Where will my rocket land?

[tdn]

Over the years it has become apparent to me that there is a great hypocrasy in voting for Ralph Nader and launching rockets. It seems like a more sensible political stance wold be to support James "cut down all the trees" Watt.

However, in an attempt to keep the peace with our Green friends, I should simply avoid landing my rockets into the evil, chloraphyl-powered, rocket eating monsters in the first place.

(I should also note that my coworker informs me that in China, retieval of rockets from trees is a cottage industry. Guys with ladders roam launch sites looking to make a few yuan.)

Anyway, what is the best way to avoid trees in a clear field ringed by trees? Is there a fast and dirty way of figuring out where a rocket will land (without having RockSim at hand)? It seems to me that for every 100 feet of altitude reached, the rocket will drift downwind 100 feet per 5mph of wind. Thus, in a 10mph wind, launching to 700 feet, I should plan on launching 1400 feet upwind of the nearest trees. This is assuming a vertical launch and an optimal recover system (descent of 11-14'/sec.).

Does this sound about right?

 

[Zack Lau]

My technique is to time the field--use a stopwatch to figure out how fast a rocket can reach the edge of the field. This works great a club launches, where you can watch a lot of rockets to get a good estimate of how fast the wind gets. Sometimes, the higher altitude winds are much faster than the surface winds.

Thus, on a windy day, the field might be only 30 or 40 seconds "long." On a calm day, the same field might be 120 seconds "long."

 

[tdn]

Originally posted by Zack Lau
My technique is to time the field--use a stopwatch to figure out how fast a rocket can reach the edge of the field. This works great a club launches, where you can watch a lot of rockets to get a good estimate of how fast the wind gets. Sometimes, the higher altitude winds are much faster than the surface winds.

Thus, on a windy day, the field might be only 30 or 40 seconds "long." On a calm day, the same field might be 120 seconds "long."

I'm not sure I follow.

Since I don't go to club launches, I have no others to compare my own launches to. But I usually start with a low altitude rocket to see how it behaves. Because it doesn't go up very high, it won't come anywhere near the tree line. Certainly I could time how long it takes to come down, but how can I tell if my next rocket (perhaps launched to 500 ft) will get as far as the trees?

 

[Zack Lau]

You may be able to extrapolate the distances--if it only goes half way to the trees, you get twice the timed distances. Thus, if it takes 15 seconds to go half way, you might rate the field at 30 seconds. But, this isn't very accurate, as the trees often shield the field from wind, so the wind above the trees is worse than the wind near the surface.

Sometimes you can get an idea of the upper level winds from hawks hovering over the field.

But, most of the time, until *someone* sends up a high altitude test rocket, you just don't know what the winds will be like.

 

[GL-P]

https://www.rocketreviews.com/tool_drift_rate.shtml

Just something to try /\ Winds can vary greatly at higher altitudes.

 

[JRThro]

Originally posted by tdn
Anyway, what is the best way to avoid trees in a clear field ringed by trees? Is there a fast and dirty way of figuring out where a rocket will land (without having RockSim at hand)? It seems to me that for every 100 feet of altitude reached, the rocket will drift downwind 100 feet per 5mph of wind. Thus, in a 10mph wind, launching to 700 feet, I should plan on launching 1400 feet upwind of the nearest trees. This is assuming a vertical launch and an optimal recover system (descent of 11-14'/sec.).

Does this sound about right?

I think it sounds about right, but we don't have to go by what sounds right.

Use arithmetic (which you might have already done) to figure out how far a rocket will drift while descending.

Altitude / Descent Rate = Time to Descend

and

Time to Descend x Wind Speed = Drift Distance

So... 100 ft altitude / 11 ft/sec descent rate = 9.1 second time to descend
9.1 sec x 5 mph x (44 ft/sec / 30 mph) = 66.7 feet of drift per 100 ft altitude

Or... 100 ft altitude / 14 ft/sec descent rate = 7.14 second time to descend
7.14 sec x 5 mph x (44 ft/sec / 30 mph) = 52.4 feet of drift per 100 ft altitude

So 100 ft of drift at 5 mph per 100 ft altitude is pretty conservative, but it should keep your rockets out of the trees.

