Decent Rates

[Mike]

Hi guys,
Ever since I've been into scratch building I've wondered how large to make my parachutes for each rocket.

I run every rocket I make through SpaceCAD mainly to chack stability and predicted altitude but it also suggests what size chute to use, the default for this calculation is a decent rate of 3m/s but this often comes up with to large chutes that I feel would drift a long way in only a small breeze.

I want to get my rockets down with limited drift but still no damage to the rocket on landing so what is an acceptable decent rate for a average strength rocket in good conditions?

Thanks in advance.

 

[Austin]

Mike,

15 feet per second is usually the norm and works well for all models, large and small. HPR models usually have a bit higher decent rate, up to 25fps, but their airframe is strong enough to handle it.

Carl

 

[kgholloway]

Mike,

Here's a simple parachute dimension calculator that I created in MS ExCel 97. It calculates the correct size of parachute, based upon the model's weight, for a descent speed of 15'/s. The X-form shape for these calculations is assumed to be the standard 3 to 1. That is the length of a leg from one side of the parachute across the center to the other side should be 3 times the distance across the leg.

Ken Holloway

 

[rstaff3]

Ken: Cool, speadsheets are good!

Mike: If this does only cross-form chutes, you might also look at InfoCentral and EMRR. Both have various calculators. InfoCentral has one that will account for multiple chutes, amain and a drogue, etc.

 

[Mike]

Thanks guys
I've sorted out my parachute problem but now have another.

I've been putting my pencil rocket through SpaceCAD but it doesn't do the hexaganol bodytube etc. and there are a few more inaccuracies so I'm trying to get it so it has a large stability margin. I know things are deemed unstable at beneath 1 caliber but when do things go overstable?

TIA

 

[airforce]

Carl, anybody, could you please define 'overstable' to me.
I would think that would be a good thing.
Thanks.
Randy

 

[kgholloway]

Mike,

"Overstable" is a realtive term. For some people you are overstable when you exceed 2 calibers of stability. For others a rocket has to be over 5 calibers stable. Personally if I have about 2 calibers of stability with the heaviest and longest motor I want to use in a rocket I'm happy.

For smaller and lighter motors the rocket will be overstable. However, that doesn't bother me as I'd rather have the rocket achieve stable flight at a lower velocity when using smaller and less powerful motors. So I worry about the minimum stability with my designs and don't bother about the maximum stability.

Ken Holloway, NAR #78336, L-1

 

[airforce]

Sorry, I still have a lot to learn.....what do you mean by calibers?
Thanks
Randy

 

[rstaff3]

A caliber is basically just the diameter of the body tube.

You can search TRF for 'stability' as overstability was discussed earlier IIRC. You should also go to the Apogee web site here and search the site. They have lots of stuff in their newsletters, which are archived and searchable.

I have never had any problems due to overstability, although these models are in theory much more sensitive to the wind conditions.

 

[MarkABrown]

1 caliber = 1 rocket diameter

The only real disadvantage that I'm aware of of a rocket being overstable is that it will have a tendency to weathercock into the wind. This can cause the rocket to leave the launch area under extreme circumstances.

On the other hand, it also means potentially less walking during recovery since after main chute deployment, the wind should carry the rocket back to the launch point.

 

[Dean]

Mark has the over-stability issue correct, in that an over-stable rocket wil be very sensitive to cross-winds. Of course, a properly stabilized rocket will weathercock in a cross-wind too. What make a rocket "over-stable" is that it will go too far. Rather than drift nicely back towards the pads, it will go into "cruise-missile" mode, depart the area at speed and probably strip the chute when it deploys, if it manages to deploy in the air.

The rule-of-thumb of 1-to-2 calibers of stability should give you a rocket that will turn slightly into the wind (as a kind of "correction"), but not lay over too far. That's the theory, at least. In practice there are other factors at work. Some designs are notorious for reacting to winds, others go exactly where you point them despite any cross-wind. Clearly the latter is prefered.

 

[kgholloway]

Dean,

I think that overstable designs have the reputation for severe weather cocking due more to the method of launching them then to their being overstable.

From what I've read in several books and articles the problem with being overstable is that at low speeds the angle of attack is changed by any cross wind placing a large rotational force on the rocket perpendicular to the fin can. When launching an overstable rocket from a rod this force can bend the rod while the rocket is traveling up it's length resulting in the rocket coming off the rod at a low angle.

This is based upon my observations with overstable rockets equiped with both launch lugs and rail buttons. When launched off of a rail there was little or no weather cocking observed. When launched off of a rod in the same weather conditions the rockets would weathercock and leave the road at up to 45 degrees from verticle.

Ken Holloway, NAR #78336, L-1

 

[Mike]

Alrighty then,
I've got to around 2 calibers but the fins seem strangley small, the picture is a screen dump from SpaceCAD of the fin design window.

Do you think these fins are large enough to make a 64cm (about 2ft 2inches) rocket stable? Just want a second opinion really.

Thanks

 

[rstaff3]

Off the top of my head they seem small to me also, bit the 2 calibers you mention depend on other factors. Give me the BT dia and length, nosecone length and shape, and I'll put it in Rsim and give you back the barrowman CP. On a 3FNC rocket, I tend to believe the design program, assuming you have entered the right materials to get a reasonable CG. You can always build it and adjust the CG then.

 

[Mike]

Thanks a lot,

Nosecone
The nosecone has a diameter of 33.7mm is 14cm long and cone shaped. There are flats sanded on it flush to the sides of the bodytube.

Bodytube
The body tube is hexagonal and made from 2.5mm balsa. The outside diameter is 33.7mm and the balsa strips are stuck round a 24mm tube. The bodytube is 44cm long

Ferrul
The ferrul bit is a 33.7mm diameter tube and 6cm long. The fins are attached to the end of the ferrul

Fins
The material I have for the fins is 2mm clear plastic.

Motor Mount
The motor mount extends behind the ferrul. It is a cylindar of balsa (33.7mm) and 45mm long. There is an 18mm hole through the centre and a motor tube goes in that.

I know I've given slightly more detail that you asked but obviously Rsim (or any design program) cannot take all the technicalities so I gave you all the details so estimations can be made.

Thanks again.

 

[rstaff3]

Well here's my approximation. I modeled the body as a tube. Nose cone is solid balsa, body is balsa with an inner 24mm Estes tube. Ferrule is a cardboard tube. Eraser is solid balsa with an 18mm mount. Added a 24" plastic chute - may be too big. Note a C6-0 is loaded.

 

[Mike]

Thanks again Dick,
So would the best thing to do be to increase the fin size to make it less 'marginal' or to add nose weight?

 

[rstaff3]

I roughly doubled the fin size and got the following results. Note that bigger fins pull both the CG and CP rearward, but the CP moves a bit faster. If it were me, I'd increase the fin size and then compare the as-built CG. If it turns out marginal, then you could add some nose weight. There's a good chance the CG of my sim is off from what yours will be. And, I didn't account for the hex tube. I don't know if this will help or hurt, but it's best to be conservative just in case.

BTW, this sims to around 900 feet with a C6 motor. Optimal delay is around 5.25 sec.

 

[Mike]

Thanks Dick,
I've been fine-tuning the fins and I think all is sorted now.

Thanks again.

 

[rstaff3]

Be sure to show us the finished product