# Launch rail velocity - simulated vs actual

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#### Richard Dierking

##### Well-Known Member
TRF Supporter
I see some threads concerned about simulated launch rail exit velocity but none about measured velocity. Has anyone compared simulated launch rail exit velocity with what was actually measured?

I have some vague ideas about how to measure exit velocity, but I'm curious about ways people think this could be measured. Probably the Myth Busters high speed video with the background grid would not work well because frequently there's too much wind for something like that and the set up would be difficult. It would have to be set up pretty easily, accurate, and of course not potentially interfere with the launching rocket.

Also, I mentioned rail, but it could be a launch rod or even a tower

Given that you know the length of the rocket, I would think you could get a pretty good estimate just by taking high-speed video focused on the rocket and the rod. Measure time from nose cone to tail clearing the top, and calculate.

Obviously, the rocket will be accelerating during this, so you'll be calculating some sort of average. There might be ways to increase accuracy of this (like use landmarks on the rocket that are closer together than tip to tip) but I'd think it'd be a good start.

Use a bullet chronograph with sensors mounted above the end of the rail.

Neil, I'm not familiar with specifications for high-speed video. Would some kind of calibration be necessary for accuracy? Also, how much would position of the camera affect the measurements? I would rather not have landmarks placed on the rocket because it might be measuring other people's rockets.
Very true about the acceleration. Wondering how actually popular sims determine exit velocity. i.e. as soon as that rocket goes over 1 to 1 it's going to start moving.

Steve, for the bullet chronograph, wouldn't the rocket have to pass through the sensing part? The rocket would be much larger than a bullet. Do you know someone that has done this for rocket velocity?

btw, I think actual exit velocities are usually lower than sims. And, that has implications. But, I need data for conclusions of course.

You could fairly easily measure this from video captures of the rocket lifting off just by measuring the length of the rail in advance and scaling. You might have to do a little interpolation.

Steve, for the bullet chronograph, wouldn't the rocket have to pass through the sensing part? The rocket would be much larger than a bullet. Do you know someone that has done this for rocket velocity?
The sensing part is just a pair of photo eyes for something like this Pact. You’d have to modify them somehow rather than sending the entire rocket through the sensor, but it shouldn’t be difficult. No, I don’t know someone who has done that. The problem might be that some chronograph have a minimum velocity that is higher that our liftoff velocities. There’s a radar chronograph (Labradar) that needs no screens, but it’s minimum velocity was 69 FPS. There are also radar guns used for sports to measure the speed of a ball that could be used. Here’s a bullet chrono:
https://shop.opticsplanet.com/pact-...DdEpP4kcrlP5Lg8-LHpP8XUqv-TVRxjEaAmiXEALw_wcB

I was considering the photographic methods, but then you don't really have numbers until you view and interpret the video. Well that, and I don't have a good camera for high speed. And, of course, the longer the time interval between say frames, the more velocity averaging there is. I would probably have to use the length nose to aft end of the rocket. I guess I could add indicating markers on the rocket, but it will not be my rocket.

For the radar instruments, I believe you shouldn't be at a high angle for measurements. There could be issues if not calibrated correctly. And good ones like the chronographs get a bit expensive.
But, this is helping me think this through. I must have known distance and time. The bullet is one object moving between two fixed points. Then, you just need the time interval.

Maybe I should focus on rockets launched from rails. Small rockets launched from rods are usually BP motors; they don't have much of a problem accelerating. And, what I find out from rockets launched from rails could apply to rods.

OK, how about a photocell (some kind of optical sensor) attached at the top of the rail. A light source (what kind?) would be directed across the open space where the launch buttons would have to pass. Since there are at least two buttons on the rocket and this distance could easily determined before loading the rocket, I wouldn't have to mess with the rocket at all. The first button passes the sensor and the time interval begins until the sensor detects the other button and the interval ends.

I need data that I can quickly enter into a spread sheet, determine the velocity and compare to the sim. If I know the rocket launch mass, and the motor with the engine file, I don't really need most of the other stuff for the sim, correct?

What do you think?
Still wondering if someone has already done something like this. It would be so nice if the wheel was already created.

The two thoughts that come to mind for me are the high speed camera method doing a curve fit on the data points, or integrating an accelerometer.

One of our fliers used to time his igniter formulas by using a flashing light on the ignition circuit and burst firing his camera (DSLR) at the zero count, from there he was able to count the frames at a known frame rate from when the light was seen until the rocket moved in another frame. Similar could be done with a long 12" blocked scale (lots of Mythbusters used one to measure movement via photo/video) and a video camera of known frame rate, probably not super accurate but probably "close enough".

