Tuesday, November 29, 2016

Crayon planning and a side comment

I had been wanting to get Estes Crayon rockets for the grandrocketeers and the latest Estes sale gave me the opportunity. I have to admit that I bought the bundle pack to give me a couple to play with. They don't need that many, do they? My ulterior motive was fueled by this post by RocketECoyote.

Since they arrived (see previous post for da' photo), I have been pondering what to do with them, with finless and an upgrade to 29mm being the first two options. Unfortunately, the RTF guts will have to be dealt with. I went back to the post linked above and, when I read it again, I'm pretty sure he mostly used the stock tube without said guts.

So, for a finless, I could cut off the fin can and replace it with BT-60 that is painted to match. This can could be re-used someday. For 29mm, I'd have to cut it off, modify it, and reattach it.  I guess that's doable. At the moment I don't have any motivation.

Five of the six rockets came in good condition. The wraps were slightly crinkled over the fin can but they are perfectly acceptable. The white one, however is a mess. The tube was oblong and the 'Estes' sticker looks like my 3 year old grandkid put it on.  I carefully squished the tube back to round and at least the cone holds it that way. The wanted to re-apply the sticker, or remove it, but he underlying white wrap was tearing so I just left it. I guess the kids won't notice. This is not worth bugging Estes about and I feel better now that I've bitched a little :)

Sunday, November 27, 2016

And now, another finless RockSim model

I thought about the shortcoming of my First RockSim model and decided to approach it differently. May or may not be better, but I did it anyway!

Instead of a BT-60 rocket with a 3" ringtail, this one is a 3" rocket with a central BT60. Since you can't attach a nose cone to an inside tube, the exposed BT-60 is separate from the BT-60 section that is inside the 3" body. The two funky parts of this one is that only the exposed part of the strakes are modeled and the front BT-60 abuts to the 3" tube without a transition section. It's not clear how RockSim handles this.

This version is much more stable than the first. With a C11-3 the margin is ~2.5 with the CP on the rear nozzle section. The expected altitude is 218' and the optimal delay is almost 3. With a D12-3, the altitude is 447' and the optimal delay is essentially the same. This still doesn't compute to me. Maybe it is because it is moving faster and RockSim thinks the drag is higher?

Since this version behaves better, my confidence is a bit higher.  I may go back to the C11 just to keep the altitude down on my semi-local field.

More finless thoughts

I entered the finless Apollo Command Service Module and actually deluded myself that there was some value other than a pre-build visual description. After my previous post, I realized there is another basic flaw with my model. That is, the outer tube is modeled as a ring fin and rocksim treats it accordingly. However, with a closed bottom and the RAIS (Ram Air Intake Stabilization) effects, it is not clear if that it acts as a ring fin. RAIS is even closer to witchcraft than Dean Black's induction stabilization (aka GDS-Gas Dynamic Stabilization). While KenECoyote has had some success, there is no rules of thumb, much less equations, to relate the size of the rocket to the geometry of the RAIS intakes and 'nozzles'.

To summarize a couple of other issues with the RockSim model:

  • The effect of the bottom nozzle on base drag is unclear.
  • The sims are not giving intuitive results.
The upshot is my mindsim has totally melted down. My inclination is to find an open area and try it. There is not much point to add fins and create just another 24mm model. I also think I should go for the D12 v. the C11. Both RAIS and GDS need airflow, the more the better.

Apollo Command/Service Module 99% done

Well, all that's left is paint touch ups, add a shock cord and 'chute, and hit it with a dull coat to protect the cardstock. The nozzle should be darker than shown below and I have already repainted it.


I updated the model RockSim model with accurate dimensions, actual mass and CG values, and the nozzle. It still has a phantom cone to simulate base drag. Final flight specs include, with a C11-3 loaded:
  • Margin of 1.16 (note this used the 'phantom cone' tweak to account for base drag).
  • Stable off a 42" rod.
  • 87' altitude.
  • Optimal delay of 1.42s.
If I move up to a D12-3:
  • Margin of 1.1.
  • Stable at 37".
  • 150' altitude
  • Optimal delay of 1.25s.
I don't have confidence in the RockSim results (not that I really expected to). Most obviously from these sim results, it doesn't seem like the optimum delay should be lower on a D12 than on a C11. Next, I'm not sure how the large nozzle will affect base drag. To further confuse matters, I also found that, if I remove the 'phantom cone', the margin with a D12 is still 0.82. After thinking about said funkiness, Psuccess is about 25%.