 

[rbeckey]

The park I fly in is a somewhat long oval of perhaps 5-600 yards by 150 yards. It is surrounded on all sides by towering hardwoods. I have learned to go to the high side of the park and gauge the wind, and select a launch site accordingly. It usually is at one end, near the edge. We haven't lost many rockets lately, though we lost several in the first few launches. It seems more of an art than science.
The field has high hills around it and if you get above them, there is no telling what the upper level winds will do. I had an Alpha III drift 400 yards on an 8 inch chute after a C6-5. It seemed to actually GAIN altitude at one point. Fortunately it blew straight down the field long ways and was retrieved by Master Recovery Specialist Sam. (Number 1 son.)

 

[DynaSoar]

Originally posted by tdn
Anyway, what is the best way to avoid trees in a clear field ringed by trees? Is there a fast and dirty way of figuring out where a rocket will land (without having RockSim at hand)? It seems to me that for every 100 feet of altitude reached, the rocket will drift downwind 100 feet per 5mph of wind. Thus, in a 10mph wind, launching to 700 feet, I should plan on launching 1400 feet upwind of the nearest trees. This is assuming a vertical launch and an optimal recover system (descent of 11-14'/sec.).

Does this sound about right?

You can predict better if you fly under more control and less at the whim of the environment. For larger birds with changeable chutes I use X-form chutes to reduce drift. For modrocs with their plastic hex chutes you can alter them to make a Y-form to acheive the same effect.

Cut a slit from the middle of every other edge, halfway to the center (see attached drawing). Stick a small piece of scotch tape at the end of each slit to keep it from tearing further. This will reduce the effective diameter of the chute, so start with one size larger (use a 15" instead of a 12", etc.).

When the chute opens it will spill these slits, preventing it from developing lift due to drift. It will fall a little faster and a lot closer.
You could cut notches instead, but with just slits the remaining pieces will "stream" and help slow the rocket.

 

[Mister Rogers]

DynaSoar's plan for a "Y" parachute is totally cool. I am going to try it for sure. Would anyone care to comment on the advantage of this as opposed to cutting a circular spillhole in the top of the chute?

 

[gerbs4me]

you could use a streamer, less drift

 

[graylensman]

yeah, this is a question with so many variables that there's no way to give a definitive answer. Our last section launch happened with pretty high winds. A Midnight Express exhibited lateral drift during boost. Heavier birds weathercocked.

 

[DynaSoar]

Originally posted by Mister Rogers
DynaSoar's plan for a "Y" parachute is totally cool. I am going to try it for sure. Would anyone care to comment on the advantage of this as opposed to cutting a circular spillhole in the top of the chute?

Side cuts will keep it from oscillating. A spill hole won't. If it swings a lot it'll drift with the wind when the swing goes that way. It'll drift less than a full chute, but still more than one with open gores (slits of cut away sections). Plus, if it swings it might spin and that causes tangles.

 

[GlennW]

My first launch is always the same rocket, essentially used as a wind tester. I use a 25 year old beat up Alpha III that I don't really care if I lose. It gives me a good idea of what the rest of them will do that day, and I can adust accordingly. Interestingly enough, although always ready to be sacrificed, the thing is still going strong!

Glenn

 

[JRThro]

Originally posted by GlennW
My first launch is always the same rocket, essentially used as a wind tester. I use a 25 year old beat up Alpha III that I don't really care if I lose. It gives me a good idea of what the rest of them will do that day, and I can adust accordingly. Interestingly enough, although always ready to be sacrificed, the thing is still going strong!

Glenn

You have a 25-year-old rocket that you don't care if you lose? Wow! Let me be the first to suggest that you retire that one, display it in a place of honor, and build a new replacement as your wind tester.

(And I've been in this hobby for less than 5 months!)

 

[Mister Rogers]

Originally posted by DynaSoar
Side cuts will keep it from oscillating. A spill hole won't. If it swings a lot it'll drift with the wind when the swing goes that way. It'll drift less than a full chute, but still more than one with open gores (slits of cut away sections). Plus, if it swings it might spin and that causes tangles.

Thanks for the informative and logical answer. I didn't consider that aspect at all.

 

[tdn]

Originally posted by GlennW
My first launch is always the same rocket, essentially used as a wind tester. I use a 25 year old beat up Alpha III that I don't really care if I lose. It gives me a good idea of what the rest of them will do that day, and I can adust accordingly. Interestingly enough, although always ready to be sacrificed, the thing is still going strong!

Yeah, but how does that help me? I don't even have a 25 year old rocket!

