Open rocket help

I'm designing a 18 mm MD rocket and open rocket says that it should reach an altitude of 2268 m (7440 ft) with an aerotech D2.3. Of course, this is with a polished finish and airfoiled fins, but I still think the altitude sims are way off. With a ~2 inch nose cone, OR predicted an altitude of 1911 m and a Cd of 0.25, but when I shortened the nose cone to 4 mm and changed the shape to a 0.5 parabolic, it increased the altitude to 2200 meters and raised the stability by 0.4 calibers. Also, when I checked the Cd, it turned out that it was only 0.2! I thought that something was wrong, so I started playing around with the nose cone shape and when I changed it to conical, the altitude dropped to 952 m. I think that something is wrong. Any ideas?


BTW, my rocket has a stability of 0.6 cal. Should it be ok with that level of stability?

Post the file please.

Oops. :blush: Here you go.

View attachment 275604

Hello?

View attachment 276386

Hello! I tried opening both files and they are all screwed up.
And by the way, you want a stability of at least 1.0

Sorry. I finally got to downloading the file

First things first. Your fins are not even attached to your body tube. OR currently does not support fins on the boat tail.

Secondly, OR does not really like sharp pointy fins like you have.

A back of the hand sim puts you at about 2000 feet. You may want to consider adding some length to your rocket. It will make it go much higher.


Attached is an example of a 18mm MD rocket that does 4K feet on a D2.3 Try a design more like that.


As always, feel free to ask more questions

Matt

View attachment D altituide rocket (3) 2.ork

This is what I see when I opened the file:

blackbrandt said:

Sorry. I finally got to downloading the file

First things first. Your fins are not even attached to your body tube. OR currently does not support fins on the boat tail.

Secondly, OR does not really like sharp pointy fins like you have.

A back of the hand sim puts you at about 2000 feet. You may want to consider adding some length to your rocket. It will make it go much higher.


Attached is an example of a 18mm MD rocket that does 4K feet on a D2.3 Try a design more like that.


As always, feel free to ask more questions

Matt

Click to expand...

There are ways of making OR put fins on a boat tail if you use phantom body tubes to match the diameter of the motor tube.

https://www.rocketryforum.com/showthread.php?62884-K-Tesh-s-OpenRocket-Tutorials has the creation of a phantom body tube...

https://www.rocketryforum.com/showthread.php?123564-K-Tesh-s-Open-Rocket-files&p=1433811#post1433811 has a file for the Velociraptor that you can look at and see how I did it.

Ok, I have another question. The aft diameter of my boat tail is 4mm, and I was wondering if that would be large enough for the flame to pass through and not touch the boat tail.

BTW, I attached a video of the D2.3 and the flame looks really small.

[video=youtube;Dt4p-drdXuQ]https://www.youtube.com/watch?v=Dt4p-drdXuQ[/video]

blackbrandt said:

Sorry. I finally got to downloading the file

First things first. Your fins are not even attached to your body tube. OR currently does not support fins on the boat tail.

Secondly, OR does not really like sharp pointy fins like you have.

A back of the hand sim puts you at about 2000 feet. You may want to consider adding some length to your rocket. It will make it go much higher.


Attached is an example of a 18mm MD rocket that does 4K feet on a D2.3 Try a design more like that.


As always, feel free to ask more questions

Matt

Click to expand...

Wait, OR does not like sharp pointy fins? Is there a bug in the software that makes it do that?

Also the new altitude that I got is 11,000 ft.

I've seen the optimization routines in OR drive to those long, skinny fins, too. I didn't believe/trust the result. I figure that kind of fin (or long rod in the logical extreme) is an edge case that just isn't modeled properly.

As for the small exit boat tail - have you figured out a way to mount the motor down there so the nozzle is right by the exit? Otherwise, I think the flame would fill up the space and the tip would act like a new nozzle. But not for long.

Lastly, what are your recovery ideas? I don't think the D2 has an ejection charge.

