Rocket flight physics

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First of all, nice page, nice graphics----you seem to have a pretty good discussion of the main factors. Then again, I am an engineer and I am kinda used to these eqns; you might need to include some baby steps (see below).

You might add that you are assuming that the rocket flies straight up as a simplifying step, and that there is no wobble from instability.

Picking a nit: is the average thrust of the motor 0.8N or is that the "simplified" value of the sustaining thrust? Calculatn of the average is going to involve integration of the area under the curve, which will be way above the heads of many readers. I use a trapezoidal representation of the time-thrust curve with about 100+ time slices (I use shorter time intervals for each step during the initial thrust peak; spreadsheets are your friend) and generally get within 1% of the advertised impulse values. You may want to mention that the start-up thrust does not count until the motor gets beyond a certain point on the curve (5% of max thrust? --not sure on that one)

On your plot of inflight forces, what are the red, blue, and green lines for? You need to step the readers through all the steps of the process.

In your calculations, it could be helpful to the "average guy" to see on the page some actual numbers put into the equatns you start with, and how to convert them to the correct units of measure for input. Again, baby steps.

Or maybe not. I have a tendency to over-explain things (I am a firm believer that a very large part of the average audience is too embarrassed to ask questns).
 
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powderburner said:
On your plot of inflight forces, what are the red, blue, and green lines for? You need to step the readers through all the steps of the process.
Click to expand...
Red = thrust, blue = drag, green = gravity. See the section titled "All together now". Or put your cursor onto the graph and look at the readings. ;)

My first question is, has anybody other than Thrustcurve.org heard of a Quest 13mm x 55mm A3T? Quest Aerospace do not appear to recognise it.

If it does exist and you put it into a Mosquito, it will hang out of the back about 1cm more than the 44mm long Estes A3-4T - what will that do to the rocket's stability?
The page also allows you to put a C6 into a Mosquito. If you try that in reality, the Mosquito will probably not go very far due to being seriously deformed. :) (It's already deformed. The page gives it a diameter of 19mm and a mass of 2g. Estes gives it a diameter of 14mm and a mass of 3g.)
Likewise, the page allows you to put an A3 into an Alpha. Unless you add extra mass to allow for some sort of adaptor, that won't go very far either as the motor will fall out. :D (The adaptor's mass does seem to be included. The page gives the Alpha a mass of 38g. Estes gives it as 23g. Even the plastic-finned Alpha III is only 34g.)

So much for all the minor nits, which the average reader won't notice and which don't really detract from the overall purpose of showing basic rocket science to a novice. This it does very well, with a nice simple explanation, interactive graphs, and links to further reading for those whose interest has been raised reading the basics here. Well done! :clap:
 
Maybe on the force discussion you might mention that 1 lbf = 4.448 newtons. Perhaps, it was there and I missed it. Otherwise, this would be a quick way for a user to get the force in pounds, if they want it. I grew up in Engineering more than 40 years ago using gc in English units for force calculations. The gc concept can be confusing and I think that beginning physics courses now avoid it and do force calculations in the metric system, hence, forces are in Newtons.
 
This looks really cool. Would you consider expanding the JavaScript to include rockets beyond the Mosquito and Alpha? Or possibly even a "user added" calculation?

The optimal delay is also very interesting...
 
Thanks for the comments guys; I've made a few updates:
  • mentioned the conversion between N and lbf, with a link to an on-line converter,
  • replaced the bogus Quest motor with an Estes C6,
  • added a "Method" sub-section that goes through the first two steps of the simulation,
  • added a legend to the "In-Flight Forces" graph, and
  • added a link to the Wikipedia page on integration.
Note that this is a toy simulator for illustrative purposes only. I do not intend to expand it, as there are already good flight simulators available. My goal is to explain how they work and most importantly to help people understand the inputs and results.
 
