open rocket - altitude simulation problems - obviously incorrect altitude.

[Kingston57980]

I am currently using open rocket to designe a rocket for a project, however the simulation says that the apogee will only be 1.13m?
i am new to this o it is probly some thing really simple that i have overlooked.
any help would be greatly apreacheted.
Hear is some of the specs of the rocket:
outer diameter - 6cm
nose cone shape - ogive
body tube length - 40cm
fin type - trapezoidal
fin hight - 6cm
motor type - Estes B4-4

Thanks!!

 

[CalebJ]

Post the simulation file when you get a chance.

 

[dhbarr]

That's likely going to be overall mass, weight in the backend ( fin material ), near-instantaneous chute deployment, or possibly a launch rod that's too short by an order of magnitude. Attaching the sim file will narrow it down pretty fast.

 

[Kingston57980]

Hi all, thank you for the responses.
I have attached the simulation files.
the fin material is carboard - I had the same idea! (overall mass is 436g with B4-4 motor) - I would change this to plywood or balsa wood for the final design.
I have also checked chute deployment as well, however, as far as i can tell the deployment will be after the ejection charge (4sec after burnout).
I have also previously checked the launch rod length and it was at 100cm (plenty longer than the rocket)
I have also tried a higher power motor in there (I even tried a H class motor) but that still only got to a apogee of 800m.
I have also tried installing the 22.02 beta version of open rocket to see if that would fix the issue, but with a very similar rocket it did not.
Thank You!

 

[waltr]

Got 3.45 ft with a B4.
Rocket weight of 438 grams is simply just too much for a B, C or even a D motor.
Try an E30 and it goes to 622ft with 20m/s off the rod.

Another point is you had the Ignition at: Launch + 6 seconds. in the Inner Tuber Motor tab.
This is not needed. Just set it to Automatic. The ignition delay feature is useful when simulating multiple stages and/or delay ignition in clusters but not for a single motor 1-stage rocket.

 

[CalebJ]

Check your body tube numbers. The wall thickness is causing OR to estimate its weight very, very high.

I tried a few quick changes:
Body tube from the original size to BT70. Close outside diameter, much thinner walls. 22g vs 191g.
Nose cone to balsa would drop it from 146g to 17g.
Fins from .118in PLA to .094 (3/32") plywood - 61g down to 22g (after adjusting the fin tabs).
Centering rings to 3/32" plywood cuts their weight in half.

Might have made a couple of other small changes... But the end result is a similar looking design at less than a quarter of the weight. With 25g of nose weight it should fly reasonably well on a C5-3.

 

[thzero]

Something like that with common parts that can be obtained from rocket vendors. Looks good on C11-E12

 

[Kingston57980]

Hi all!
Thank You so much for all of you suggestions!
This is my designe so far, i have tried to incorporate as many of your suggestions as I can.
If you have any more suggestions I will not be ordering the parts for another few days so anything else would be greatly apricated!
(Yes - i know there is a clipping issue with the nose cone - i have been assured by the website that they will fit - NC-60a and BC-60)
Thanks!
Edit: correct file this time!

 

[CalebJ]

Much better.
Quick thought... you might consider using a 24mm motor mount. You can always adapt down to 18mm, but it gives you a lot of flexibility.

The fins need to be positioned and/or the fin tabs recalculated.

 

[thzero]

Why are the fins so far forward, your stability is questionable at best. Also the centering rings and the fins and the fin tabs don't all line up

I'd adjust the fins so that they are like -.25 from bottom of the tube, same with bottom centering ring, top centering ring is -.3.54 (so basically flush with the to of the motor tube), then make motor tube flush with bottom of the tube, and redo the fin tabs. Btw, no idea why the fin rotation is set to 25 degrees. also you need to use a bigger motor tube if you are going to fly 24mm as opposed to 18mm motors. Fins are too thick and too heavy. Use 3/32 bass wood or aircraft plywood, not quite as light as balsa but bit stirdier, you can even paper them for a bit more strength

Both motors you have are aerotech reloadables, so you need to have the casings, which means you need a motor retainer. That adds more weight, and you would need to adjust the rear centering ring upwards and the motor tube down a bit to make room for it. Added a Estes 24mm retainer and moved the motor mount back .25 from body tube.

 

[CalebJ]

Agree with everything thzero said. For what it's worth, here's the version I was tinkering with yesterday.

 

[Kingston57980]

Thank You!
After reading what you've said about re-loadable motors -
I think I have decided to go with a single-use Kilma D9-7 motor - because they are single use, light and most importantly, cheap
there are also a bit more powerful than other D class motors that I have looked at and will fit in a 18mm inner tube. (any links to inner tubes would be appreciated!)
Iv manged to find some 1.5mm(1/16in) basswood, however, I'm worried this might be too flexible?
I am also wanting to put a twist/angle on the fins later to test gyroscopic stability - would this still be alright for that.
Also is the motor just held in by a friction fit or is it adhered in with something? And how do you adhere the shock cord please?
- After tinkering for a little bit this is what I have got ↓
Thanks!

 

[neil_w]

Have you ever built a rocket before? It is good to have a bit of experience building kits to become acquainted with standard techniques before designing your own.

 

[CalebJ]

Better and better.

1/16 would make me nervous, but others here can certainly give you better feedback on that choice. Be sure to hit the 'calculate automatically' button on those whenever you make a change that could affect them.

Parachute may need to be a bit bigger to protect the fins on landing.

 

[Kingston57980]

Hi @neil_w
no this is my fist time - if you could point me in the direction of any techniques that would be brilliant!
@CalebJ ok, if they break I'm not massively bothered - not planning on doing many flights before moving to a tri-motor rocket- but I will get a bigger chute.
Planning on just using this rocket as a benchmark and to get my baring's before moving on to a tri-motor design.
Thanks!
Edit: iv made the fins smaller to lower the stability as it was at 3cal, now at 1.73.

 

[neil_w]

Hi @neil_w
no this is my fist time - if you could point me in the direction of any techniques that would be brilliant!

Best way to learn is to build a couple of standard kits.

 

[dhbarr]

Thank You!
After reading what you've said about re-loadable motors -
I think I have decided to go with a single-use Kilma D9-7 motor - because they are single use, light and most importantly, cheap
there are also a bit more powerful than other D class motors that I have looked at and will fit in a 18mm inner tube. (any links to inner tubes would be appreciated!)
Iv manged to find some 1.5mm(1/16in) basswood, however, I'm worried this might be too flexible?
I am also wanting to put a twist/angle on the fins later to test gyroscopic stability - would this still be alright for that.
Also is the motor just held in by a friction fit or is it adhered in with something? And how do you adhere the shock cord please?
- After tinkering for a little bit this is what I have got ↓
Thanks!

Klima is available where you live?