The budget can wait!

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Rex Fynns
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The budget can wait!

Postby Rex Fynns » Sat Jul 01, 2017 10:56 pm

G’day Gentlemen and Ladies too,

It seems the bug has recently bitten and thank you all for being part of forum that I can hopefully learn from.

A little background, I reside in SA which is probably not the best state to start from given the present Acts and Regs that govern model rocketry here and lack of clubs (?), but on the plus side I have ready access to large flat open salt pans safely away from population. Think Woomera-ish.

I am also fortunate to have several decades of involvement with firearms and hand loading as well as being a career pilot. In short just the right amount of knowledge to be a menace, hopefully my professional sense of applied safety keeps me risk low, I feel it will.

Like most newbies I guess, the usual Estes kits launched and lost and now a burning desire to scratch build a Mach plus rocket and engine in the budget world of MD airframes.

The game is afoot as they say and componentry is arriving almost daily from a variety of suppliers.

I have decided to shy away from computer simulators at this stage for as far and as long as I can as I am finding one of the most gratify aspects is eyeball engineering. It has been, all be it somewhat narcissistically a bit of a buzz to mull over a build or design issue for a few days and narrow down a solution to find with a bit post research (google) it is the accepted solution.

However I still have a hundreds of questions.

Perhaps one quick one here if someone can point me in the right direction, I recently purchased 1.2m of 29mm FRP tube from a notable local rocket hardware supplier however upon weighing it, it came in at 250g/m and looks significantly over built. Does anyone know of a local source of CF 29mm tubing with better g/m weight ratio?

Regards
Rex

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SpaceManMat
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Re: The budget can wait!

Postby SpaceManMat » Sun Jul 02, 2017 2:21 pm

Welcome to the forum Rex,

If you're looking to scratch build a Mach busting 29mm rocket then I would recommend you are better off with a sim, it will certainly increase your chance of success. If not them there are a few things you need to make sure of to give yourself a reasonable chance.

Firstly your fins need to have a span bigger than the tube diameter, this is a general rule to ensure there is enough fin area in clean air. I would also look at not using a delta fins as they have a large percentage of their area close to the body.

Also you are going to want to have to check on your rockets stability. Again general rule of thumb says Center of Gravity should be a minimum of 1 body diameter in front of Center of Presure at all times, which is easy enough to check in a sim. Without it you can make some assumptions and give your self a bit of margin. For a standard rocket 3 or 4 fin rocket The Center of Pressure is usually around the start of the fins. To get Center of Gravity simply balance the rocket on your finger with motor, chute etc all loaded. As long as the Center of Gravity is about 6 cm in front of the fins or more then I think you should be fine. You should however be much more cautionious if you go over Mach 1.5 as Center of Presure can do some interesting things.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Al777
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Re: The budget can wait!

Postby Al777 » Sun Jul 02, 2017 7:26 pm

Hello Rex & Welcome,

I am also new to this game & Forum, I look forward to watching your progress.

Dose your firearms experience "hand loading" extend to old Black powder muskets / Long arms ? or is it the modern center-fire type rifles ?
(WARNING questions may follow if you answer yes to Black powder)

I also Love eyeball engineering , but find simulation & calculation a beneficial clarification.

Welcome & Cheers,
Alex

Rex Fynns
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Re: The budget can wait!

Postby Rex Fynns » Sun Jul 02, 2017 10:06 pm

G'day Matt,

Thanks for the tip re. proportionate fin size, handy rule of thumb to know, I just had a look at the fin size I had eyeballed for a 25mm prototype and it turns out they have a span of 36mm. The 3 fin plan form is a scalene trapezoid with 57 degree LE sweep back and TE sweep of 35 degrees, root chord 105mm and tip chord 20mm. It will be pretty a thick section of 3.2mm carbon/balsa composite sandwich with maximum thickness at around 50% chord with slight tapering thickness toward the tip. Attachment could be challenging at this stage just thinking a sanded to body radius root with microballoon in west system epoxy filleting. This prototype won't be a Mach challenger just a bloody quick stability tester.

I hear what you’re saying re. simulator benefits however the exercise is about testing what I have learnt over the years and using old fashioned text book maths to get me in the paddock so to speak.

G'day Alex,

LOL....sorry centrefire rifles, I'll take a punt....trying to source black powder are we? Anyone with a class B licence can purchase but it is becoming thin on the ground, Pyrodex is the new Godex (I think) and when all else fails it can be made the recipe is straight forward. If you need a commercial source supplier in SA PM me.

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SpaceManMat
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Re: The budget can wait!

Postby SpaceManMat » Mon Jul 03, 2017 9:40 am

If you want to learn the math then Google Barrowman. I suspect NASA may have an online app that will calculate Center of Pressure for you in their educational section. The other rough method people do is to create a card board cutout of the rocket and mount it horizontally on a vertical axel at the Center of Grarity, then test it with a fan. If the rocket cutout keeps pointing into the fan it should be good, if it wants to turn away from the fan it's bad (unstable). It's just an approximation as it does not take into account the 3D geometry of the actual rocket.

FYI with black powder alternatives keep in mind that they need to be well contained to be effective. I recommend you do lots of ground testing to ensure you can get it to work consistently.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Rex Fynns
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Re: The budget can wait!

Postby Rex Fynns » Mon Jul 03, 2017 5:11 pm

G'day Matt,

I'll follow on the CP/CG discussion over in the "How To" forum might be able to offer a good (?) redneck eyeball method, the How To section will likely expose it to a wider audience for critiquing.

Rex

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Re: The budget can wait!

Postby Kryten » Tue Jul 04, 2017 9:15 am

Before I started using simulators:
My initial designs used the TLAR method ("That Looks About Right"), but I always did a swing test to ensure that it was stable before launching.
I also used the cardboard cut-out method - but this was to determine the CP, not for a rudimentary wind-tunnel test. The CG of the cut-out corresponds to the CP of the model.
I then started using VCP, then OpenRocket when I upgraded from Windows XP.
I wouldn't consider trying to make a supersonic rocket without running a sim.
"Pub, ah yes. A meeting place where people attempt to achieve advanced states of mental incompetence by the repeated consumption of fermented vegetable drinks" (“Timeslides”)

october sky
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Re: The budget can wait!

Postby october sky » Fri Jul 07, 2017 5:41 pm

Welcome to the forum Rex and I'm sure you will enjoy the awesome hobby of rocketry !

As we often say here at the forum....there is no such a thing as a dumb question....I sure recall my early years and so many good people here to help to get it right... :D

Enjoy.....

Ari
QRS Inc. NO: 003

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