One of our fliers used to time his igniter formulas by using a flashing light on the ignition circuit and burst firing his camera (DSLR) at the zero count, from there he was able to count the frames at a known frame rate from when the light was seen until the rocket moved in another frame. Similar could be done with a long 12" blocked scale (lots of Mythbusters used one to measure movement via photo/video) and a video camera of known frame rate, probably not super accurate but probably "close enough".
One was used in the Mythbusters "Greased Lightning" 30' fireball test.

Photogates are made to precisely measure this kind of thing. (I grew up playing with them in my Dad's physics classroom.)

https://www.pasco.com/products/guides/photogates
If you space two a known distance apart, say at the end of the launch rail and 1-2 feet below, all you need is something sticking off the rocket, like a fin, to pass through each gates to get an exact time it took for the rocket to traverse that distance.

Velocity = Distance / Time

You're just some until conversions away from having a directly measured speed (remember significant figures!).

From there, you could compare your calculated speed to less direct measures like the velocity at 6 ft AGL (or however long your launch rail is) as recorded by a flight computer. You could also try to extrapolate from video frame rates or bursts of still photos from a camera, as has already been suggested, but that seems a bit messy to me.

That's how I would measure it but is knowing this precise number really important? The speed-off-rod recommendations are just a rule of thumb to make sure your rocket is going fast enough that the fins can generate enough lift to make aerodynamic corrections.

If the first rail button leaves the rail and the rocket is not aerodynamically stable yet, is it better or worse that the 2nd button is still on the rail?

For the photogate, I wonder how significant differences in the fin width at the point of sensing would be? Awh, but if it was measured when the fin started at the two points. Is that it?
I like this kind of timing thing because it spits out a number if you used an Arduino.

That’s what the bullet chronograph is, two photogates, a counter, and a simple program to convert to velocity.
If you already have an arduino you can save some money and use that but at the expense of your own development time.

That's how I would measure it but is knowing this precise number really important? The speed-off-rod recommendations are just a rule of thumb to make sure your rocket is going fast enough that the fins can generate enough lift to make aerodynamic corrections.
Good point.
There's a dispersion of flight off a rail. I've seen it. That's when I starting thinking the sim velocity off the rail wasn't what was happening. I'm thinking the dispersion is greater the shorter the rail. I kind of came to this conclusion for leaning pads away from spectators like we should do (at institutional types of launches). Anyway, if you lean the pad away and rockets still seem to go over the crowd, the dispersion is too great and you have to lean the pad farther to account for the dispersion. Correct? What if you could use a longer rail and lean the pad away less? Or, would it make no difference?

See, I'm getting better Steve, I loaded your message while I was typing this one.
Being retired, my time is priceless. The problem is my capabilities.
I appreciate the help on this. It really helps me.

Now that I think about it: a smartphone app could do this very easily processing the incoming video instantaneously. Would just need to calibrate measurements based on the length of the rod in the picture. Probably an easy task for someone who knows what they’re doing.

If you’re truly leaning the rod or rail away from the flight line and rockets are flying above the crowd on the way up, I would look at other things than velocity off the pad, unless the only ones that fly over the pad are obviously slow.
I’d look at wind direction, pad rigidity, and things like that.

f you’re truly leaning the rod or rail away from the flight line and rockets are flying above the crowd on the way up, I would look at other things than velocity off the pad, unless the only ones that fly over the pad are obviously slow.
I’d look at wind direction, pad rigidity, and things like that.
Yes. True. But, do you see what I mean about the dispersion?
So, really, if doing something like this, ambient conditions would be good to note, and measurements should be related to what the rocket actually did; what path it took.

It would be great if you could launch one rocket over and over with different motors and rail lengths but I'm not going to do that. Well, unless I get a big bump in my social security benefits.

Have you seen layered photographs? (I'm not a photo guy, so I don't know all the techniques and names for them.) I've seen them where you can see the apparent angle of the rocket sequentially off the pad on the same photograph. Why do some rockets go off the pad so wonky? Blast deflector? Unintended thrust vectoring as the motor pressures up?

I'm not trying to imply that this is not a complicated thing. Just trying to understand it better. Seems we don't have a lot of data on this.

If the first rail button leaves the rail and the rocket is not aerodynamically stable yet, is it better or worse that the 2nd button is still on the rail?
It is worse. In all cases you would prefer both to be released at the same time.