Nice introduction. You do some good build videos. Clear and concise. Maybe consider adding a short video walking through the forces and equations. One nit; you explain how force of gravity changes with mass, but miss the opportunity to demonstrate that in your Java SIM as propellant is used during the motor burn. Would be hard to see anyway at the scale shown, I suppose.
 
Just a small suggestion, when you mention the mass at the beginning you put weight in parenthesis to describe the mass. Weight is a force so you may want to consider removing that line.
 
adrian said:
Red = thrust, blue = drag, green = gravity. See the section titled "All together now". Or put your cursor onto the graph and look at the readings.
Click to expand...

I was trying to encourage him to provide more complete documentation on the page, without the user having to mouse or click various items. You know, the "conventional" way to present a chart.
 
My own experience when interacting with people wanting to understand these equatns is that most folks get tripped up over proper use of units. To engineers, this is as simple as breathing. To other folks, it is a great mystery.

Thus my comment about showing the step-by-step through a sample calculation. I would also suggest showing the conversions between Imperial and Metric units of measure, such as if the rocket was weighed in ounces, the gravitational constant was metric, and the motor data was in the typical Estes mix of both. Many people need to learn (re-learn?) the gory details of how to get all the data lined up in consistent units or else the equations only produce gibberish.
 
WRosales said:
Just a small suggestion, when you mention the mass at the beginning you put weight in parenthesis to describe the mass. Weight is a force so you may want to consider removing that line.
Click to expand...

However, it hurts people's heads to try to work with concepts like pound-force and pound-mass. If you are going to work in the Imperial system I strongly recommend using pounds for force and slugs for mass. The downside of this is: since so many others in the rocketry world seems to use "pounds" for everything, you are only going to educate/train your students in the proper scientific use of these terms, and then when you turn them loose in the world they will get confused all over again when they run into all these other folks.

There is a reason why it's called rocket science, not rocket sloppy.
 
powderburner said:
However, it hurts people's heads to try to work with concepts like pound-force and pound-mass. If you are going to work in the Imperial system I strongly recommend using pounds for force and slugs for mass. The downside of this is: since so many others in the rocketry world seems to use "pounds" for everything, you are only going to educate/train your students in the proper scientific use of these terms, and then when you turn them loose in the world they will get confused all over again when they run into all these other folks.

There is a reason why it's called rocket science, not rocket sloppy.
Click to expand...
I say just "ok, if you're used to thinking in imperial units, here's the conversion. But do your math in SI."
 
Absolutely the best simple explanation of a rocket flight simulation program I have ever seen.

:wave::wave::wave:

I really like the simplicity of the explanations. They're short and to the point. I actually like the links to Wiki because you don't have to clutter the page with the details but they're for anyone who wants to look at them.

The graphics are cool. One option might be to have a metric/imperial option switch and possibly a non-dimensional option switch for the plot labels.

metric units are: m/s, m/s2, and s;, imperial unit are: ft/s, ft/s2, and s; and non-dimensional unit are Mach and g.

What I don't know is whether the non-dimensional unit option may cause confusion and distract from the excellent presentation.

Bob
 
Under thrust it says 1.9 sec burn with a high of 14 N etc. That doesn't match the graph that is shown.


Under gravity it says m is the remaining mass of the rocket. I would drop the word remaining unless you want to bring in a discussion of the changing motor mass.
 
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Thanks for the comments; I've made some additional updates:
  • More discussion of units at the top
  • I had forgotten to upload the correct thrust curve image
  • Added a mention of calculating Fg and remaining mass
  • Acceleration, velocity and altitude chart tool tips show S.I. and imperial units
  • Slightly reworked the method to use force more consistently
 
John,

As with all your rocketry projects, I like this a lot, but I have a minor bone to pick with the statement "CD is the coeficient of drag, essentially a fudge factor."

