Flight Speed

[Mike]

Hi all,
I'm planning to build quite a large scratch built rocket and the launch speed for it is pretty low. I know I need to be going 13.5m/s at the end of the launch rod for a stable flight and for every mph of wind another 9 fps which is about 3m/s. This means that when flying in 5 mph winds I need a launch speed of 28.5m/s, this makes for quite a long launch rod with slower acceleration rockets.

Do I have all of this right and is there a way of reducing the required flight speed, perhaps by making the rocket more stable?

Thanks

 

[rstaff3]

Mike, just re-verifying on info the 44 ft/sec is the rule of thumb presented for up tp 5mph. You have to add if the wind is over 5 mph.

I don't have any better guidance, but will say I have launched stable rockets (margin of 0.8 to 1.1 depending on the sim) with less that that velocity. I hesitate to provide the numbers since I don't have a better numerical rule of thumb and I don't have an accurate estimate of the winds.

 

[daveyfire]

There's a fix that always seems to work for me: put a bigger motor in it

Seriously, though, there was an article in an issue of HPR about Wind Caused Instability. E-mail me and I'll see if I can find it to send to you.

HTH.

 

[Dean]

Note also that making a slow accelerating rocket more stable (nose weight or bigger fins) can make things worse. Besides adding weight, it makes the rocket more likely to weathercock (turn upwind) as it leaves the rod.

The whole issue results from a high 'angle-of-attack' when the rocket leaves the rod and begins free-flight. What this means is that the rocket is not pointed directly into the wind that it feels. If a rocket leaves the rod at 20mph, and is hit with a wind gust of 20mph, the wind it experiences is hitting it at a 45 degree angle off the nose.

If the rocket is stable, or over-stable, it will rotate into that wind. The more 'stable', the faster it responds to the wind angle. If the rocket doesn't get up to speed quickly it could lay well over towards the horizontal before it stops rotating. This, as you can imagine, is a bad thing.

As mentioned above, the subject of Wind-Caused Instability is another factor. As the angle of attack on the rocket changes, the CP can move around. Usually it moves forwards, which can make a rocket go unstable. Plenty of otherwise stable designs will start doing loops if the angle of attack gets above 15 degrees. I've seen several Estes Phoenix and Heatseeker kits launch into a breeze and turn 20-30 degres *downwind* before stabilizing. Big fins on a Phoenix..., yet the rocket turned away from the wind on launch. That's wind caused instability. I've seen the Saturn 5 model do the same thing. At a club launch one was launched twice with perfect flights. The owner tried it a third time later in the day when the wind was up a bit and it went unstable right off the rod; same model, same motor. Never got more than 20 feet off the ground.


The moral of the story is that acceleration is a good thing, but if the winds are very calm, you can get away with slow launches. I launch my Phoenix with an E30 in a wind, or an E15 if it's calm.

 

[ibeblip]

Hi Mike:
Another rule of thumb is the "Thrust to Weight Ratio".
The rocket motor should produce a minimum of 5-6 times as much thrust as the total weight of the rocket vehicle when fully prepped and ready for flight.

 

[Mike]

Thanks, you've all helped a lot and she should be fine to fly as long as the winds aren't too strong.