Sky Shark 218- 1940s Sci Fi Build Thread for the Buck Rodgers Challenge!

[ljwilley]

Did some simulations on Rocsim and it shows that a flight should be fine with a couple of B6-4s. I have brought the CG forward in Rocsim to make it stable enough to run simulations. The rocket weighs 188 grams without any motors loaded (and with nose weight). I have added the Rocsim file below. Even though it is being problematic and is incomplete, it is good for doing simulations on motors and is mostly good for getting dimensions on tubes, etc.
I probably wont get full credit on the rocksim file, but I'm ok with that. Not trying to do this too competitively.

 

[ljwilley]

Added a coat of grey paint to the nose cones. I decided to leave the pod tailcones silver.

 

[jqavins]

So... I'm having a bit of trouble with Rocsim...

Try contacting Apogee; Tim is pretty good about helping with RocSim difficulties. (I'll take a look too, and I'm sure I won't be alone.)

Just as proof for the successful swing test, I have a video. Sorry about the low light...

The video seems to be broken. It's just five seconds of black blank screen with audio.

 

[ljwilley]

Try contacting Apogee; Tim is pretty good about helping with RocSim difficulties. (I'll take a look too, and I'm sure I won't be alone.)


The video seems to be broken. It's just five seconds of black blank screen with audio.

Thanks. The video works fine for me, not sure what's wrong. It is only 5 seconds, because I usually have more trouble posting videos that are longer than that. I'll try posting it as an unlisted you tube video. It should come up then.

 

[ljwilley]

Just in case the video of the swing test isn't working, here's one that should work.

 

[ljwilley]

Painted the rocket, and glued on the pod noses/tail cones with CA. Launch lugs and recovery equiptment will be next!


It's far from perfect, but think it looks cool!

 

[ljwilley]

Idk if I said this already or not, but I added some engine blocks with wood glue earlier. Anyway, I just finished adding the shock cord and launch lugs with CA. The next thing to do is to launch the rocket!

 

[jqavins]

Painted the rocket, and glued on the pod noses/tail cones with CA. Launch lugs and recovery equiptment will be next!
View attachment 676309

It's far from perfect, but think it looks cool!

Looking good.

 

[ljwilley]

Looking good.

Thanks!

 

[neil_w]

Looks pretty cool, good luck with the flight.

 

[ljwilley]

Looks pretty cool, good luck with the flight.

Thanks!

 

[ljwilley]

Today I attempted to fly it, but had some bad results. Both motors went off, but the rocket went unstable. I'm not quite sure how such a convincing swing test can lead to such a bad flight, but luckily the rocket is fixable. I will try to fly the rocket again once I fix it.

 

[les]

Did you do a swing test in both axis?
You've got nice long fins with the pods in one direction, but fairly stubby fins in the other axis.

 

[ljwilley]

Did you do a swing test in both axis?
You've got nice long fins with the pods in one direction, but fairly stubby fins in the other axis.

I did not. I will do that before the next flight. Thanks!

 

[bad_idea]

I envy those of you who build quickly. I drew up a rough plan for an entry in the challenge, but even if I made it my main focus, it wouldn't be built and flown in anything like two months. I seriously have to improve my build rate.

 

[billpr]

She looks great. like the design. Colors pop out.

 

[pugachu]

I envy those of you who build quickly. I drew up a rough plan for an entry in the challenge, but even if I made it my main focus, it wouldn't be built and flown in anything like two months. I seriously have to improve my build rate.

Im the same way. The only reason why I got so far on mine, is i was off work for a week (too many built up vacation days.. use it or lose it policy)

 

[jqavins]

Today I attempted to fly it, but had some bad results. Both motors went off, but the rocket went unstable. I'm not quite sure how such a convincing swing test can lead to such a bad flight, but luckily the rocket is fixable. I will try to fly the rocket again once I fix it.

I've had the same experience. No difference between the axes of this one. The swing test included lightly brushing the grass to make it tumble and it decisively recovered. And yet is wend skywriting when launched.

 

[jqavins]

Try contacting Apogee; Tim is pretty good about helping with RocSim difficulties. (I'll take a look too, and I'm sure I won't be alone.)

Well, I finally got around to looking at the RockSim file. I think the results are believable. The ventral and dorsal fins are pretty small, and a boat tail always pushes the CP forward.