In my experience the most difficult concept to grasp by many folks using simulation software is the meaning and importance of Cd. They don't understand that knowing the actual Cd of the rocket they build is vital to accurate simulations. Since most of the simulations they do are of rockets that are only in the planning stages, the Cd used by the sim program is a generic Cd which is in the midrange of what a typical simple rocket might have. It is simply a guesstimate of this crucial number. But real Cd's can vary widely, so the sim with a generic Cd is often way off when it comes to calculating the altitude of a rocket which has actually been built.

I can't tell you how many times I've heard people say that the sim program was terrible in predicting the altitude of their rocket even though they plugged in all the actual weights and sizes of all the components. They have no idea how crucial knowing the actual Cd is. Additionally, most general users of sim software have no idea they can measure and calculate the real Cd of their rocket, or that they can then plug that number back into the simulator for far more accurate simulations.

The unfortunate thing about this is that except for choosing a motor with more impulse, choosing the slowest motor possible, and building a rocket of minimum diameter, virtually everything else you can do to make your rocket go higher involves tweaking the Cd. So if you are interested in altitude, understanding Cd is vital.

Yes, most sim programs "fudge" the Cd because they have no idea what the real Cd will be. But the number itself is anything but a "fudge factor." It's a vital numerical descriptor of how well you've designed and built your rocket.

Steve (who spends way too much time with his teammates living in the incredibly complicated world of Cd trying to represent the U.S. better in altitude events)
 
I've used the term "fudge factor" here regarding Cd but didn't mean the whole term, maybe it stuck. Cd is sometimes given to be used with just the diameter of the rocket, not the entire frontal area. In that case, Cd(dia) = Cd(area) x fudge, where the fudge is a number greater than one which corrects for the difference between the actual frontal area and the area of the body diameter. "Fudge" is just another coefficient, really, but not being consistent about it lets in extra messiness. It is valid to use Cd(dia) in the sense that if you measure the actual altitude of a rocket, and then find the Cd that produces that altitude based on diameter area, it will work for simulations of that design using body diameter. But you can't drop it into the equation using actual frontal area and get correct results, and it causes extra confusion and work to guesstimate Cd for an unusual design. It's also one reason why stated Cd's for rockets often run almost double the Cd's of basic aerodynamic shapes.

As to the article, I like the "All Together Now" section, but didn't read the warning to not let the equations freak you out the first time ...
 
Related, is there a sim software where I can enter a Cd override? Seems likely useful for rockets that are a bit un-Barrowman.
 
Gus said:
... I have a minor bone to pick with the statement "CD is the coeficient of drag, essentially a fudge factor."
Click to expand...
You are absolutely right; Cd is critical to getting accurate simulations. I've struggled in the past with how to describe it so, but haven't yet found one.

From Wikipedia:
"The drag coefficient of any object comprises the effects of the two basic contributors to fluid dynamic drag: skin friction and form drag. The drag coefficient of a lifting airfoil or hydrofoil also includes the effects of lift-induced drag. The drag coefficient of a complete structure such as an aircraft also includes the effects of interference drag."

I'd love a short (1 paragraph) that gave a better explanation, but haven't yet found one. (Arguably, most definitions are equivalent to "fudge factor" anyway: "dimensionless quantity that is used to quantify ...".)

If you know of a good description, I'd love to summarize it in the article and add a link in the Further Reading section.


bill_s said:
Cd is sometimes given to be used with just the diameter of the rocket, not the entire frontal area.
Click to expand...
I show calculating the frontal area (A) separately from the Cd since it's simple and completes the drag equation.
 
