Page 3 of 6

Re: Altitude Records Online

Posted: Thu Apr 26, 2012 7:42 pm
by PK
makeit wrote:
If you are getting close to Mach 1 then it will go higher if you add more weight.

hey PK, if you have the time and you are not joking could you please help me understand that?


If you take a mass up in the air then it gains energy. To understand this, first place a brick on you head. Then place the same brick on your head by having someone drop it from the second floor, the brick had more energy when it was on the second floor....

So a rocket is really a simple machine for turning the potential energy of the chemicals in the motor, into the potential energy of the rocket at altitude.
The more efficient we can make that energy conversion, the higher our rocket will go.

Drag is a force that comes from energy that is transferred from the rocket motor, via the airframe and into the air. Once imparted, the air is warmer, or moving faster, and that energy is no longer available for gaining altitude. Drag is a function of the speed of the rocket squared.
So a rocket doing 200Km/h is experiencing 4 times the drag of a rocket doing 100Km/h. So more speed=WAY more drag; ergo speed is bad right? Well almost...

There's one more thing, and it's the tricky bit. "As soon as a rocket leaves the ground, it is in freefall and has to generate more thrust than its weight (plus its drag) if it wants to Accelerate.
Imagine a 10Kg rocket on the pad with a D12 in it... What's going to happen when you push the button?
5..4..3..2..1..Zip, Nada, Nothin'.
You need 100N of thrust just to stop that rocket falling backwards..... So.. The slower a rocket flies, the longer it has to pay the price of gravity... If that 10Kg rocket got to 1000 feet in 1second, then it would have wasted 100NS of impulse(energy) on overcoming gravity. If it took 10 seconds, then it would have wasted 1000NS (a J class motor). Our 10Kg rocket with a D12 wasted 20NS and got nowhere!
So more speed = less gravity penalty; ergo speed is good right? Well almost...

From the above you can see that there is an optimum velocity to travel at to thrust to a maximum altitude.
BUT! You should consider that the vast majority of a rockets flight is taken up by the coast phase. And here comes the clincher:

During Coast...
The drag issue still applies: More speed is bad.
But the more mass you have at burnout, the further you go.
This is why bullets aren't made from polystyrene. For the same diameter and speed, a lead bullet has heaps more mass than a polystyrene bullet. It will go further and do more damage when it gets there.

This works out quite well for us. We can slow down our rocket by adding mass until drag comes down to an acceptable level. Then, at burnout, that mass helps our rocket coast further..
It takes a bit of simulating, but try a few different weights for your rocket an see if it makes a difference...
PK

Re: Altitude Records Online

Posted: Thu Apr 26, 2012 8:11 pm
by makeit
Wicked!! :D Thanks PK, Your reputation holds true.

Re: Altitude Records Online

Posted: Thu Apr 26, 2012 8:41 pm
by Space Mark
If you're using OpenRocket, put a "Mass Component" in the nosecone of your rocket (no weight needed) and then go to Analyze -> Rocket Optimisation.

Now expand out the Nose cone -> Mass component and select "Mass" in the "Available parameters:" tree view. Click the Add button. Because you started with 0, and 0*anything = 0, you need you set the maximum. Go for something crazy to start with like double your rockets total weight.

Make sure "Optimised value" in the optimisation settings is Apogee altitude, then click "Start Optimisation".

A rocket i had prepared earlier suggested 93.3grams of weight would be excellent to gain altitude.

Re: Altitude Records Online

Posted: Thu Apr 26, 2012 9:06 pm
by Forbury
Thanks for that information, nice way to tweek a rocket.

I added some weight to the nose cone where the payload is (in the open rocket), and the altitude did increase by 10 to 15%. I did however notice the speed decreased.

I have done several simulations with various motors and found some will go up to mach 1.3, to 1000m, here im not going for altitude but speed.

Another simulation is just for height, at 1300m which there is some speed sacrifice.