There used to be a feature in RockSim that plots the static margin as a function of wind angle. In Version 11 I can't find it. I'd be surprised if the feature were totally removed, so I sent a contact form to Apogee to ask about it. The static margin on the main screen is (supposed to be) the minimum value from that computation, and it's 1.39 with a pair of C6 engines loaded, so it ought to be stable, but I'd still like to see that graph.

I also tried simulating 2×C6-3 and 2×C5-3 engines. The altitude with twice the impulse of the B engines is way more than twice that with the B engines, which suggests to me that this rocket is just underpowered with a pair of Bs. And the rod exit speed is a good bunch higher with the C5s than with either the B6s or the C6s (no surprise) and the rod exit speed could explain your unstable performance on that first flight.

If I were in your shoes, I would get some C5-3 engines for the next launch.

Will your repairs include reattaching the dorsal/ventral fins? If so, I'd suggest making slightly bigger ones. In a separate copy of you your file, I increased the fin size by just 10% in each dimension, so 21% bigger in total area, and it made a big difference in the stability margin.

 

[ljwilley]

Well, I finally got around to looking at the RockSim file. I think the results are believable. The ventral and dorsal fins are pretty small, and a boat tail always pushes the CP forward.

There used to be a feature in RockSim that plots the static margin as a function of wind angle. In Version 11 I can't find it. I'd be surprised if the feature were totally removed, so I sent a contact form to Apogee to ask about it. The static margin on the main screen is (supposed to be) the minimum value from that computation, and it's 1.39 with a pair of C6 engines loaded, so it ought to be stable, but I'd still like to see that graph.

I also tried simulating 2×C6-3 and 2×C5-3 engines. The altitude with twice the impulse of the B engines is way more than twice that with the B engines, which suggests to me that this rocket is just underpowered with a pair of Bs. And the rod exit speed is a good bunch higher with the C5s than with either the B6s or the C6s (no surprise) and the rod exit speed could explain your unstable performance on that first flight.

If I were in your shoes, I would get some C5-3 engines for the next launch.

Will your repairs include reattaching the dorsal/ventral fins? If so, I'd suggest making slightly bigger ones. In a separate copy of you your file, I increased the fin size by just 10% in each dimension, so 21% bigger in total area, and it made a big difference in the stability margin.

The CG in the Rocsim file is way up at the top because I put it there to simulate motors. It would take hundreds of grams of nose weight to get it where it is on the Rocsim file. I might just try to make the fins bigger.

 

[jqavins]

Well, that explains why changing the nose weight didn't seem to make any difference.

Still, I think the B6 engines are just not pushing the rocket hard enough. When it went unstable, was that right away?

 

[ljwilley]

It had enough thrust to get off the pad fast enough, but it was VERY clearly unstable. It just did not want to fly straight. Here's a video:

 

[ljwilley]

When I fly it again, it will either be C motors or B6-2s.

 

[neil_w]

What I saw:

1) It pitched over to the side, suggesting indeed it was the verticals (non-wings) that were insufficient
2) I could be wrong but it looked like the wings were flapping like crazy once it started to pitch. I wonder if you have enough rigidity in the wings.

 

[ljwilley]

What I saw:

1) It pitched over to the side, suggesting indeed it was the verticals (non-wings) that were insufficient
2) I could be wrong but it looked like the wings were flapping like crazy once it started to pitch. I wonder if you have enough rigidity in the wings.

I looked at the video on my computer and slowed down the video- they are not flapping around, the whole rocket was spinning out of control and it looked kind of weird. Those fins are plenty strong, they were just fine. But you are definitely right about that first one.

 

[jqavins]

I'll stop harping on it (after this) but I really do think more speed off the rod would help a lot. C5; in thrust we trust.

 

[ljwilley]

Added more fins. I will do more seeing tests and fly it on some B6-2 motors.

 

[ljwilley]

Did 3 swing tests. One with the main fin horizontal, one with the new fins horizontal, and one in between. All were stable!

 

[ljwilley]

Successful flight!!!! I launched it on the B6-2s. It was perfectly stable, had plenty of thrust, and the parachute opened nicely. Unfortunately, as I sort of expected, one fin cracked and one broke off when the rocket landed. Luckily, the damage is easily fixable. If I fly it again, it will probably be on some C motors, maybe composite, idk. I know I will lose points for broken fins, but I'm not here to compete anyway, just here to have fun!

I won't have time to fly it again, so this is probably the last big update of my submission. (I will update the Rocsim file before officially considering this submitted). Thanks everyone, and good luck with your projects!

 

[SkyFire]

Cool rocket! Congrats on a successful flight!
Here's my Skyshark;