Help, what am I doing wrong!

[Lowpuller]

Ventris with E30 Calculations

E30 per NAR E30, 33.6N-sec

20 Ounces estimated weight, equals 1.25 lbs.
1 pound = 0.455Kilograms
Therefore mass or m = 0.569 kilograms

1 Pound Force = 4.448 Newtons

1 G = 9.8m/s^2
6 G needed for a safe launch
Therefore 6 Gs = 58.8 m/s^2 needed for a safe launch

F=ma
m = 0.569 kilograms
a = at 5Gs = 58.8 m/s^2
F = 33.44 kgm/s^2

1 Newton or 1N = 1kgm/s^2

Therefore F = 33.44N, or F needed for a safe launch

Impulse of an E30 = 33.6 N-sec

Thrust or Force or F produced by the E30 at 1/2 second or 0.5 seconds equals:
F = 33.6 N-sec/.5sec = 67.2 N, however per Aerotech's Thrust Curve this is only 44.48 Newtons.

Therefore it appears the E30 has enough Thrust or Force to lift the Ventris, however we also need to know the rocket's speed at the end of the launch rod.

Assuming 5 feet of usable rod or rail or 1.5m of usable rod/rail

Assuming constant Thrust at 44.48 Newtons, F=ma or a=F/m

We now know F=44.48 Newtons and m=0.569 Kilograms so a=44.48N/0.569kg= 78.17m/sec^2

Speed at time, after launch is v=at

Height after time is h=1/2*a*t^2, solve for t, t^2=h/1/2a, t=sqrt(2h/a)

a=78.17m/sec^2, h=1.5m

t=sqrt(2*1.5m/78.17m)

t=0.20 seconds

v=at, a=78.17m/sec^2, t=0.20 seconds

v=78.17m/sec^2 * 0.20 seconds = 15.31 m/s

So it looks like the Ventris will fly as modified, and leave the rail at a decent speed assuming no wind, after I weigh the final build, I plan to see if the E30 will lift an 808 as well.

Thanks to all who help me figure this out, please let me know if anything else needs revision.

 

[shreadvector]

I stopped reading after I read your pounds to kilograms line.

There are 2.2 pounds per kilogram. You have it backwards. Very, very backwards.

No poiint in reading anything past that line.

 

[Rich Holmes]

F=ma
m = 2.75 kilograms
a = at 5Gs = 58.8 m/s^2
F = 44.47 kgm/s^2

2.75 * 58.8 = 161.7

 

[Zebedee]

Yup - 20oz rocket = 0.57kg not 2.75kg

There's an easy short cut I noticed the other day:

For a 5:1 thrust to weight ratio you need a 3:2 ratio of N thrust to Oz weight. E.g. 30N will lift approx 20oz (it's actually 21.4oz but 3:2 keeps it simple).

Since the motor concerned has a higher initial thrust than it's average of 30 you should be fine but, as always, it's good to check the thrust profile during the first few tenths of a second to be sure.

 

[shreadvector]

m does not equal 2.75 kilograms.
This is a fatal and serious error.

2.75 * 58.8 = 161.7

 

[Rich Holmes]

20 oz = 1.25 lb = 1.25 / 2.2 = 0.56 kg mass (not 2.75 kg) or 0.56 * 9.8 = 5.45 N weight

6G requirement -> 6 * 5.45 = 32.7 N thrust (not 44.47). Acceleration is 32.7 / 0.56 = 58.8 m/s^2 = 6 * 9.8 m/s^2

Total impulse of E30 (33.6 N s) is irrelevant, because we're interested only in the thrust in the first fraction of a second. Per the thrust curve at https://www.nar.org/SandT/pdf/Aerotech/E30.pdf in first 0.2 s the average thrust is about 42 N.

If we approximate with constant thrust = 42 N and constant mass = 0.56 kg then acceleration a = 42 / .56 = 75.5 m/s^2.

Then speed at time t after launch is a * t and height is 1/2 a * t^2.

For 1.5 m launch guide, 1.5 = 1/2 * 75.5 * t^2 so t = sqrt (1.5*2/75.5) = 0.2 seconds. Then speed is 75.5 * 0.2 = 15.1 m/s.

