# Safe Lift Speeds?

### Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

#### moocrew

##### Well-Known Member
How do you determine weather or not the rocket you are launching will be fast enough for the fins to take effect at the tip of the rod?

Im modding my DC Gemini a bit and wanted a really really low flight..im actaully trying to make this the lowest in the fleet, So I decided I would drop two 1/2A's or two A10's and I wanted to know how to determine if I could get the desired alt. without breaking something...(windows... )

Thnx
-moo

i use rocksim

lol i kick my own butt for not being clear.

I do use Space Cad to help with that rod length calcs....but I've found that they aren't very correct half of the time.

I just wondered if there was anyother way of figuring length or nessecary speed?

this page has some basic guidelines I'm not sure how well it will translate to such a low impulse as a 1/2 A..

Like Elapid,I rely on rocsim and look for atleast 25 mph off the rod

https://www.info-central.org/index.cgi?propulsion

the estes educator site also has good info

Originally posted by moocrew
How do you determine weather or not the rocket you are launching will be fast enough for the fins to take effect at the tip of the rod?

You know the swing method of stability testing, by swinging it around you on a string?

Start out slow and go faster very gradually until it goes stable. Swing at that rate several times and find out how many circles it makes in a given number of seconds. Divide by the number of seconds you test, and that's how much of a circle it's going per second (almost certainly a fraction of a circle per second if you're using a long enough string).

Now multipy the length of the string by pi to get the circumference of the circle.

Then multiply that fraction-of-circle-per-second number by the circumference, and you feet per second. That's how fast it was going when it went stable. That's how fast it needs to be going when it leaves the rod, no matter what the length.

Now you can use SpaceCad or another method to calculate the speed with a given engine at whatever time steps you're calculating with (SpaceCad does tenths of s second, I think), and see at what time/altitude it's going the necessary speed. That altitude is the needed rod length.

I trust SpaceCad's speed calulations OK, but not so much the stability estimates. I always swing test.

thanks for the info guys this should keep my rocs on the right path.

I second that Dyan! I've had more problems the answers from Roc-sim with micros, I do not trust it at all anymore, I've reverted back to swing testing everything designed. Than check it against what Roc-Sim tells me. I'm confident now the errors have to do with being very accurate with the measurements and weights entered in the program. Appartently; at least with Micro models a very small difference in the actual measurement, actual part weight and/or EXACT placement of the part make the diffenence between a good CP/CG and sim or a unstable model. May not se as sensative on Standard size stuff but for low power models I'll stick with swing testing.

Replies
3
Views
520
Replies
21
Views
2K
Replies
6
Views
1K
Replies
34
Views
3K
Replies
9
Views
1K