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  #1  
Old 09-26-2021, 04:25 PM
rraeford rraeford is offline
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Default Semroc Hydra VII Question

I got one of these some time ago and pulled it out to build. I notice on the Semroc site, it describes this kit as “improved.” Does anyone know how it was improved? It doesn’t say in the kit description. I know in other kits like the Orbitsl Transport, they made the fins interlock to aid in construction. This kit is quite a bit of work and I wonder if it might be worth getting the new fin set for it (if there is one). Thanks.
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  #2  
Old 09-26-2021, 09:06 PM
Andy63 Andy63 is offline
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Quote:
Originally Posted by rraeford
I got one of these some time ago and pulled it out to build. I notice on the Semroc site, it describes this kit as “improved.” Does anyone know how it was improved? It doesn’t say in the kit description. I know in other kits like the Orbitsl Transport, they made the fins interlock to aid in construction. This kit is quite a bit of work and I wonder if it might be worth getting the new fin set for it (if there is one). Thanks.


I have the PDF instructions from the original SEMROC web site. You can get the instructions by clicking on "instructions" for the Hydra VII.

But here is what I can gather on the changes:
- There are only two chutes in the improved kit. They are CP12/24 rather three 12" chutes. Looks like you cut one down to 12" and the other to 18"
- Decals in the improved kit are listed as DKV-29A & B whereas the original is DKV-29
- Nose weight is included with the improved kit
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  #3  
Old 10-02-2021, 08:56 AM
PaulK PaulK is offline
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The original definitely needed nose weight if flying on 7x C6-7. Not sure I'd agree the new decals are improved, but I painted in the original Red/White/Blue scheme, so no decals on the fins anyhow.

Some recommendations, based on the original instructions
1) Build with 3 motor tubes open, not just 1. This provides a little redundancy in case the center motor doesn't light.
2) Strengthen and seal the joint where the 6 lower tubes meet the upper tube, I simply put fillets of epoxy in the valleys after building, before finishing. This has held up to at least 20 launches of 7 motors (I lost count).
3) Use a single larger parachute, with a sufficiently long shock cord. IIRC, it used to call for the sections to come down separately? Chasing 1 section is much easier!
4) Build and finish the bottom section without ring and outer fins, keeping gluing surfaces free from paint.
5) Add a baffle (or some other method) to attach the shock cord higher up. The original method for attaching the shock cord to the central motor tube will only last a couple flights. I used the SEM-EB-16 from Semroc.

I've seen folks leave all 7 tubes open, but that puts too much force on the middle joint, and will cause damage.

I always fly with 3 central B6-4 or C6-7 motors, and 4 outer plugged B6/C6 motors, by adding an epoxy cap to the booster motors, so they don't kick out. I don't mind kicking motors, but 4 is too many at one time for me.

Here's the response from Carl (March 2006) when I asked about leaving more tubes open and getting a good seal by the nose cone halves:

Paul,

A lot of this is personal preference. I always leave three open, central and
one on either side. Many people leave all seven open and use delay type
engines in all seven. Lots of noise at the top, but the parachutes are
assuredly out!

If the booster engines are a tight fit, they do not eject, but "vent" back
through the nozzle. From my experience, the burnoff of the delay charge is
far more damaging than the ejection charge.

I always use extra yellow glue on the nose cones and upper tubes to help
with hot gasses, but ejecting the engines before they can do damage is still
the best on the rocket if it is permissable in your area to eject the
engines.

Thanks for your interest and support!

Regards,
Carl McLawhorn
NAR#4717 L2
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Last edited by PaulK : 10-02-2021 at 09:16 AM.
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  #4  
Old 10-02-2021, 12:40 PM
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ghrocketman ghrocketman is offline
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Eject them and forget about them. They are bio-degradable cardboard.
Enuff said.
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