# Test Post

### Help Support The Rocketry Forum:

Test Post

#### mrblack859

##### Member
Does anyone know how the equation for the area of a nozzle throat is derived? Equation configured for imperial pounds depicted below.

#### Gary Byrum

##### Overstable By Design
Post tested.....Kellogg's tested and General Mills tested sir. All checks out. Everything a cereal killer could ask for.

#### Incongruent

##### Well-Known Member
Does anyone know how the equation for the area of a nozzle throat is derived? Equation configured for imperial pounds depicted below.
It is derived by locking a bunch of smart monkeys with physics degrees in a room.

#### mrblack859

##### Member
It is derived by locking a bunch of smart monkeys with physics degrees in a room.
I don't have any monkeys!

#### Incongruent

##### Well-Known Member
Post tested.....Kellogg's tested and General Mills tested sir. All checks out. Everything a cereal killer could ask for.
I hear that those cereal killers are a violent brunch.

#### Incongruent

##### Well-Known Member
I don't have any monkeys!
Evolved monkeys.

Oh and why not?
Did they go bankrupt?
Yes?

Good. That means that there will be no more monkey business.

#### fyrwrxz

##### latest photo
Finally- a REAL 'test post'...no seriously-he's testing us! I, myself, have never seen a square root and I've core sampled a lot of soils....

#### Incongruent

##### Well-Known Member
Oh and in case you haven't picked it up, I'm joking. Sooner or later (but you don't get to choose) someone who can answer the question probably will.

Also, welcome to the forum. You'll like it here... if you don't dislike or feel neutral to it.

#### Incongruent

##### Well-Known Member
Finally- a REAL 'test post'...no seriously-he's testing us!
But I didn't get a review!

Oh, and...

"Is this for a grade?"

#### mrblack859

##### Member
Oh and in case you haven't picked it up, I'm joking. Sooner or later (but you don't get to choose) someone who can answer the question probably will.

Also, welcome to the forum. You'll like it here... if you don't dislike or feel neutral to it.
I'm liking it already!

#### BuiltFromTrash

##### Well-Known Member
Lol Incongruent.

mrblank859 I am guessing you have friends on here that you want to talk to outside of club launches, eh? None the less, have fun!

#### mrblack859

##### Member
Lol Incongruent.

mrblank859 I am guessing you have friends on here that you want to talk to outside of club launches, eh? None the less, have fun!
I've never been to a club launch! I've never built my own rocket either. I'm reading and learning as much as possible right now. I just started a rocketry club with some friends at my university, and we have designed a 3in mid power rocket that uses a 29mm cesaroni (one grain for first launch). We will be building it this spring and hopefully launching it in Sylacauga, AL! We hope to eventually design and build our own liquid bipropellant engine in a few years. I'm trying to learn all the hard maths right now. I don't know anyone in the rocketry hobby yet. :cry:

#### dhbarr

##### Amateur Professional
I've never been to a club launch! I've never built my own rocket either. I'm reading and learning as much as possible right now. I just started a rocketry club with some friends at my university, and we have designed a 3in mid power rocket that uses a 29mm cesaroni (one grain for first launch). We will be building it this spring and hopefully launching it in Sylacauga, AL! We hope to eventually design and build our own liquid bipropellant engine in a few years. I'm trying to learn all the hard maths right now. I don't know anyone in the rocketry hobby yet. :cry:
http://www.nar.org/find-a-local-club/nar-club-locator/

