F-Motor Record???

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Re: F-Motor Record???

Postby jhurst7 » Sat Sep 24, 2016 2:17 pm

How much would a standard ejection charge weigh? Just so I can simulate it.
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Re: F-Motor Record???

Postby SpaceManMat » Sat Sep 24, 2016 2:38 pm

jhurst7 wrote:How much would a standard ejection charge weigh? Just so I can simulate it.


The exact size will depend on the volume of air you have to presurise. Somewhere around a gram in your case.
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Re: F-Motor Record???

Postby jhurst7 » Sat Sep 24, 2016 3:09 pm

Ok, thanks Matt.

Also, does anyone know if 2μm paint is realistic?
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Re: F-Motor Record???

Postby OverTheTop » Sat Sep 24, 2016 3:15 pm

There is a rule of thumb calc from Modern High Power Rocketry.

BP charge (grams) = diameter of parachute compartment (inches) x diameter of parachute compartment (inches) x length of parachute compartment (inches) x 0.006

This is my starting point for charges normally. You can go up or down on this, depending on your recovery configuration.

Ground test to make sure. The backup charge is always larger. A parachute is of no use to you in the airframe. "Blow it out or blow it up!"
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Re: F-Motor Record???

Postby Viking » Sat Sep 24, 2016 3:29 pm

2u would be near impossible. That's 0.002 of a mm. Close to perfect. A glass like Polished or mylar finish on CF or FG would go close.
A good paint job would fit OR's 60u option, a very good one 20u.
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Re: F-Motor Record???

Postby jhurst7 » Fri Sep 30, 2016 11:04 am

Ok thanks for that,

I've changed the design a bit as the F10 wasn't available, now I am working on a 24mm design using the CTI 73-F30.
It will be an electronic deploy with a 15cm parachute. With no wind at all the apogee is 1762m and with some pretty heavy winds of 25km/h and standard deviation of 10km/h the apogee varies from 1730m to 1750m. With that heavy wind, the drift is around 400m. The altimeter I am thinking of using is the Stratologger. The maximum acceleration is 25gs.
I have a few more questions:

1.What is a good body tube thickness for something this small?
2.What is the g-force rating on the Stratologger? 25gs doesn't seem like too much, but I just want to check.
3.What are some trackers that I can use to track my rocket? I'm not sure if it is required, the rocket comes down pretty quickly 14m/s and only drifts 400m, would I actually need the trackers?
4. Is a ground hit velocity of 14m/s too much?
5. I've used a simple, swept rectangle, an Apogee rockets website said that this was a good shape to use for subsonic flight, does this seem good.
6. Should I reduce the sweep if the rocket is coming down so fast so that the fins don't break off?

That's all for now.
Thanks

-James
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Re: F-Motor Record???

Postby Jordz » Fri Sep 30, 2016 2:15 pm

jhurst7 wrote:
1.What is a good body tube thickness for something this small?
2.What is the g-force rating on the Stratologger? 25gs doesn't seem like too much, but I just want to check.
3.What are some trackers that I can use to track my rocket? I'm not sure if it is required, the rocket comes down pretty quickly 14m/s and only drifts 400m, would I actually need the trackers?
4. Is a ground hit velocity of 14m/s too much?
5. I've used a simple, swept rectangle, an Apogee rockets website said that this was a good shape to use for subsonic flight, does this seem good.
6. Should I reduce the sweep if the rocket is coming down so fast so that the fins don't break off?

-James


1) Depends on the material and the acceleration. If its a long slow burn cardboard could be fine, a high specific impulse burn may require a lot stronger material. Wall thickness is entirely dependant on how much strength you are willing to sacrifice for weight loss.

4) Putting safety aside for a second, this again comes down to question 1. A thick walled rocked will survive 14m/s easily, a thinner walled tube may not survive.

5) Reducing sweep will increase drag, sacrificing total height. Id suggest that a small increase in weight (for fin strength) would be a better sacrifice than reducing sweep.

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Re: F-Motor Record???

