L2 build

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SpaceManMat
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Re: L2 build

Postby SpaceManMat » Tue May 24, 2016 10:34 pm

Well the K1440 is out, but even that L2 flight is questionable at 1.35. I'll leave it to someone more experienced than myself to give their thoughts on that. As you can clearly see the approach to M2 has a dramatic effect on CP. Even though your CG is moving forward as fuel is burnt it's still being rapidly over taken by CP.
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cryoscum
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Re: L2 build

Postby cryoscum » Wed May 25, 2016 8:39 am

SpaceManMat wrote:Well the K1440 is out, but even that L2 flight is questionable at 1.35. I'll leave it to someone more experienced than myself to give their thoughts on that. As you can clearly see the approach to M2 has a dramatic effect on CP. Even though your CG is moving forward as fuel is burnt it's still being rapidly over taken by CP.


Mat is spot-on with this, though the problem is easily fixed with a bit of extra weight in the nose. This adds stability margin, but also reduces the max velocity, both of which will help to alleviate the problem.
I tend to finalise each build by doing the sim again, double checking all air frame dimensions and overriding the CG and mass with actual measured numbers. It is truly the only way to be sure what's going to happen. I also then try to keep the stability margin at no less than 1.25 at Max V and this would mean a margin on the pad of maybe 2.5 or 3, depending on the rocket/fin design etc.
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camacd
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Re: L2 build

Postby camacd » Wed May 25, 2016 11:07 am

Thanks for that advice guys.

I have not made the fins yet, so its not too late to make a significant change to the design to get the stability back. Up until now I have not been aware that the stability is so profoundly affected by the velocity.

I'll do some more iterations on the design and hopefully get it to work at the higher velocities.
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Re: L2 build

Postby camacd » Wed May 25, 2016 11:18 am

SO I have modified the fins to increase stability. They are quite a bit bigger than the original design - almost twice the surface area.

Now the stability curve for the K1440 is showing minimum stability on the pad, then increasing before dropping back to approx cal 3.0. Perhaps I've gone a little too far. I am still acieving excess of M2, so goal still in sight!

For all the other motors, the stability increases right through the burn, so its only the K1440 that gives the issues.
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K1440 stability rev B.JPG
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Re: L2 build

Postby SpaceManMat » Wed May 25, 2016 1:32 pm

Yeah you don't want to over do it either. An overstable rocket will overreact to disturbances. Given the large range of your stability margin you will want to set the minimum stability at no more than 1.5. Like Nic says down to 1.25 is ok, but you really want to make sure your simulation is correct. The other issue you can have with oversized fins is they may be more susceptible to fin flutter and that will cause fin failure pretty quickly at m2.
QRS: 124
AMRS: 32 L2 RSO
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Re: L2 build

Postby camacd » Wed May 25, 2016 2:25 pm

Are there any rules of thumb regarding the best shapes for fins to achieve stability and minimise risk of flutter? My instinct says to lengthen the base cord and shorten the height, but the sim often reduces stability when I increase the base. And also the height is clearly the most dominant dimension so reducing it by 5mm drastically reduces the stability. I start with a shape that "looks right", then I tweak it to try to optimise stability against drag but have no science behind it.
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Re: L2 build

Postby Lister » Wed May 25, 2016 3:24 pm

cryoscum wrote:Mat is spot-on with this, though the problem is easily fixed with a bit of extra weight in the nose. This adds stability margin, but also reduces the max velocity, both of which will help to alleviate the problem.

Wouldn't this be an easier fix?

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Re: L2 build

Postby OverTheTop » Wed May 25, 2016 3:36 pm

Are there any rules of thumb regarding the best shapes for fins to achieve stability and minimise risk of flutter?

You typically need the exact opposite of wide and skinny for them not to flutter.

Here is one I have got some good speed out of. Check out the shape of the fins. They are a good indication of what is needed IMHO:
viewtopic.php?f=6&t=4758

Having said all that, if you get the construction right to get any resonances out of the fin, and make it strong enough, you could use just about any shape you like :wink:

Another option is to increase the number of fins, and make them smaller. There is usually a drag penalty to be paid for this due to the increased frontal area.

The faster you go the more sweep back you need on the leading edge to get the "critical mach number" up. That keeps the losses down that are due to the shock wave generated by the leading edge. The effective curvature (about the thickness) of the fin is reduced on a swept wing, as it only sees the air traveling at a velocity of the freestream velocity times cosine(sweep angle) in a direction normal to the leading edge.

