what launch angle?

[jayarrG7]

It was a bit more wind than my first launch, so I angled the launch rod into the wind. Instead of helping, the Estes Patriot took a relatively low path.
What is to be considered when choosing the proper angle to launch the rocket?

 

[FastCargo]

That depends on a lot of factors...strength of the wind, size of the engine, configuration of the rocket, etc. The thing to remember is to not go over 30 degrees from vertical.

FC

 

[llickteig1]

It was a bit more wind than my first launch, so I angled the launch rod into the wind. Instead of helping, the Estes Patriot took a relatively low path.
What is to be considered when choosing the proper angle to launch the rocket?

I'm sure there is a real technical explanation, but in general, a normally stable rocket will "weathercock" into the wind. It is a little bit counter-intuitive when you're new, but think of it this way: the fin area has the highest surface area so the wind blows the fins outward causing the rocket to tilt into the wind.

There is also a situation called "rod whip" that can happen when a rocket is launched in breezy conditions where the launch rod bends and sort of flips the rocket on recoil. A higher thrust-to-weight ratio and not excessively long launch rod can help avoid this problem. The direction can be somewhat random, but is usually parallel with the wind direction - either into or with the wind.

Your best bet is to stay pretty close to vertical for the best flight profile.

--Lance.

 

[luke strawwalker]

As has been mentioned, there are a lot of factors that go into it. Here's a few pointers--

1) the design of the rocket, its weight, and stability margin play a crucial role. If the rocket has greater than 2-3 caliber stability (also called "overstable"--more than one body diameter or so between the CG and CP) it is more prone to weathercocking than rockets at about one caliber stability.

2) if the rocket is heavy or being flown on a motor at the lower power end of those appropriate for the rocket, it's more prone to weathercocking.

3) if the rocket is weighed down with extra payload, it's more prone to weathecocking.

4) The stronger the wind is, the greater the effects of weathercocking (naturally). Also, the further the rocket is likely to drift under parachute (or streamer-- remember, the rocket is essentially "immersed" and descending in and through a moving river of air, which will carry it 'downstream' in direct proportion to the wind speed.

5) Therefore, to minimize the effects of weathercocking (defined as the rocket's tendency to fly 'into the wind' (upwind) we have several choices:
A) use a larger motor or one with higher thrust/shorter duration in the same impulse class. (of course this is also dependent on the field size, flying conditions, etc.)
B) make the rocket lighter, so that it accelerates faster and to a higher peak velocity, thereby making it better able to 'penetrate' the wind. Avoiding heavy payloads on windy days is a good idea.
C) ensure that the rocket is not "overstable" and that the CG-CP relationship is about one body diameter between the CG and CP.
D) not flying rockets particularly prone to weathercocking in windy conditions.

6) OF course, you can't do many of these things except when building a particular rocket, and some designs just ARE NOT going to fly well (if at all) in windy conditions no matter what you do, due to the design peculiarities...

7) Angling of the launch rod is greatly affected by the previous factors. The "optimum" angle is determined by a combination of all these factors-- wind speed, rocket stability margin, high or low motor thrust, rocket weight, recovery device size and type, desired trajectory, the rocket's acceleration rate under thrust (fast or slow on takeoff), etc.
A) the heavier the rocket, the more "overstable" it is, the lower the motor thrust, the slower the acceleration, and the smaller the recovery device, the less angle you need to tilt the rod into the wind-- these factors mentioned all tend to push the rocket's flight path over into the wind anyway.
B) the lighter the rocket, if it has one-caliber stability, the higher the motor thrust, the faster the acceleration, and the larger the recovery device, the MORE you have to angle the rod, as these factors all tend to help the rocket "penetrate" the wind and fly more or less straight. SO, if you want to recover the rocket in the same county, you'll have to angle the rod a little more to get it initially 'pointed' upwind once it takes off.

8) Be aware that it is almost NEVER required to use the full 30 degrees of tilt allowable by the NAR safety code. If it's THAT windy, you shouldn't be flying anyway. The only case where "large" angles are really required is if the wind is quite high, the rocket has single-caliber stability, is a high-performance design (minimum diameter, three fins, well streamlined, etc), has a high-thrust motor (about the biggest possible for the rocket, and a fast-burning (short duration) motor with a high peak thrust which will accelerate the rocket VERY fast, the rocket has a large recovery device with a slow rate of descent, and the wind speed is pretty high. Most of the time, angles in the 0-20 degree range are MORE than enough.