http://www.tripoli.org/Prefectures/mapID/5

Cheapest midpower is 29/40-120 ; if you agree to attend a club launch as a group, I'll send you a free complete casing. Wildman has reloads for $10-15 / flight. I can also recommend Loki 38/120 casing, three loads for that which require no hazmat fees or certs. You have to provide your own igniters, though. Enjoy! #### jlabrasca ##### Well-Known Member Peach Tree City... Go Wolves? Go whatever the team mascot of Clayton State is? I've never been to a club launch! I've never built my own rocket either. I'm reading and learning as much as possible right now. I just started a rocketry club with some friends at my university... I am new here too, so I won't pile on with the hazing -- NASA is probably as good a place to start as any, if you don't have a fluids or stat. mech professor you can bother https://www.grc.nasa.gov/www/k-12/airplane/astar.html I am wondering, though, if this is homework from the club's faculty advisor? #### mrblack859 ##### Member #### mrblack859 ##### Member Also here is our rocket design. The OpenRocket file and the part spreadsheet are the only things that matter. The fin template is out of date, and the rock design word doc is just brain storming junk lol. #### mrblack859 ##### Member http://www.nar.org/find-a-local-club/nar-club-locator/ http://www.tripoli.org/Prefectures/mapID/5 Cheapest midpower is 29/40-120 ; if you agree to attend a club launch as a group, I'll send you a free complete casing. Wildman has reloads for$10-15 / flight.

I can also recommend Loki 38/120 casing, three loads for that which require no hazmat fees or certs. You have to provide your own igniters, though.

Enjoy!
I've contacted the Atlanta Tripoli Prefecture and they said launching with Phoenix Missile Works out of Sylacauga, AL is our best bet. If you see my rocket design post in this thread, you can see the motor case we are planning on using, as well as the rest of our rocket! We would love all the help you have to offer!

#### jlabrasca

##### Well-Known Member
Here is what I have assembled so far...Part of our google drive
So you know about the educational stuff from GRC, which makes your formula "configured" for imperial units just a little more weird.

What, exactly, are you trying to ask and how much math ("maths" -- are you an exchange student?) do you have?

If you've got undergraduate mechanics and a term or more of Diff Eq behind you, you can go back pretty far in the derivation. If you've passed an upper division fluids or thermo class, the derivation might make sense to you. If you just want to know what the terms in the equation mean, then reading through the NASA site will provide a more complete answer than you're likely to get here. You'll have to do some algebra to match it up with whatever source gave you the equation in the top post, but all of the terms are defined in the GRC slides (and in the badly formatted notes below the slides).

Last edited:

#### mrblack859

##### Member
So you know about the educational stuff from GRC, which makes your formula "configured" for imperial units just a little more weird.

What, exactly, are you trying to ask and how much math ("maths" -- are you an exchange student?) do you have?

If you've got undergraduate mechanics and a term or more of Diff Eq behind you, you can go back pretty far in the derivation. If you've passed an upper division fluids or thermo class, the derivation might make sense to you. If you just want to know what the terms in the equation mean, then reading through the NASA site will provide a more complete answer than you're likely to get here. You'll have to do some algebra to match it up with whatever source gave you the equation in the top post, but all of the terms are defined in the GRC slides (and in the badly formatted notes below the slides).
I've taken multivariable calculus and linear algebra. No fluid mechanics or thermo classes. I found a source that provides the equation that is configured for SI units. I read pretty thoroughly what each term of the equation means so that I can calculate and plug in all the variables, but I don't know how the formula was made, which is what I'm interested in. I've read through a fluid mechanics forum to try to find good starting material as an introduction into fluid mechanics. If you have any material to recommend, do tell! Also, I'm not an exchange student

#### jlabrasca

##### Well-Known Member
I've taken multivariable calculus and linear algebra. No fluid mechanics or thermo classes. I found a source that provides the equation that is configured for SI units. I read pretty thoroughly what each term of the equation means so that I can calculate and plug in all the variables, but I don't know how the formula was made, which is what I'm interested in. I've read through a fluid mechanics forum to try to find good starting material as an introduction into fluid mechanics. If you have any material to recommend, do tell! Also, I'm not an exchange student
Clicked on your rocket design link -- guess its "Go Eagles!" then?

Its going to read like more hazing, but my recommendation is that you get a little further in your math and physics classes. There are not Cliff's Notes nor any Quick Start Guide for Fluid Mechanics -- there is just a long slog through thermodynamics, statistical mechanics, and continuum classical mechanics.

