A Tale of a Scratch Built...

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Last night I was hunting through my parts boxes for the stuff I needed to clone a Big Bertha, needed for this Saturday's demo. While I was raiding my store of body tubes, several of the tubes comprising part of the whistle pods of the Estes Screaming Mimi fell out and scattered all over the place - again :eek: (I use Michaels' coupons to get Screaming Mimi's for cheap and cannibalize them for BT-60's, D' engine motor mounts, and, of course, the popular PNC-60AH nose cone used in so many older Estes kits).

I therefore determined to remedy this slightly annoying situation...

Looking at these 4" BT-20 tubes, I naturally thought "tube rocket." And since I need to make more use of the RockSim7 I shelled out almost 100 bucks for, I resolved to sim the rocket before building it.

This was almost the case... I couldn't resist test-fitting the tubes around a BT-50, and, before I knew it, 7 of them were glued around the base of an 18" long BT-50 core. ;)

Then I got around to RockSim. It turns out that you can't sim a tube rocket "stock", as RockSim has no provision for tubular fins. Fortunately, Bruce Levison has written a series of excellent articles on how to do so, and these are available as newsletters from the Apogee website . I started out with the piece on simulating side pods with RockSim 7 (#119), and decided to adopt the method which yielded the more conservative estimate of the CP (See figure 3 on page 3). This in turn led me to newsletter #113, "Simulating Fins on Fins." After this bit of reading, I was able to design the rocket.

However, the RockSim results showed the bird to be unstable (1 caliber stability WITHOUT engine), which did not jive with my intuition, which thought that 18" of body tube + a PNC-50K nose cone would yank the CG well forward. To make sure I was doing things correctly, I simed the rocket in the afore-mentioned figure 3 and came out right on the nosey. So what was up? :confused:

A check of the figure 3 rocket showed that it was using an identical nose to my tube creation; however, its mass was calculated to be 0.3 oz, and a PNC-50K nose is listed in the catalog as having a mass of 0.13 oz. A new look at the nose showed that I was not dealing with a "true" PNC-50K, as my nose had a shoulder of 0.75", compared to 0.5" given in the catalog. Which weight was closer? If 0.13 oz was, then RockSim said I needed nose weight or a longer tube or a new nose. I found myself wishing for a scale, as I wanted to make use of the nose (I have 4).

Then I remembered that there was a discussion on TRF about the weight of pennies. A quick search of the forums gave the weight of new pennies as 2.5 g (~0.08 oz). I rigged up a simple balance using a ruler and a T-shaped aluminum sanding block. The nose was definitely heavier than 3 pennies, but less than 4, placing its weight between 2.4 and 3.2 oz. I was happy :D

Then my eye caught sight of one of the red "clear" plastic tubes I had purchased from Custom Rockets sometime back, and I decided to jazz up the rocket a bit by turning it into a payloader. The appropriate mods were made to the RockSim simulation, which gave a peak altitude of around 750' on a C6-5. Here is a screenshot of the final design:


I have attached the RockSim file for those of you with version 7.

Even though something was still bugging me about the design, I finished building the bird and went to bed, which happened around 2 AM.

Then, while taking my morning shower today, an image of RockSim's tail view flashed up in my brain, and I realized my mistake :mad: . Here's the picture, class - can you find the boo-boo? (Ignore the fake fins within the core - that's not it)


If you said "The dummy didn't think that the launch rod would be very snug passing beneath the tube fins - not much clearance there", you're right. A test showed that the rocket would not even come close to moving freely on the rod. Things got a little looser when I slid the rod back and forth several times, but not enough. Frustrated, I set my Frankenstein aside, and checked my email, where I saw a note from Sandman... The name produced a flash of inspiration, and soon I had constructed a tool consisting of a 4" strip of sandpaper CA'd around a 1/16" wooded dowel. Needless to say, the bird soon moved freely on the rod, unless it was bent.