I kind of came to this conclusion for leaning pads away from spectators like we should do (at institutional types of launches).
Yup. At most institutions they get upset when “One Flies Over the Cuckoo’s Nest.”

Sorry, should have explained what I meant by "institutional". The first time I went to BALLS, I was thinking about how to lean the pad away and I heard someone say, "this is balls, we go straight up." BALLS is not institutional. Lots of spectators, lots of rules (necessary btw), cookie-cutter type of launch is institutional. And, please, no one be offended. I should come up with a different term.

Also, I would like to stop discussing leaning and even flight dispersion at least for now. My fault. I would like to focus on obtaining data.

So, I'm going to try the launch button sensor. I'll see how well I can detect a rail button out of a 1010 rail.

Is there someone out there that is pretty good at programming Arduino?
When I come up with the program, I would like to have it reviewed.

I'm thinking short program. Detect the first button, time to the next one, and display the elapsed time on a LCD and record. Then, reset for the next measurement. I would note the rocket mass, airframe diameter, motor, igniter, and the distance between the rail buttons. For the launch and flight, take burst photos of the launch, and note the apparent trajectory. I guess I could ask the flyer to provide info on sim launch exit velocity if they had it.

Thoughts?

There's a simple high speed video method. Put marks not on the rocket but on the rail. Pick any convenient, identifiable point on the rocket, such as the trailing point of the root edge of a fin, and use the scale on the rail to measure the distance traveled in the last frame-to-frame interval that you can. Divide. Done. You can see this done in many a crash test video and about one in three Myth Busters episodes.

Sorry, should have explained what I meant by "institutional". The first time I went to BALLS, I was thinking about how to lean the pad away and I heard someone say, "this is balls, we go straight up." BALLS is not institutional. Lots of spectators, lots of rules (necessary btw), cookie-cutter type of launch is institutional. And, please, no one be offended. I should come up with a different term.

Also, I would like to stop discussing leaning and even flight dispersion at least for now. My fault. I would like to focus on obtaining data.

So, I'm going to try the launch button sensor. I'll see how well I can detect a rail button out of a 1010 rail.

Is there someone out there that is pretty good at programming Arduino?
When I come up with the program, I would like to have it reviewed.

I'm thinking short program. Detect the first button, time to the next one, and display the elapsed time on a LCD and record. Then, reset for the next measurement. I would note the rocket mass, airframe diameter, motor, igniter, and the distance between the rail buttons. For the launch and flight, take burst photos of the launch, and note the apparent trajectory. I guess I could ask the flyer to provide info on sim launch exit velocity if they had it.

Thoughts?
For rockets intended to fly over 25k, my FAA COA requires that they be launched vertically. It has nothing to do with anything other than that. I’ve attempted to explain that we prefer they fly away from the flight line (I’m a proponent of angling away) but FAA has their own procedures to follow. I’ve seen the same requirement in the Special Provisions in other COAs as well.

For rockets intended to fly over 25k, my FAA COA requires that they be launched vertically. It has nothing to do with anything other than that. I’ve attempted to explain that we prefer they fly away from the flight line (I’m a proponent of angling away) but FAA has their own procedures to follow. I’ve seen the same requirement in the Special Provisions in other COAs as well.
Are TRA practices for leaning rockets different than NAR? I thought the PM was responsible to assure that pads leaned away.
Anyway, no worries. I'm the same way. I even lean my rockets away at FAR when we are in bunkers. I intend to lean my rockets away and have active flight control go to vertical after a predetermined time. Well, at least that is my plan.

Sticking to trying light path detection of rail buttons exiting the rail first. Ordered IR LED and sensor to begin testing. If it doesn't work, I'll switch to video method or something else.

So, Steve, how many days left? Not that you are counting.

There's a simple high speed video method. Put marks not on the rocket but on the rail. Pick any convenient, identifiable point on the rocket, such as the trailing point of the root edge of a fin, and use the scale on the rail to measure the distance traveled in the last frame-to-frame interval that you can. Divide. Done. You can see this done in many a crash test video and about one in three Myth Busters episodes.
Have you ever seen the camera they used for the high speed video on Myth Busters? These types of cameras cost thousands of dollars. One of them for Myth Busters cost \$100K! I heard that FAR was going to purchase a high-speed video camera. I'll check.

Anyway, thank you for responding with this suggestion, but seems expensive and would require reviewing the video and more calculations. Still, I will consider it, although I would certainly have to "invest" in another camera.