Postby jhurst7 » Fri Sep 30, 2016 8:40 pm

Jordz wrote:
1) Depends on the material and the acceleration. If its a long slow burn cardboard could be fine, a high specific impulse burn may require a lot stronger material. Wall thickness is entirely dependant on how much strength you are willing to sacrifice for weight loss.

4) Putting safety aside for a second, this again comes down to question 1. A thick walled rocked will survive 14m/s easily, a thinner walled tube may not survive.

5) Reducing sweep will increase drag, sacrificing total height. Id suggest that a small increase in weight (for fin strength) would be a better sacrifice than reducing sweep.


1) I'm thinking of using fibreglass and making the components myself, except maybe the tailbone retainer but I'm still working that part out. Maximum acceleration is 25gees, what kind of thickness would be required on that?
4) I might make the parachute a bit bigger as long as I have a good tracker, which I'm still open to suggestions on.

Thanks
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Re: F-Motor Record???

Postby Happy Heyoka » Sat Oct 01, 2016 12:23 am

jhurst7 wrote:Ok thanks for that,
1.What is a good body tube thickness for something this small?


you should check out the FAI competition rockets - they're incredibly light weight and use some pretty interesting construction techniques. I think the competition classes specify a particular motor so they design exactly to those specs.

One of the rocket mags had an article that I can't seem to lay hands on.
https://www.google.com.au/search?q=FAI+competition+rockets
http://www.ibiblio.org/pub/archives/rec.models.rockets/ARTICLES/vellum_tube_talk.txt

they mostly use smaller motors than you're talking about but you might get some good ideas.
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Re: F-Motor Record???

Postby martymonsta » Sat Oct 01, 2016 10:03 am

Just a couple of general things to consider.

I'm taking a educated guess that you are simulating using a 6" parachute with the Cd set to 0.8? Are you sure that the Cd is 0.8 because it if it isn't your decent rates could be way off (faster in the sim then in reality).

Open rocket also has one major flaw. It simulates wind speed the same regardless of altitude, when I reality this is not the case. For example today at the Cedar Grove launch site [url]windyty.com[/url] forecasts wind on the surface to be 3.6m/s (13km/h), @330ft AGL 5.2m/s (19km/h), @2000ft AGL 6.4m/s (23km/h) and @5000ft AGL 15m/s (54km/h). Thus with this kind of information your rocket is going to be traveling sideways faster then it is coming down during the first part of it's decent and I calculate that you could have a LZ 1.4km from apogee but that will vary by the hour.

As for tracking, are you talking about GPS? Remember that if you were to go with the chute option for recovery you are trying to visually track something the size of a 30cm ruler next to DVD at 1.7km :shock: . If you go with a streamer you could end up tracking something the size of a 20c piece.

Just thought that these were some thing that were worth considering.
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Re: F-Motor Record???

Postby jhurst7 » Mon Oct 03, 2016 4:38 pm

martymonsta wrote:Just a couple of general things to consider.

I'm taking a educated guess that you are simulating using a 6" parachute with the Cd set to 0.8? Are you sure that the Cd is 0.8 because it if it isn't your decent rates could be way off (faster in the sim then in reality).

Open rocket also has one major flaw. It simulates wind speed the same regardless of altitude, when I reality this is not the case. For example today at the Cedar Grove launch site [url]windyty.com[/url] forecasts wind on the surface to be 3.6m/s (13km/h), @330ft AGL 5.2m/s (19km/h), @2000ft AGL 6.4m/s (23km/h) and @5000ft AGL 15m/s (54km/h). Thus with this kind of information your rocket is going to be traveling sideways faster then it is coming down during the first part of it's decent and I calculate that you could have a LZ 1.4km from apogee but that will vary by the hour.

As for tracking, are you talking about GPS? Remember that if you were to go with the chute option for recovery you are trying to visually track something the size of a 30cm ruler next to DVD at 1.7km :shock: . If you go with a streamer you could end up tracking something the size of a 20c piece.

Just thought that these were some thing that were worth considering.


