack |
| H128W-14A | 175 | (Not given) | 151 | 1.4 | White Lightning |
There are differences in every field. This level of inconsistency is infuriating, especially for an already unintuitive motor system. I have more faith that the product pages are correct, as they also provide a thrust profile that appears to align with the stated specifications.
Out of the frying pan, into the flame trench
The pain was not quite over yet. A further issue occurred in that OpenRocket lists two AeroTech motors with a designation G75J and two marked H128W, and in both cases neither seemed to match the above statistics nor the thrust profile. Consequently, I had to digitise the thrust profile. Interestingly, this is provided by AeroTech in pounds-force, despite their own motor designations being in Newtons. Perhaps this is because AeroTech appear utterly adverse to consistency.
Here is the H128W in the process of being digitised:
Engauge is an excellent graph digitiser. From this I created CSV files, converted from lbf to N, and translated them into the RASP file format (.eng), which OpenRocket readily accepts. The integrated values from these profiles, as seen once loaded in OpenRocket, are a much closer match than the built-in profiles.
In the extreme improbability that a reader has both reached this far in this rambling blog post and also needs these thrust curves for OpenRocket, I include them here. Note that these are ASCII text files.
All fired up: Finally, onto the simulation
Now that I had at last narrowed the selection and acquired what I sincerely hope is the correct data, I could finally run the OpenRocket simulations and compare the new motor selection (AeroTech) against the old (Cesaroni):
| Brand | Cesaroni (Old) | AeroTech (New) | Cesaroni (Old) | AeroTech (New) |
|---|---|---|---|---|
| Motor name | G54 Red Lightning | G75 Black Jack | H54 White Longburn | H128 White Lightning |
| Motor class | G | G | H | H |
| Impulse (N·s) | 159 | 155 | 168 | 175 |
| Average thrust (N) | 53.3 | 75 | 53.6 | 128 |
| Max thrust (N) | 122 | 76 | 103 | 151 |
| Burn time (s) | 2.99 | 2.0 | 3.13 | 1.4 |
| Apogee (m) | 485 | 350 | 508 | 498 |
| Apogee (ft) | 1,591 | 1,148 | 1,667 | 1,634 |
| Velocity off-rail (m/s) | 14.4 | 16.7 | 12.7 | 14.6 |
| Stability (cal) | 2.41 | 2.25 | 2.36 | 2.33 |
| Thrust to weight | 5.10:1 | 5.93:1 | 5.08:1 | 11.54:1 |
| Max velocity (m/s) | 101 (M 0.30) | 90.8 (M 0.27) | 104 (M 0.31) | 131 (M 0.39) |
| Max acceleration (m/s²) | 104 (10.6 G) | 53.8 (5.5 G) | 80.9 (8.2 G) | 129 (13.2 G) |
The motor choices are certainly acceptable. Comparing back to that first blog post with the requirements for thrust-to-weight and velocity off-rail, the AeroTech motors are better on both counts because they are both faster burning than their counterparts. This is less favourable for my preference for a slow burn, but good for stability.
H128W with a 1 m/s wind.
Conclusion: Mission accomplished?
I am sad to be losing some burn time, and mildly infuriated with the impenetrable complexity of the AeroTech system, but ultimately I have hope this is the right choice. The AeroTech motors have great availability and support in the UK because of Wizard Rockets, and thus availability and support triumph desired performance and simplicity.