Here is a photo of the rocket, which I have christen Algol, after the "Demon Star" in the constellation of Perseus. Primer and paint will have to wait until the weather clears.


Lessons learned:

1) 4" is way too long for tube fins on a rocket of this size.
2) Make sure you have enough clearance for the launch rod.
3) TRF is quite handy for resolving problems.
4) RockSim is cool.
5) Murphy is gonna shaft you, especially on a "simple' build.

As an example of the latter, it just occurred to me that I could have used a piece of that 1/16" dowel to create a launch lug stand-off and run the rod through one of the fins, avoiding all the mess I went through - Duh!

But I ain't gonna change a darn thing...
Originally posted by SpaceGarbageMan
Here's the RockSim file...

A great design, but I would make sure you put a payload (altimeter!?) in the payload section it each time it flys, or add some nose weight (between 5 and 10 grams). Do you know the actual weight of the clear payload section? Its hard for me to believe it only weighs 2.7 grams; if it weighs more, this design should fly nice and stable.

I reworked the RockSim file for you. I get marginal stability (static margin 0.77) on a C6-5; those tube fins are heavy! I also pushed the parachute and shock cord further down toward the motor mount for a worst case simulation.

You had seven fins selected for the Fake fin set for CP, you should have only used six, sorry if my article is not clear on this. I also set the override mass to 0.001 instead of 0.028 grams which should not make a lot of difference (the Tube #'s take the weight of the tube fins into account for the CG estimation).

I also set the fin sets to be rotated 360/7 = 51.428571 degrees apart from one another now take a look at the fake fins in the core! I also set the launch lug to a radial position of 25.7 degrees.

You don't need a launch lug at all, you already have 7! Just launch this one off a 1/2 inch wide 36 inch long section of PVC pipe placed through one of the tube fins (e.g. 1/2 inch diameter launch rod).

Bruce S. Levison, NAR #69055

Thanks for re-doing the RockSim file - I made the naiive assumption that the number of fins in the CP fake fin set should be equal to the number of tubes. My bad... :eek:

You are correct in that the payload section weighs more (I need to expand my parts database); however, I filleted the tube joints with glue to achieve a "smoother" look, which added weight to the rear. The measured CG is almost exactly 12.75" from the nose tip - when I modified the RockSim file you produced by slightly increasing the thickness of the payload tube until the CG's match, I get a stability of 0.96 calibers with a C6-5 loaded. Much better ;)

You are right about the launch lugs, except that a standard rod is a snug fit in any of the 7. If I had it to do over, I would place the lug so the rod passes through one of the tubes. I'll have to take a look at the 0.5" PVC pipe - didn't think of that :)

I have been using RockSim more of late... I am working on a pumpkin oddroc (using the pumpkin described in this thread) - basically, an elongated Fat Boy modified to use 24 mm engines. In the attached RockSim file, I used a elliptical nose cone + the same shape transition to simulate the pumpkin nose. Is there a better way to sim such a shape? I would like to be able to design a modroc like the Sputnik Too, which is a styrofoam ball with wooden dowels as the stabilizers.

Thanks for your help and suggestions! :D They are greatly appreciated.

Here is a link from the EMMR Rocksim Library with an example of a ball (pumkin! :) ) type nose cone. The Rocksim file was created by Tim Van Milligan. I used it to help with a boiler plate / prototype scratch model of the Discovery from 2001 A Space Odyssey.
Originally posted by rkt2k1

Here is a link from the EMMR Rocksim Library with an example of a ball (pumkin! :) ) type nose cone. The Rocksim file was created by Tim Van Milligan. I used it to help with a boiler plate / prototype scratch model of the Discovery from 2001 A Space Odyssey.

Here is the link he met to post, that page just got modified with some new material


Sputnik is a different story/beast, it is simulated more like a spool rocket due to the base drag being the lagest contributing factor to its stability.

Bruce S. Levison, NAR #69055