Yes, this was the situation. If it is 1.4km from launch/apogee (hopefully around the same area), then I do need electronic tracking, and I'm hoping not just a beeper, although if it gets the rocket back I'll use it.

If I get a good tracker, I'll put in a bigger parachute, just need to find one now.

Happy Heyoka wrote:you should check out the FAI competition rockets - they're incredibly light weight and use some pretty interesting construction techniques. I think the competition classes specify a particular motor so they design exactly to those specs.


Thanks for that, some good reading.

I think I've decided to go with a 1mm thick wall on both sides (OD 26mm, ID 24mm).


If anyone has any information on the G-Force rating for the Stratologger, that would be great.
Also, after reading through Lister's 24mm thread, I'm a bit confused as to why he and the person that broke the G-motor record used the G150 instead of the G65. There was something mentioned about the offset mass of the moonburner. I was just clarifying what that meant.
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Re: F-Motor Record???

Postby Viking » Mon Oct 03, 2016 5:44 pm

jhurst7 wrote:...
Also, after reading through Lister's 24mm thread, I'm a bit confused as to why he and the person that broke the G-motor record used the G150 instead of the G65. There was something mentioned about the offset mass of the moonburner. I was just clarifying what that meant.


The moonburner has an offset core which looks like the attached image (Borrowed from: http://www.rimworld.com/nassarocketry/p ... BASICS.pdf)

That means the propellant mass is offset from the axis of the rocket.
I have an unsubstantiated theory that a moonburner also has offset thrust through the nozzle due to the offset core, which could be more of an issue than the offset mass. Someone else here more knowledgeable than I can probably confirm/deny that.
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Re: F-Motor Record???

Postby SpaceManMat » Mon Oct 03, 2016 6:32 pm

As to which motor is best, really what you want to do is to produced optimismed simulations of all the motors you are interested to see what produces the highest altitude.

Usually what you are looking for is the highest impulse F motor. But there are always exceptions, such as can you fly a reasonable size F motor with a smaller diameter case. Or can you sacrifice a few Ns for a longer burning motor. Or swap to a lighter motor might pay off.

Best way to simulate a buch of motors and optimise the model for each.
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Re: F-Motor Record???

Postby OverTheTop » Mon Oct 03, 2016 6:41 pm

I have an unsubstantiated theory that a moonburner also has offset thrust through the nozzle due to the offset core, which could be more of an issue than the offset mass. Someone else here more knowledgeable than I can probably confirm/deny that.


I was just pondering such a thing yesterday with an M840... :)

The offset core, in the case of the M840, burns into a short "combustion chamber" between the end of the grains and the convergent part of the nozzle. That short length of no grain allows the pressure across the throat to be more even, and the mass-flow and thrust coaxial with the motor.

I was pondering the effect of gluing the M840 with the grains further aft. I think it would have the possibility to skew the position of choked flow in the throat slightly, causing the mass-flow to be not inline with the motor axis.

The effect would probably be short-lived, improving as the core opens up. I don't have a feel for how much sideways kick the thrust would actually cause (depends on how tightly the sonic portion of the flow is tied to the throat), particularly as it exits the rail.
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Re: F-Motor Record???

Postby jhurst7 » Fri Oct 21, 2016 7:59 pm

That's some great food for thought there.

With some changes, adding in weight for a tracker, my apogee is 1700 on a very, very windy day.
The drift is around 850m on that very windy day, however, it could probably be more as the winds will be higher at a higher altitude, so the drift could be as much as 1.3km.

The beeper I am thinking of using is the Pratt Hobbies Microbeacon. https://www.apogeerockets.com/Electroni ... icroBeacon

I am going to start body tube construction tomorrow, using some Diggers 1.0m² Woven Sheet Fibreglass Cloth.

So, a few more questions:

1. Has anyone used Diggers Fibreglass Cloth before? Would they recommend it?
2. Has anyone used the Microbeacon before? I saw it on Lister's 24mm MD, so does anyone who did anything over there know how well it would be able to be heard from a distance?

That's all for now.
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