Some great suggestions here. I like the idea of interchangeable 'chutes for differing weather conditions. Looks like I'll have to invest in some swivel hooks.

I also like rbeckey's idea of putting the launch site at the windward side of the field -- this effectively doubles the margin of safety. But how likely is it that high altitude winds are blowing in the opposite direction? I know that they can do this, but how high up?

Of course, there are two ways to predict this -- watch what the clouds are doing, and launch a test rocket.

 

[rbeckey]

One caveat about the windward edge of the field. An overstable or slow climbing rocket can arch INTO the wind fairly severely, especially if it is gusty. That can put the recovery area BEHIND you in the woods. I made the mistake once of angling the rod into the breeze and a custom built 2.6x42 inch ring fin rocket almost went horizontal on an E9, into the wind. Fortunately recovery was safely made, and a lesson learned.

 

[tdn]

Originally posted by rbeckey
One caveat about the windward edge of the field. An overstable or slow climbing rocket can arch INTO the wind fairly severely, especially if it is gusty.

Good advice. I think a good rule of thumb for me is that on windy days, I leave those types of rockets at home.

 

[JRThro]

Originally posted by tdn
Some great suggestions here. I like the idea of interchangeable 'chutes for differing weather conditions. Looks like I'll have to invest in some swivel hooks.

Wal-Mart (and other places, I'm sure) has #10 snap swivels in packages of 12 (I think) for a dollar or two, in the fishing department. They are perfect for attaching streamers or parachutes to model rockets, and I use them all the time.

 

[teflonrocketry1]

This article was written before RockSim had the ability to calculate the landing range from the launch pad (Oct. 2000).

https://www.apogeerockets.com/education/newsletter24.asp

It gives some general guidelines on how to recover a rocket close to the launch pad. I have given public demonstrations where I ran the calculations several hours (or the night) before the flight and hit the launch rod or caught the model from where I was standing on its way down. I sucessfully have recovered many flights at Camp Stambaugh in Canfield, Ohio which has a parade field (about 300 feet in diameter) surrounded by a lake and trees.

The keys are to know the wind speed and direction, the altitude the model will likely achive, and the descent rate on the recovery device. Knowing these ahead of time will help you select the right motors and launch guide angle to recover the model rocket within a given launch site.

Bruce S. Levison, NAR #69055

 

[DynaSoar]

Originally posted by JRThro
Wal-Mart (and other places, I'm sure) has #10 snap swivels in packages of 12 (I think) for a dollar or two, in the fishing department. They are perfect for attaching streamers or parachutes to model rockets, and I use them all the time.

I'd add that you should make sure you get the locking type. They have a bend on the end of the wire loop of the snap so that they don't pull out of the clip as easily. A small Estes/Quest chute will tend to be the first thing to fail under high stress, but pulling the clip apart is the second.

 

[JRThro]

Originally posted by DynaSoar
I'd add that you should make sure you get the locking type. They have a bend on the end of the wire loop of the snap so that they don't pull out of the clip as easily. A small Estes/Quest chute will tend to be the first thing to fail under high stress, but pulling the clip apart is the second.

Good point. That's the kind I have, but I didn't realize there was any other kind.

 

[GlennW]

Well, John and tdn, you don't need it to be a 25 year old rocket, just one that you don't care if you lose. You could go out and get a new Alpha III, which is the same as mine really, just with different colors, and use that. If you lose it, it's cheap enough and easy to build a new one in no time.

As far as retiring it and displaying in a place of honor, I already posted a rant in another thread about how I feel about retiring rockets, so I won't get into it here, but let me just say this rocket is way too ugly to display anyway in its present abused condition!

Glenn

 

[shockwaveriderz]

I would first goto https://www.terraserver-usa.com/ and get both an aerial view and a topo map so you can see elevations.... you will also be able to measure true distances.....behind that tree line may be a very large field.....or open space.....

you will want to pick a launch site such that the long field direction will be in the same plane of the wind direction...

the idea is to situtate the actual launch site so that you can take account of weathercocking if needed..,... You can launch your model with the wind or against the wind......depending on the weight/thrust ratio of the model rocket itself, will determine where it goes in 3-d space........

this will aid with overall recovery efforts....

The major point I am trying to make here is that you need to be flexible where you launch from and therefore where you will recover from....

wind speed and wind direction play a very large role in where the model will eventually land....recovery system can largely determine how far the model will go.....