What I read already involved a high speed camera, for whatever value of high speed was intended. The main point is the scale placed on the rail, and whatever speed camera you can use will have to do.

How many feet per second is the predicted speed? If you have a "regular" 60 fps camera, you can divide the speed by 60 for feet per frame. Somewhat faster than that - 120 at least - isn't too terribly hard to beg, borrow, or steal, and should give you a decent answer.

Remember, motors vary from piece to piece and batch to batch, so it's probably not hard to get the technique's margin of error well below that of the motor. And better than that is pointless.

What I read already involved a high speed camera, for whatever value of high speed was intended. The main point is the scale placed on the rail, and whatever speed camera you can use will have to do.

How many feet per second is the predicted speed? If you have a "regular" 60 fps camera, you can divide the speed by 60 for feet per frame. Somewhat faster than that - 120 at least - isn't too terribly hard to beg, borrow, or steal, and should give you a decent answer.

Remember, motors vary from piece to piece and batch to batch, so it's probably not hard to get the technique's margin of error well below that of the motor. And better than that is pointless.
I use a very convenient app called Video Stopwatch on my iPhone. It allows you to scroll through a video and select two images and then it calculates the elapsed time.

Are TRA practices for leaning rockets different than NAR? I thought the PM was responsible to assure that pads leaned away.
Anyway, no worries. I'm the same way. I even lean my rockets away at FAR when we are in bunkers. I intend to lean my rockets away and have active flight control go to vertical after a predetermined time. Well, at least that is my plan.

Sticking to trying light path detection of rail buttons exiting the rail first. Ordered IR LED and sensor to begin testing. If it doesn't work, I'll switch to video method or something else.

So, Steve, how many days left? Not that you are counting.
NAR practice and TRA practice is identical when it comes to following the special provisions of our COA: do what FAA says.
In all other instances I think our guidelines are pretty much the same. Tilt pads slightly away from the flightline. If rockets go over the spectators on the way up, halt operations, figure out what’s going on, and adjust the range as necessary.
I’m not counting and I’ll really miss being president.

I’m not counting and I’ll really miss being president.
Sorry, just teasing and hope you know that. I haven't been on TRF very long but have seen some challenging threads; actually pretty rough at times. Always appreciated your input and you've done a great job as TRA President. Wondering if we'll see the same level of input here.

Have you ever seen the camera they used for the high speed video on Myth Busters? These types of cameras cost thousands of dollars. One of them for Myth Busters cost \$100K! I heard that FAR was going to purchase a high-speed video camera. I'll check.
Many phones nowadays have slow-motion modes that'll do something like 240 fps. Some will do higher. That should be good enough to get the information you need. I was never thinking of a dedicated high-speed camera like that.

Consider 50 fps speed off the rod. If you use a 1 foot reference marks at the top of the rail, then it would take 1/50 s to traverse it. With a 240 fps camera that'll be about 5 frames, which should be good enough to get a pretty good read on it.

Or something like that.

I created a set-up using an IR break beam sensor and Arduino. The code is done and seems to work. There's a lot involved with this project, but basically looking at simulated vs actual rail exit velocity. Using a 1010 rail (1") which accommodates rockets with 1/4" rail buttons first. Data collected will be mass of the rocket, distance between buttons, motor, and the rail length. And, a basic description of how the rocket flew off the rail. I would like to have a corresponding photo sequence of the rocket exiting the rail, but I would need some help with that. I'm thinking of using a new 6' rail for these tests.

I uploaded the Rocksim file for a couple Madcow rocketry kits and tried some motors. The two kits for example sims I'm using are the Tomach and the Patriot. Both are 38 mm motor mount, but different in airframe diameter, length, and number of fins. For the same motors, I obtain the same rail exit velocity, which makes sense because it's at launch, so different aerodynamic forces are not significant. Correct?

Here's some rail exit velocity data for the Tomach as a 6 lb. rocket (without motor):
I154J 40 fps
I211W 57 fps
I600R 94 fps

If the buttons were 24" apart, the elapsed time would be 50 ms, 35 ms, and 21 ms.

I'm going to get a 8' rail and drop a block of wood with rail buttons 12" apart and see what the measured vs. known free fall velocity is. The break beam sensor will be at the bottom of the rail of course. Maybe electromagnetic release of the block at the top of the rail; that would be fun. This is also where using a camera with elapsed time would be useful. I'll look into the app for my phone.

I did run into an issue with measuring outside in direct Sun because it uses an IR sensor. But, I'm working through that problem. I added some short plastic tubes on the IR source and detector and will try again tomorrow.