This following topic is for the info posted here. viewtopic.php?f=56&t=3599

Re: Altitude Records Online

Posted: Fri Apr 27, 2012 1:14 am
by Space Mark
Exactly right Forbury, as PK explained, its a tradeoff :)

Re: Altitude Records Online

Posted: Fri Apr 27, 2012 8:03 am
by PK
Speed is easy. Minimum diameter, Minimum mass, minimum fin size, minimum length. In that order.
PK

Re: Altitude Records Online

Posted: Fri Apr 27, 2012 10:03 am
by cryoscum
makeit wrote:...PK, Your reputation holds true...


Hehe, ScienceSpice strikes again! :lol:

Re: Altitude Records Online

Posted: Fri Apr 27, 2012 1:25 pm
by october sky
PK = Sheldon ! :wink: And PK, no comments about the waistline.... :P

For a dumb A.. like myself, I feel like the 'ol Maxwell Smart when the boss explained to him his assignment....did you get that Max....I lost you after "Now Max...." Many of you would remember this, specially the "oldies". :lol:

Re: Altitude Records Online

Posted: Fri Apr 27, 2012 10:32 pm
by ogivemeahome
october sky wrote:... did you get that Max...

Max was too busy looking at 99. Come to think of it, so was I!

While I'm here ...
I've been working out some equations for predicting rocket performance (that don't require a computer); speed at the end of the launch rod, maximum speed (using quadratics). So far the values agree (more or less) with what comes out of OpenRocket.
Still trying to work out the delay period (what comes out isn't right). Would anyone like a copy? (For critique/constructive criticism)

Re: Altitude Records Online

Posted: Sat May 19, 2012 6:06 pm
by Forbury
Launch on 6th may went well, got a D12-5 to about 1500 ft, and also lost the dam rocket.
Now rebuilding the same rocket, and going for the E30-7, and F72 record with onboard altimeter. Will post with results.

Re: Altitude Records Online

Posted: Sat May 19, 2012 10:41 pm
by Rockoon
That's rotten luck Forbury :x . I hope you didn't lose an altimeter in the process.
Was it too windy when you launched?

Re: Altitude Records Online

Posted: Thu Jun 28, 2012 8:01 am
by ogivemeahome
Rockoon, I've submitted a claim form for the 'Frisky Frog'.
(see thread here): http://www.ausrocketry.com/forum/viewtopic.php?f=10&t=3548
Please disregard the 'staged' submission (oops). :?
I delayed submitting due to doubts about validity - can the reading be trusted?
Opinions welcome.

Thanks.

Re: Altitude Records Online

Posted: Sat Jun 30, 2012 10:59 am
by Rockoon
:D I have your rocket posted now. Sorry for the delay between responses. It's been so busy round here lately.
And for a A8 to hit over 500' is quite hard to do really.
In our neck of the woods in Canada, I can't ever remember seeing a 5 delay A in stores here. Just a 3 :roll:
So, can it take a B motor you think? Curious to see what it could do in another motor class.
Let's see...FYI. I have a U.K. rocket record of 975' on a B6-6. It does sound reachable mate.

Anyways, good luck on your next attempt.
Cheers

Re: Altitude Records Online

Posted: Sat Jun 30, 2012 1:11 pm
by ogivemeahome
Rockoon wrote:... And for a A8 to hit over 500' is quite hard to do really. ...
So, can it take a B motor you think? ...


It's hard to find the 'sweet spot' in the design and to stay within the 'weight budget'. Once that's done, some good weather helps. :wink: I'm curious to know how much variation is there with the specified thrust - maybe I got lucky and had one on the high end of the distribution?
Re: the B, the fins are a bit on the flimsy side - the extra speed might be a problem. A few more flights with the A8-5 to see if it was a fluke, then I'll reinforce the fins a bit and try a B.

Re: Altitude Records Online

Posted: Tue Oct 27, 2015 11:05 am
by Lister
Is there a current aussie list of altitude records that I can see.. have been working on a couole of designs and would like to add my name to the list 8)