You can't use v = d/t because that is valid only for constant speed.

 

[Rich Holmes]

m does not equal 2.75 kilograms.

And even if it did, it wouldn't be 44.47 N thrust.

 

[Lowpuller]

Shreadvector,

Please look again, I couldn't see the forest for the trees.

I'm gonna keep fixing this until I figure it out.

Thank you,

 

[Lowpuller]

Rich,

Thank you for your help, can you clarify a couple of things please?

6G requirement -> 6 * 5.45 = 32.7 N thrust (not 44.47). Got it.

Acceleration is 32.7 / 0.56 = 58.8 m/s^2 = 6 * 9.8 m/s.Ok I need a little help here, I believe your saying a=F/m, I get the left hand side of the formula, and the middle, but you lost me on the right hand side of the formula this just appears to be 6 Gs, what am I missing here

For 1.5 m launch guide, 1.4 = 1/2 * 75.5 * t^2 so t = sqrt (1.4*2/75.5) = 0.2 seconds. Then speed is 75.5 * 0.2 = 15.1. Where does the 1.4 come from, should this be 1.5, for the length of the rail?

Thanks you again,

 

[Rich Holmes]

Yes, the 1.4 should be 1.5. I fixed that in my post above.

32.7 / 0.56 = 58.8 m/s^2 = 6 * 9.8 m/s^2 : 32.7 is the force and 0.56 is the mass; 58.8 m/s^2 is the acceleration. But we got the force in the first place by using 6 times the weight of the rocket, or 6 times the mass times the gravitational constant 9.8 m/s^2. So the 58.8 we got by dividing the force by the mass should just be 6 * 9.8, and indeed it is.

 

[Lowpuller]

Awesome, thanks for the clarification.

I am gonna try to clean my original post up now, hopefully a good example for others once corrected.

And the good news it will fly on an E-30, so I can fly it on my club.

I plan to weigh it once complete and then see if I can lift an 808 also.

Thanks again for all the help!

 

[bobkrech]

Let's start over. You do not understand the difference between acceleration, velocity, and distance.

A model rocket needs a minimum launch velocity, v =< ~32 ft/s (~21 mph) to be aerodynamically stable on a day where the wind is not exceeding 5 mph.

A high power rocket needs a minimum launch velocity, v =< ~48 ft/s (32 mph) to be aerodynamically stable on the same day.

Acceleration is how fast the velocity is changing in a second.

The equation that relates velocity to acceleration is v = a x t which means that the acceleration multiplied by the time is the velocity.

The acceleration of gravity is ~32 ft/s per second. For this special case a = 32 ft/s/s = 1 G. This make it very convenient because G is a dimensionless variable, as it the ratio of any 2 numbers with common units, including the ratio of forces which for rocketry is thrust and rocket weight.

Weight is a force that equals a mass x 1 G acceleration. In the English system, weight is a force measured in pounds (the unit of mass in the English system is a slug.) The force developed by a rocket motor is also measured in pounds in the English system. The thrust to weight ratio of a rocket is the thrust of the rocket motor divided by weight of the rocket, both in pounds. A rocket motor that develops 5 pounds of thrust and weights 1 pound has a thrust to weight ratio T/W = 5.

Once we know the T/W ratio of any rocket, we can easily calculate the a of the rocket and the time it take to reach the minimum v required for a safe launch.

To determine the upward acceleration of the rocket, we need to subtract the weight of the rocket from the thrust, so a in units of G = (T/W) - (W/W) = (T/W)-1.

Our 1 pound rocket launched with a motor developing 5 pounds of thrust accelerates upward at a rate of (5/1) - 1 = 4 G. In real English units that's 4 x 32 ft/s/s = 128 ft/s/s.

As the acceleration a multiplied by the time, t in seconds equals the velocity v. You can determine the time required to reach the velocity by taking the equation v= at and dividing both sides by a. v/a = at/a = t

If your rocket needs to reach v = 32 ft/ and it is accelerating at 128 ft/s/s then the time to reach the velocity is t = v / a = 32 / 128 = 0.25 seconds.