It really doesn't get any more basic than the stuff on the Glenn Research site. Unless you are planning to do something innovative and exciting with your liquid fuel motor, plugging-and-chugging (and some competent supervision and advice from somebody on the faculty) will get it built. If you are planning to do something innovative, you will need to know a lot more math (and even more help from a competent advisor).

And SI units aren't "imperial" units.

Best of luck to you.

#### mrblack859

##### Member
Clicked on your rocket design link -- guess its "Go Eagles!" then?

Its going to read like more hazing, but my recommendation is that you get a little further in your math and physics classes. There are not Cliff's Notes nor any Quick Start Guide for Fluid Mechanics -- there is just a long slog through thermodynamics, statistical mechanics, and continuum classical mechanics.

It really doesn't get any more basic than the stuff on the Glenn Research site. Unless you are planning to do something innovative and exciting with your liquid fuel motor, plugging-and-chugging (and some competent supervision and advice from somebody on the faculty) will get it built. If you are planning to do something innovative, you will need to know a lot more math (and even more help from a competent advisor).

And SI units aren't "imperial" units.

Best of luck to you.
Go Eagles! I'm a Philosophy major and English minor, so I was planning on reading the textbooks on the subject. Does that sound like a feasible plan? The equation I posted used pounds instead of slugs, which meant that you had to use imperial units unless you used conversion factors so that you could use SI units, which would be an unnecessary pain. SI unit version that I found is a bit different but easier to use:

Edit: no gc factor, which would have been used to convert pounds to slugs.
Edit 2: here is a thread that explains the problem: http://www.thespacerace.com/forum/index.php?topic=653.0

Last edited:

#### Lowpuller

##### Well-Known Member
Pounds are weight

Slugs are mass

#### jlabrasca

##### Well-Known Member
The equation I posted used pounds instead of slugs, which meant that you had to use imperial units unless you used conversion factors so that you could use SI units, which would be an unnecessary pain. SI unit version that I found is a bit different but easier to use:

Edit: no gc factor, which would have been used to convert pounds to slugs.
Edit 2: here is a thread that explains the problem: http://www.thespacerace.com/forum/index.php?topic=653.0
WOW -- that thread is a crazy find. It looks the OP there "Superman12" googled his way to a scan of a pamphlet on liquid fuel rocket motors (c. 1966), and that it made him angry.

So, like I said -- more math and physics.

It may be that you know this (it is the bulk of the thread to which you link) and aren't expressing it clearly, but slugs and pounds are both imperial units. Pounds are weight, slugs are mass. The conversion factor g is some standard value for the acceleration due to gravity near the earth's surface (in ft/s/s).

In SI units ("metric" units in standard american english), it'd just be mass (kilograms in MKS, grams in CGS). The extraneous g is confusing in a rocket equation because it seems like it might have something to do with the motion of the rocket body away from the earth. It doesn't. Its just how you get from quantity by weight to quantity by mass. In 1960-something, balances that read out in kilograms would be less common than balances that read out in pounds, so it'd kind of make sense to dimension your formulae in feet-pounds-seconds. The folks responsible for the content on the Glenn Research Center "Beginners Guide To Rockets" are writing for a contemporary readership, who are accustomed to kilograms and meters. Stop looking at Krzycki and restrict your attention to the GRC content.

FWIW, the pamphleteer's choice of units are kind of odd even for 1960-something. Krzycki has the ideal gas constant figured in foot-pounds (energy) per pound-mass per Rankine (which is absolute temperature scaled to the divisions on a Fahrenheit thermometer, to whichKrzycki appends an erroneous "degree" symbol, and which NOBODY but Rankine ever used).

The formula in your top-but-one post -- in whatever system of units -- predicts the cross-sectional area of the narrowest part of the nozzle for a propellant modeled as an ideal gas at a given flow rate (in pound-mass/second in imperial units, in Kg/s in SI units) and a given pressure at the location of the constriction (throat).