9) As the angle increases, the more likely the rocket will fly a "parabolic arc" trajectory (cruise-missile as it's sometimes called). The rocket will not fly very high but will be moving at a pretty high rate of speed when the recovery charge fires and deploys the recovery device-- this can damage the rocket or strip the chute, cause zippers, etc. (launched at a 45 degree angle from vertical, a rocket will theoretically fly 1/2 as high but twice as far away from the pad as one launched vertically. A vertically launched rocket should, theoretically, approach a complete stop at apogee, which is why it's preferable to deploy the recovery device at apogee when the rocket is travelling the slowest. For a rocket launched at a significant angle, or weathercocked severely, the rocket will fly an "arced" flight path and still be moving pretty fast at the apex of the arc. Additionally, the rocket will gain acceleration once it's "over the top" of the arch, and gain speed with every second it continues to descend (that is why you usually use a shorter delay on windy days). Ejecting at the apex of the arc is bad enough due to the additional speed, but going "over the top" and ending up deploying a couple seconds into a ballistic arc descent gaining a lot of speed is a lot worse!)

10) It's not unheard of to tilt the rod slightly "downwind" for rockets that are prone to weathercock-- BUT NOT BY MUCH!!! This will help rockets take off pointed WITH the wind, which will then straighten them up a bit so they fly a more 'straight up' flight path. You don't want the rocket going TOO high when it deploys it's recovery gear though or on a windy day it's likely to drift into the next county or clear to OZ...

Hope this helps...

Later! OL JR

 

[Che]

Great post Luke Strawwalker, very comprehensive. I'll have to re-read when I am completely awake.

BTW- I miss BocktoberFest. Well, at least we can get a little Shiner out here in the desert.

 

[adrian]

To some extent, weathercocking is good. The rocket tips over and flies into the wind, then drifts back towards you after the parachute opens.

If, after angling the rocket into the wind, it then tips over too much and flies at a low angle, you probably want to launch it vertically - you don't need to aim it into the wind if it's going to weathercock and fly into the wind anyway.

I've heard of the trick of angling a heavy rocket with the wind to reduce weathercocking. One of mine weathercocks heavily due to a combination of mass and large horizontal fins. On one occasion I tried angling it slightly with the wind, and it worked - the rocket went up straighter, then drifted a long way, and the only reason I got it back is because the wind had changed direction. Previously that length of drift would have taken it two fields away into some very long plants where I would probably not have been able to see the rocket even if I was practically standing on it; however, the new direction took it to a landing site with short grass, so I got the rocket back after a nice long walk. Bottom line - don't try this unless you have plenty of recovery room!

 

[KennB]

The first time I noticed a launch rod angled with the wind was at a TARC qualification day CMASS sponsored. As stated above, the rocket came off the rod and weathercocked into the wind with the result of a nice vertical flight that provided the team with something much like their calculated profile.

I would guess that is an old trick for anyone who flies in altitude competitions. Yes, it provides you with a good walking opportunity afterwards but you do get a much more predictable boost and coast. Also, if you angle too much into the wind and get more of a ballistic arc, you'll likely have a high speed recovery deployment with all the destructive issues that brings with it. :cry:

 

[mattress67]

Great info as I can attest what seems intuitive is not so... Recently launching a Big Daddy on a +10mph day, I thought angling the rod into the wind would help things. Instead my BD quickly became a cruise missile with the ejection charge firing after impact.

 

[cobra1336]

I seem to build overstable rockets, so if there is any wind, I go LOW or I make sure I have good speed to make the rocket stable quickly.

 

[MarkII]

Angle it slightly, slightly into the wind. The safety code says no more than 20° but I wouldn't go past 10°, because the more angle you use, the more that you are flirting with weathercocking. If you are worried about excessive drift, then either use a streamer, choose a motor that won't take it so high or else put the rocket away and choose another one. Long rockets and those with a lot of fin area are especially prone to weathercocking. Also, skip the long-burn motors; you need to use something that will punch the rocket into the sky.

 

[JoeG]

Rule 1. Listen to the guys from Kansas.

Rule 2. Refer back to rule one.

After attending my first Kansas LDRS I was very impressed with the ability of these guys to deal with the wind. On the first day of the launch the wind was in the 20mph range. I saw one person walk up to Bob Brown and ask if he thought the wind would die down. His reply was, "It already has."

I watched Chuck, who worked the away pads tirelessly every day all day, and Lance, who seemed to be everywhere all the time, help set up over stable rockets by pointing them downwind slightly. The looks on some of the owners' faces was priceless.

I think it's an art after so much experience with the wind. My guess is that each one of these ended up being much closer to the pad than they would have been pointed into the wind.

Thanks for the lessons guys. There is no substitute for experience...unless it's GPS and a four wheeler.