See here for how to calculate what sweep angle you might use:
https://en.wikipedia.org/wiki/Swept_win ... c_behavior


Post subject: Re: L2 build
cryoscum wrote:
Mat is spot-on with this, though the problem is easily fixed with a bit of extra weight in the nose. This adds stability margin, but also reduces the max velocity, both of which will help to alleviate the problem.

Wouldn't this be an easier fix?

Depends on what the flyer is trying to achieve with the design.
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camacd
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Re: L2 build

Postby camacd » Wed May 25, 2016 6:19 pm

Just for clarity, my objectives for this design are twofold

1- get my level 2.

2 - go as fast as i possibly can with a basic goal of Mach 2.

The first objective should not be too big of an issue (touch wood), if I do my homework and keep everything well documented, ad build carefully.

The second is a very big step from my current highest speed of mach 0.75 or so. However, in designing this I have simulated a number of motors so I can use the same rocket to gradually build up speed from about Mach 0.6 for an I motor, up to M2+ for the K1440.

This might seem to be overly ambitious, but I reckon nothing ventured nothing gained. I'm already learning a great deal.
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Re: L2 build

Postby Lister » Wed May 25, 2016 6:28 pm

If you add some nose weight to your sim with smaller fins what does it do? If you add weight it will go a little slower under boost but will maintain speed for longer and you will get more altitude up to a certain point.. every rocket has a different optimum weight but for straight speed keep it as light as possible while still being stable.

if it was my rocket I would use smaller fins and a bit of nose weight.

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OverTheTop
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Re: L2 build

Postby OverTheTop » Wed May 25, 2016 8:37 pm

For max speed you need minimum mass. Design everything around that. If you want to go more conservative along the way maybe make provision for removable noseweights. What do you think?
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Re: L2 build

Postby Lister » Wed May 25, 2016 9:28 pm

Removable nose weight :lol:
Attachments
2016-05-25 21.28.13.jpg
2016-05-25 21.28.13.jpg (208.74 KiB) Viewed 1282 times
2016-05-25 21.26.27.jpg
2016-05-25 21.26.27.jpg (145.57 KiB) Viewed 1282 times

camacd
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Re: L2 build

Postby camacd » Thu May 26, 2016 8:17 pm

SO a couple of iterations later and I think I've hit on a reasonable compromise. 4 fins, with a relatively long base, good sweep back, and then optimised to reduce the height to get the best result.

The stability varies during the flight but is a minimum of about Cal 2 at the high velocity which should be ok.

The lower power motors give more stability but hopefully not too much to cause problems.
Attachments
K1440 stability rev c.JPG
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Re: L2 build

Postby camacd » Mon May 30, 2016 3:27 pm

About time I made some further progress.

I have now prepped the aft airframe section, motor mount and fins.

The fin design has changed as a result of the recent dialog above, where I learned about the effects of high velocity on stability. I have tweaked it to also make the most of the materials at hand. I cut the fins to near final shape, then clamped them together and dressed them down to the final shape as one, so they are all identical. Then I dressed the edges to achieve nice sharp leading and trailing edges.

I cut the fin slots with a cutting wheel on my 5" grinder. It is a very aggressive tool for a job that requires such precision, and I'd prefer not to use it as it has potential to do a lot of damage very fast, but I don't have access to any machine tools. Fortunately the cutting disc is slightly thinner than the fins, so after I finished the grinding, I set to with a very thin bladed file and opened the slots out very carefully to ensure they were nice and straight, and a snug fit for the fins.

The I put the motor mount in place, and checked the depth of the fin tabs to match exactly the inner tube. I had deliberately left the tabs with a slight excess, which I now filed off to get a perfect fit..

So now it is all ready to glue together.
Attachments
IMG_1843 comp.jpg
All components ready for trial assembly.
IMG_1844 comp.jpg
Trial fit. Everything sits snugly in place without any glue. Nice.
IMG_1844 comp.jpg (147.78 KiB) Viewed 1243 times
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Re: L2 build

Postby october sky » Mon May 30, 2016 3:41 pm

Cam, I want to just say that I really like the way how clearly you mark all your items with numbers, arrows and lines.....sounds a bit silly but it is the BEST way to know how and where all goes together ! Well done mate ! 8)
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