The distance traveled over that time is d = (v/2) x t where v/2 is the average velocity (note the initial velocity is 0 and the final velocity is v so the average is v/2).

For our rocket with a T/W = 5, t was 0.25 seconds, so d = (32 (ft/s)/2) x 0.25 s = 16 ft/s x 0.25 s = 4 ft. The value of this calculation is that the launch rod contact length for this rocket should be at least 4'.

The same problem can be done in metric units. The unit of mass is the kilogram, The unit of length is the meter, the unit of mass is the kilogram, and the unit of t of time is still seconds. (This is know as the MKS system.) In the MKS system, the Newton is the unit of force. Weight is measured in Newton and so is the rocket thrust. The equations are the same as in the English system, but the numeric values are different. G in metric units is 9.8 m/s/s

The conversion factors are 1 pound force = 4.45 Newton and 1 Newton = (1/4.45) pound force = 3.6 ounce force.

So let's solve your problem.

Your rocket weighs 1.25 pounds. You have a 4 foot launch rod and you want your rocket to have a thrust to weight ratio of 5:1 to accelerate upward at 4 G and you have a 4' launch rod. Your motor needs to generate an average of 5 x 1.25 pounds of thrust = 6.25 pounds = 6.25 pounds x 4.45 Newtons/pound= 27.8 Newtons.

In theory a motor with a 30 N thrust should work with this rocket, if the thrust is applied for a sufficient length of time for it to reach a safe apogee where a recovery device can be deployed to land it at ~16 ft/s velocity.

The question you need to ask is whether the rocket is accelerating long enough to reach a safe apogee and deploy a recovery device to allow it to descend at ~ 16 ft/s.

In round numbers, the E30 has a total impulse = 34 N-s. Since you have the average thrust and the total impulse, you can estimate the burn time if it is an ideal motor. t = TI/T = 34/30 = 1.13 seconds. Your rocket has T/W =30N/(1.25lb*4.45lb/N)= 30/5.6 = 5.7. It accelerates upward at (5.7-1)= 4.7 G for 1.13 seconds. This yields a burnout velocity = 4.7 x 32 x 1.13 = 170 ft/s at a height of ~ (170/2)x1.13= 96 feet. In the ideal world, where there is no atmosphere, you rocket is only slowed by gravity. The coast time is 170 ft/s/ 32 ft/s/s = 5.3 s during which you would have gained an additional (170/2) x 5.3 = 450 ft of altitude for an apogee of 96 + 450 = 544 ft. If your parachute deployed perfectly at apogee it would take 544 ft / 16 ft/s = 34 seconds to reach the ground from apogee.

Sounds good, and it's actually too good. We live in a real world where your loaded rocket probably weighs more than the Estes estimate, where the thrust curve is not a square pulse so you have additional gravity losses because the real burn time is longer, there is an atmosphere and your rocket has drag meaning it will not reach the burnout velocity and altitude you calculated, nor will it reach the apogee you calculate because drag is greater than gravity. Because of this the ejection delay needs to be shorter than you calculated.

The simple simulator at ThrustCurve.org says the motor does not provide enough velocity off the rod for stable flight, but the criteria they use is 50 ft/s which is a bit high. It predicts and an apogee of 465' and a required ejection delay of 4 seconds assuming the dry weight is 20 oz.
.
If your rocket weight increases to 1.33 pound, the apogee drops to 390' and the delay to 3.9 seconds, so you would select an E30-4 motor.

You can launch this rocket/motor combo straight up, but you should only launch it if the wind does not exceed 5 mph. The rocket will weathercock in a high wind and reach a lot lower altitude and you may have recovery issues.

You would have a better flight with a 29 mm F40+ motor than a 24 mm E30 because the higher thrust and apogee potential.

Bob

 

[Lowpuller]

Thanks Bob,

The good news, I'm launching off a 6' rail, that I have already derat
ed to 5'.

I have also started a majority of the build and my weight includes all components, I even added washers to the scale compensate for remaining putty and paint. I also guessed high when adding washers.