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You've listed mainly wiki sources for "Further Reading" but you might consider adding Estes TR-10 and TR-11, both available for download on YORP

https://www.oldrocketplans.com/pubs/Estes/estTR-10/TR-10.pdf

https://www.oldrocketplans.com/pubs/Estes/estTR-11/TR-11.pdf

from TR-10 "CD is a dimensionless “aerodynamic drag coefficient” that depends upon the shape and the surface smoothness of the object."

from TR-11 "One factor that needs a little explanation is the drag coefficient, CD. This term has no dimensions; it is simply a number used to describe how the shape of the body and its angle to the wind influence drag. All shapes that move through the air possess drag coefficients: your hand, autos, airplanes. and, of course, your model rockets. If we can find the value of CD for a rocket, we'll be able to compute its actual aerodynamic drag in pounds or grams."
 
rocketguy101 said:
You've listed mainly wiki sources for "Further Reading" but you might consider adding Estes TR-10 and TR-11, both available for download on YORP
Click to expand...
Those are really helpful. I've removed the offending "fudge factor" and used parts of the two descriptions you quoted. Plus I added links a those Estes technical reports.
jcrocket.com/flight-physics.shtml#drag
 
Nice summary John.

Similar to yours, I found these pages written by Randy Culp helpful starting point when I was putting together my own software a while back; they might make some additional fodder for your Further Reading Section:
https://www.rocketmime.com/rockets/rckt_sim.html
https://www.rocketmime.com/rockets/rckt_eqn.html

The first is similar to yours presented perhaps a bit differently, while the second has some nice links and presents equations for a less accurate, but non-iterative solution for altitude and max velocity based on some simple approximations (Fehskens-Malewicki solution).
 
Nice work, John, especially on the graphics. Physics can be a tough sell, I know — I've been teaching college Physics since I retired in 2009 — and maybe you could explain to readers that "ma" means mass m multiplied by acceleration a, because (in my experience) a lot of them don't remember (or never learned) their Algebra.
 
AndyC said:
I found these pages written by Randy Culp helpful starting point when I was putting together my own software a while back
Click to expand...
Thanks, those are great pages, and I added links in my references section.

Professor_Mant said:
Nice work, John, especially on the graphics. Physics can be a tough sell, I know — I've been teaching college Physics since I retired in 2009 — and maybe you could explain to readers that "ma" means mass m multiplied by acceleration a, because (in my experience) a lot of them don't remember (or never learned) their Algebra.
Click to expand...
For that first equation, I spelled it out in words. Does this explanation really require algebra? I organized things so that the unknown was always on the left and the right-hand side was simple arithmetic.
 
Better references than Wikipedia:

For everything you want to write about:

https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/shortr.html

It would be great if you could put some of the Glenn Research Center content into a more easily accessible form. The speaker's notes under the slides are poorly formatted for reading, and the whole site is hard to navigate.

-- aerodynamic drag for people in hurry

https://physics.info/drag/

-- the deep dive into drag
https://hoernerfluiddynamics.com/table-of-contents-fluid-dynamic-drag-1965-edition/
(pirated PDFs of this are available online, but buy a copy)

-- an outline of Newtonian Mechanics
https://hyperphysics.phy-astr.gsu.edu/hbase/hph.html#mechcon

Some quibbles:

The force of gravity is the attraction between bodies with mass, which are separated by some distance. Weight is the force exerted on one of the bodies due to the gravitational attraction between the bodies. If the distances over which the bodies move towards, or away from each other are small compared to the distances which separate the centers of mass of those bodies, then we may treat weight acting on any of those bodies as a constant force and associate it with a constant acceleration g. That is to say, g is the acceleration due to gravity and mg is weight -- neither of these quantities is "gravity".

Edit: HOLY COW -- necro-thread. Have to remember to scroll UP before posting. I stand by my reply though. If John Coker decides to revisit this project, I'd be happy to pitch in.
 
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Regarding the elusive Cd or drag explanation, you could mention that there are a variety of theoretical mathematical methods for predicting some of the aerodynamic drag forces and there are experimental methods such as measuring those drag forces in a wind tunnel. Published databases of drag predictions and measurements are used by the engineer as estimates for similar bodies in similar flight conditions. The non-dimensional drag coefficient is a useful way to scale drag forces to different sized bodies in different flight conditions in a consistent manner.

Now, I see the bit about the zombie thread.
 

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