Charles_McG said:

I've seen the optimization routines in OR drive to those long, skinny fins, too. I didn't believe/trust the result. I figure that kind of fin (or long rod in the logical extreme) is an edge case that just isn't modeled properly.

As for the small exit boat tail - have you figured out a way to mount the motor down there so the nozzle is right by the exit? Otherwise, I think the flame would fill up the space and the tip would act like a new nozzle. But not for long.

Lastly, what are your recovery ideas? I don't think the D2 has an ejection charge.

Click to expand...

1. Removing the long fins would make me lose a ton of altitude, and I would like to keep them, but at the same time I would also like to have accurate results. So if anyone has any info about this fin shape I would like to know, since these fins greatly improve my performance.

2. I haven't seen the nozzle of the D2.3, but I assume that it's the size of the other 18/20 loads, which is really short. (see attached video below) So I don't think that I'll be able to butt the motor up against the edge of boattail. However, the video that I attached in my earlier post shows the flame on the D2.3 to be little smaller than 4 mm (maybe 2-3 mm) but I wasn't sure and wanted to get confirmation from other members that I was OK.


[video=youtube;9-MCleI6pVY]https://www.youtube.com/watch?v=9-MCleI6pVY[/video]



3. The rocket will be recovered by tumbling. I'm a little scared of the fins breaking off, (mostly because of the long, sticklike design) so I might paint the rocket in bright, vibrant orange or red colours and then launch it in the winter so that it lands in the snow and doesn't get damaged.

TRFfan said:

2. The rocket will be recovered by tumbling. I'm a little scared of the fins breaking off, (mostly because of the long, sticklike design) so I might paint the rocket in bright, vibrant orange or red colours and then launch it in the winter so that it lands in the snow and doesn't get damaged.

Click to expand...

What makes you think it's going to tumble if there is no ejection charge?

TRFfan said:

1. I haven't seen the nozzle of the D2.3, but I assume that it's the size of the other 18/20 loads, which is really short. So I don't think that I'll be able to butt the motor up against the edge of boattail. However, the video that I attached in my earlier post shows the flame on the D2.3 to be little smaller than 4 mm (maybe 2-3 mm) but I wasn't sure and wanted to get confirmation from other members that I was OK.


[video=youtube;9-MCleI6pVY]https://www.youtube.com/watch?v=9-MCleI6pVY[/video]



2. The rocket will be recovered by tumbling. I'm a little scared of the fins breaking off, (mostly because of the long, sticklike design) so I might paint the rocket in bright, vibrant orange or red colours and then launch it in the winter so that it lands in the snow and doesn't get damaged.

Click to expand...

With the stability of this rocket being 2.0+ it will NOT tumble, it will fly a ballistic arc aka Lawndart, unless something is done to upset the stability at apogee like MAYBE breaking in the middle. Also the rockets body has zero weight. According to the file I downloaded "d altitude (3).ork"

rharshberger said:

With the stability of this rocket being 2.0+ it will NOT tumble, it will fly a ballistic arc aka Lawndart, unless something is done to upset the stability at apogee like MAYBE breaking in the middle. Also the rockets body has zero weight. According to the file I downloaded "d altitude (3).ork"

Click to expand...

1. In the revised model, the rocket has a stability of 1.41 cal and flies to 11,000 ft, so I'm trying to come up with a solution to have it come in nice and gently but without losing altitude. If anyone has any ideas please advise. I don't have a solution yet and I'm hoping that someone will come up with one.

2. The body tube's diameter is 0 is because it is just a flying case with a nose cone and a boat tail fitted over the motor case. All the details are in the file and I will post it once I get to my computer (I'm typing on my Ipad now)

3. I was thinking to loosely fit the nose cone onto the motor casing in flight and when apogee occurs, have the rocket turn over so that the nose cone slides off with a 6" parachute in it. Of course, the airstream could cause some problems with the loosely fitted nose cone so I wanted to know if anyone has any feedback on this.