The bad news, our site is limited to E Engines.

The really bad news, I'll probably hang an 808 on it, if the numbers work out, 1st flight without camera of course just to verify.

I do have a question about the minimum velocity numbers you quoted for MPR and HPR rockets, aren't both of these numbers largely dependent of the cross sectional area of the rocket, I particular fin size.

And lastly, can you recommend a manual method for calculating CP? I am familiar with estimating cross sectional area, but wondered if you know a better method. It's a long story but I'm limited to an iPad and iPhone so Open Rockets is out. I'm getting CG using a wedge, crude but effective. I have heavily modified the motor mount and I am confident I have brought the CG far lower than original design.

Thanks for your input.

 

[CarVac]

Thanks Bob,

The good news, I'm launching off a 6' rail, that I have already deranged to 5'.

I have also started a majority of the build and my weight includes all components, I even added washers to the scale compensate for remaining putty and paint. I also guessed high when adding washers.

The bad news, our site is limited to E Engines.

The really bad news, I'll probably hang an 808 on it, if the numbers work out, 1st flight without camera of course just to verify.

I do have a question about the minimum velocity numbers you quoted for MPR and HPR rockets, aren't bother of these numbers largely dependent of the cross sectional area of the rocket, I particular fin size.

And lastly, can you recommend a manual method for calculating CP? I am familiar with estimating cross sectional area, but wondered if you know a better method. It's a long story but I'm limited to an iPad and iPhone so Open Rockets is out. I'm getting CG using a wedge, crude but effective. I have heavily modified the motor mount and I am confident I have brought the CG far lower than original design.

Thanks for your input.

It's called using OpenRocket, Rocksim, or RASAero, and marking it on your rocket with a Sharpie. Unlike CG which changes, and also can be determined at the field by balancing e.g. on a pencil, that won't really change and so you can mark it permanently on your rocket.

 

[Rich Holmes]

And lastly, can you recommend a manual method for calculating CP? I am familiar with estimating cross sectional area, but wondered if you know a better method. It's a long story but I'm limited to an iPad and iPhone so Open Rockets is out. I'm getting CG using a wedge, crude but effective. I have heavily modified the motor mount and I am confident I have brought the CG far lower than original design.

Have you modified the shape of the rocket? If not then the CP should be in the same place as a stock Ventris, which it says here is 38.7 inches from the front for what it's worth.

If you have, and if you really have no access to a computer, then you're back to doing it the way people did it "back then": silhouette balancing, swing testing, and/or calculating it by hand. The Barrowman formulas are messy but can be computed with pen, pencil, and calculator. See here. Actually at the bottom of that is a spreadsheet, and if you can get that into a spreadsheet app on your iPad, you can do it that way.

 

[matthewdlaudato]

FWIW, I never use pounds or ounces (old habit from having 2 degrees in Physics). I have a decent digital scale that displays mass in grams to get accurate data on my rockets (which I typically put on the scale only after construction and painting are complete). If you ask me how much my rocket weighs, I will invariably give you a funny look, and respond "well, I don't know. But its mass is X".

Since motors in our hobby are specified in Newtons and Newton-seconds, for thrust and total impulse, respectively, T/W can be calculated immediately without conversion:

T/W = (rated thrust of motor)/mg = (rated thrust of motor)/(m*9.8 m/s/s)

If you're mostly concerned with the first few milliseconds of the flight (rod clearing time), look at the thrust curve and determine the thrust delivered in (say) the first 200ms and use that as T. As Bob K mentions, the curve is never a clean square pulse.

 

[bobkrech]

Thanks Bob,

The good news, I'm launching off a 6' rail, that I have already derat
ed to 5'.

I have also started a majority of the build and my weight includes all components, I even added washers to the scale compensate for remaining putty and paint. I also guessed high when adding washers.

The bad news, our site is limited to E Engines.

The really bad news, I'll probably hang an 808 on it, if the numbers work out, 1st flight without camera of course just to verify.

I do have a question about the minimum velocity numbers you quoted for MPR and HPR rockets, aren't both of these numbers largely dependent of the cross sectional area of the rocket, I particular fin size.

And lastly, can you recommend a manual method for calculating CP? I am familiar with estimating cross sectional area, but wondered if you know a better method. It's a long story but I'm limited to an iPad and iPhone so Open Rockets is out. I'm getting CG using a wedge, crude but effective. I have heavily modified the motor mount and I am confident I have brought the CG far lower than original design.

Thanks for your input.

The minimum launch velocity is a function of many variable that I have yet to see anyone calculate in detail, but as an educated guess, the more stable the rocket, the lower the minimum launch velocity should be.

As was previously mentioned, unless you changed the shape of the rocket, the CP will not change. Adding weight to the rear end of the rocket is a bad thing because you move the CG aft, and reduce the CG/CP ratio. If the ratio of the CG-CP separation distance divided by the airframe diameter is below 1 for this rocket with the motor loaded, don't launch it or you are likely to have a bad day. When this happens, you are stuck adding weight to the nose to get the ratio to 1 or greater. Since you are stuck with E motors, you do not have much leeway.

You should download and use Open Rocket or another simulation program. You need to make an accurate drawing to get a good value for CP, but you can use the mass override feature and actually weigh the assembled rocket, and manually determine the CG and input these values manually into the simulation package. (Saves a lot of time.) If you do this for the rocket without the motor installed, and then repeat it with the motor installed, you can then load the data for the empty rocket into the sim, load in the motor using the sim and make sure the new sim calculated CG matches your measured CG value for the rocket loaded with the motor.

https://www.rocketmime.com/rockets/rckt_eqn.html is an excellent website that shows the math behind the sims in a manner most folks will understand.

Bob

 

[blackbrandt]

The minimum launch velocity is a function of many variable that I have yet to see anyone calculate in detail, but as an educated guess, the more stable the rocket, the lower the minimum launch velocity should be.

I have to say, I think this statement would be better stated by replacing should with can. You always want the launch velocity to be as high as possible. "Should" implies that you want it to be lower if it is more stable.


Matt

 

[shreadvector]

As a 'rule of thumb' statement, the word "can" is correct.

As a comment on the expected results of the theoretical calculation or formula for minimum launch velocity, the results given by that calculation 'should' be lower for a more stable rocket.

I have to say, I think this statement would be better stated by replacing should with can. You always want the launch velocity to be as high as possible. "Should" implies that you want it to be lower if it is more stable.


Matt

 

[dward]

There is an easy way to bypass all the pounds and ounces confusion. Here is a calculation for the maximum mass for a rocket with 30 Newtons of thrust using a 5:1 ratio: https://www.google.com/search?q=53+newtons%2F5*9.8+m%2Fs%5E2+&oq=53+newtons%2F5*9.8+m%2Fs%5E2+&aqs=chrome..69i57j0.60925j0j7&sourceid=chrome&espv=210&es_sm=119&ie=UTF-8#q=30+newtons%2F(5*9.8+m%2Fs%5E2)+in+grams

Here it is again using dynes for force and carats for mass: https://www.google.com/search?q=53+newtons%2F5*9.8+m%2Fs%5E2+&oq=53+newtons%2F5*9.8+m%2Fs%5E2+&aqs=chrome..69i57j0.60925j0j7&sourceid=chrome&espv=210&es_sm=119&ie=UTF-8#q=3e6+dynes%2F(5*9.8+m%2Fs%5E2)+in+carats

If you want more information about this wonderful tool then you can read this article: https://scitation.aip.org/content/aapt/journal/tpt/43/6/10.1119/1.2033529
If you don't want to pay to read it, then you can always download the preprint for free: physicsthegoogleway.davidward.org/&#8206;

Play around with it and see if you can find my easter egg!

 

[Rich Holmes]

Huh, I use Google as a calculator all the time, but didn't know it knew all those physical constants. Thanks!

Kind of lame that it doesn't seem to know a name for 9.8 m/s^2 though.

 

[blackbrandt]

It does. Type in (for example) 8 g's to ft/s^2.

You're welcome.