BTW, I don't think that this should be in the rocketry electronics and software section. Anyone think I should move it to the LPR section?

Edit: I've edited my post above (https://www.rocketryforum.com/showthread.php?129860-Open-rocket-help&p=1536161#post1536161) so it has a few extra details in it.

timbucktoo said:

What makes you think it's going to tumble if there is no ejection charge?

Click to expand...

See post #16.

Y'know, I was just thinking about this project and how cool it would be if it got off the ground and really flew. Now that would be a real sight.

TRFfan said:

See post #16.

Click to expand...

As its been stated, the motor you chose had no ejection charge so how do you propose the NC separation?

TRFfan said:

........3. I was thinking to loosely fit the nose cone onto the motor casing in flight and when apogee occurs, have the rocket turn over so that the nose cone slides off with a 6" parachute in it. Of course, the airstream could cause some problems with the loosely fitted nose cone so I wanted to know if anyone has any feedback on this..........

Click to expand...

Post #16, 3.

TRFfan said:

1. In the revised model, the rocket has a stability of 1.41 cal and flies to 11,000 ft, so I'm trying to come up with a solution to have it come in nice and gently but without losing altitude. If anyone has any ideas please advise. I don't have a solution yet and I'm hoping that someone will come up with one.

2. The body tube's diameter is 0 is because it is just a flying case with a nose cone and a boat tail fitted over the motor case. All the details are in the file and I will post it once I get to my computer (I'm typing on my Ipad now)

3. I was thinking to loosely fit the nose cone onto the motor casing in flight and when apogee occurs, have the rocket turn over so that the nose cone slides off with a 6" parachute in it. Of course, the airstream could cause some problems with the loosely fitted nose cone so I wanted to know if anyone has any feedback on this.

BTW, I don't think that this should be in the rocketry electronics and software section. Anyone think I should move it to the LPR section?

Edit: I've edited my post above (https://www.rocketryforum.com/showthread.php?129860-Open-rocket-help&p=1536161#post1536161) so it has a few extra details in it.

Click to expand...

A loosely fitted NC will still be held on by air pressure alone, my opinion is that the current configuration is a rocket powered lawn dart, and you will probably get more altitude by actually using a section of airframe and recovery to achieve optimum weight not to mention that without an altimeter there is no way to get a accurate altitude.

The thing is, motor weight is optimum weight, so to get the highest altitude I have to design as light as possible.

TRFfan said:

The thing is, motor weight is optimum weight, so to get the highest altitude I have to design as light as possible.

Click to expand...

I don't know what to say then. I highly suspect that due to the ballistic arc the rocket will follow the airspeed and pressure against the nose cone will not allow it to simply slide off with out some assistance, and with the design you are simming only a custom delay would be long enough to activate a ejection charge at max simulated altitude to blow the nose cone off, which means a EX motor of some sort, even if the nose cone was blown off it may not produce enough drag to make the rocket unstable enough to tumble.

Ok here is the OR file.

View attachment 279266

And here is another, more simple model that I've been working on.

View attachment 279267

rharshberger said:

A loosely fitted NC will still be held on by air pressure alone, my opinion is that the current configuration is a rocket powered lawn dart, and you will probably get more altitude by actually using a section of airframe and recovery to achieve optimum weight not to mention that without an altimeter there is no way to get a accurate altitude.

Click to expand...

The altimeter I'm planning to use is the Micro peak altimeter (https://www.apogeerockets.com/Elect...imeter?zenid=5b8abbb73e42c5dbffcbd39dbe95f12b) and it only weighs 1.9 grams.

TRFfan said:

The altimeter I'm planning to use is the Micro peak altimeter (https://www.apogeerockets.com/Elect...imeter?zenid=5b8abbb73e42c5dbffcbd39dbe95f12b) and it only weighs 1.9 grams.

Click to expand...

For some reason I always forget about that little bugger.

Does anyone here think that this project will work? I hope it will.

Not safely, no.

The rocket isn't in the final design phrase yet, and I'm still working on the recovery system.