 

[Rich Holmes]

But it doesn't do 2 N / g. Or rather it does, but in that context it interprets g as grams and spits back 2000 m/s^2 instead of 0.204 kg.

 

[pythonrock]

FWIW, I never use pounds or ounces (old habit from having 2 degrees in Physics). I have a decent digital scale that displays mass in grams to get accurate data on my rockets (which I typically put on the scale only after construction and painting are complete). If you ask me how much my rocket weighs, I will invariably give you a funny look, and respond "well, I don't know. But its mass is X".

Since motors in our hobby are specified in Newtons and Newton-seconds, for thrust and total impulse, respectively, T/W can be calculated immediately without conversion:

T/W = (rated thrust of motor)/mg = (rated thrust of motor)/(m*9.8 m/s/s)

If you're mostly concerned with the first few milliseconds of the flight (rod clearing time), look at the thrust curve and determine the thrust delivered in (say) the first 200ms and use that as T. As Bob K mentions, the curve is never a clean square pulse.

FWIW Sorry but you have it backwards. An old balance scale will give you mass ( it compares one mass with another and so is not affected by G). All other scales with springs or piezo elements. which is certainly what your digital scale has, actually measure the force an object places on them. If you took it to the moon the measurement would be much less but, of course, the mass has not changed. It reads grams because the manufacturer assumes you will be using it where G=1 exactly and people are not familiar with newtons.

 

[Rich Holmes]

FWIW Sorry but you have it backwards. An old balance scale will give you mass ( it compares one mass with another and so is not affected by G). All other scales with springs or piezo elements. which is certainly what your digital scale has, actually measure the force an object places on them. If you took it to the moon the measurement would be much less but, of course, the mass has not changed. It reads grams because the manufacturer assumes you will be using it where G=1 exactly and people are not familiar with newtons.

As long as we're being nitpicking pedants, Newton's constant G = 6.67e-11*N·(m/kg)^2 everywhere. g = 9.8 m/s^2 is the gravitational acceleration on Earth; of course the gravitational acceleration on the Moon is smaller.

But even I don't go around pretending not to know "weigh" can mean "measure the mass of" in everyday usage just to score nitpicking pedant points.

 

[Lowpuller]

Geeeeeesh! I just asked a simple question about making a toy rocket fly and you guys have brought all this physics and mathematics crap into it!!

Just joking by the way I love it.

Now leave me alone I'm going to the moon with my battery powered digital scale to weigh my rocket so it will weigh less and fly higher on a smaller motor. Remember I am restricted to E engines.

 

[Rich Holmes]

No FAA waiver required!

 

[scsager]

The bad news, our site is limited to E Engines.

I like math, and I enjoy solving a good problem, however, It's possible to get a good estimate of the Ventris performance without doing any math.

Estes does test fly their kits with various motors. They do this because they want you to have a good experience. They publish a list of recommended motors for each kit.

Could your Ventris fly on an E30? Maybe it could, but I (personally) would not try it on the first flight. Find a bigger/better launch site.

This from the Ventris page @ Estesrockets.com

Technical Specifications
Ventris Pro Series II

Length: 46.25 in. (117.5 cm)
Diameter: 2.5 in. (6.4 cm)
Estimated Weight: 15.6 oz. (442.3 g)

Laser cut Plywood Fins, Screw on Motor Retainer, 24 in. (61 cm) Nylon Parachute Recovery

Recommended Engines: F26-6FJ, F50-6T, G40-7W, G80-7T
.

 

[Lowpuller]

Scott,

Where is the excitement in that?

 

[matthewdlaudato]

FWIW Sorry but you have it backwards. An old balance scale will give you mass ( it compares one mass with another and so is not affected by G). All other scales with springs or piezo elements. which is certainly what your digital scale has, actually measure the force an object places on them. If you took it to the moon the measurement would be much less but, of course, the mass has not changed. It reads grams because the manufacturer assumes you will be using it where G=1 exactly and people are not familiar with newtons.

You are correct, I use grams out of habit, but the scale I'm using is providing mass via a conversion based on force, not via comparison to a known mass.