If you were my student, before I could teach you why this formula predicts the area of the constriction for a given flow rate, given gas properties, given temperature, etc. I'd have to teach you what the gamma term (ratio of heat capacities) means, and why it shows up in the formulae for throat temperature and throat pressure (equations 8 and 11 from your source pamphlet) -- which discussion we would have near the middle of a semester-long 300-level or 400-level Thermo class. You might see gamma in the thermo unit of a 200 level class, but all of that detail about pressures at the Mach transition wouldn't show up until you'd had DQ -- and even then it'd be a "special topics" kind of thing in a semester crowded with equations of state, and the many flavors of free energy, and reversible/irreversible cycles, and all the other stuff we inflict upon thermodynamics students. Until then, its just going to be words.

Again, you can build a liquid fuel rocket engine without knowing where this formula comes from. It would be prudent, however, to make connections with the faculty in the physics and/or engineering programs before you start buying materials.

Last edited:

#### jlabrasca

##### Well-Known Member
Go Eagles! I'm a Philosophy major and English minor,
I think I was busy typing while you were editing...wish I'd waited.

You are asking if you can teach yourself rocket science in your spare time while pursuing a degree in a completely unrelated field. There is a name for what you are proposing. The shorter answer is that you can teach yourself enough to duplicate what others have done. It is not practical, or necessary, to try to teach yourself to the level of expertise required to understand why others have done as they have done. You can build and fly a liquid fuel rocket. You will not know everything that informed the design of that rocket.

#### mrblack859

##### Member
WOW -- that thread is a crazy find. It looks the OP there "Superman12" googled his way to a scan of a pamphlet on liquid fuel rocket motors (c. 1966), and that it made him angry.

So, like I said -- more math and physics.

It may be that you know this (it is the bulk of the thread to which you link) and aren't expressing it clearly, but slugs and pounds are both imperial units. Pounds are weight, slugs are mass. The conversion factor g is some standard value for the acceleration due to gravity near the earth's surface (in ft/s/s).

In SI units ("metric" units in standard american english), it'd just be mass (kilograms in MKS, grams in CGS). The extraneous g is confusing in a rocket equation because it seems like it might have something to do with the motion of the rocket body away from the earth. It doesn't. Its just how you get from quantity by weight to quantity by mass. In 1960-something, balances that read out in kilograms would be less common than balances that read out in pounds, so it'd kind of make sense to dimension your formulae in feet-pounds-seconds. The folks responsible for the content on the Glenn Research Center "Beginners Guide To Rockets" are writing for a contemporary readership, who are accustomed to kilograms and meters. Stop looking at Krzycki and restrict your attention to the GRC content.

FWIW, the pamphleteer's choice of units are kind of odd even for 1960-something. Krzycki has the ideal gas constant figured in foot-pounds (energy) per pound-mass per Rankine (which is absolute temperature scaled to the divisions on a Fahrenheit thermometer, to whichKrzycki appends an erroneous "degree" symbol, and which NOBODY but Rankine ever used).

The formula in your top-but-one post -- in whatever system of units -- predicts the cross-sectional area of the narrowest part of the nozzle for a propellant modeled as an ideal gas at a given flow rate (in pound-mass/second in imperial units, in Kg/s in SI units) and a given pressure at the location of the constriction (throat).

If you were my student, before I could teach you why this formula predicts the area of the constriction for a given flow rate, given gas properties, given temperature, etc. I'd have to teach you what the gamma term means, and why it shows up in the formulae for throat temperature and throat pressure (equations 8 and 11 from your source pamphlet) -- which discussion we would have near the middle of a semester-long 300-level or 400-level Thermo class. You might see game in the thermo unit of a 200 level class, but all of that detail about pressures at the Mach transition wouldn't show up until you'd had DQ -- and even then it'd be a "special topics" kind of thing in a semester crowded with equations of state, and the many flavors of free energy, and reversible/irreversible cycles, and all the other stuff we inflict upon thermodynamics students. Until then, its just going to be words.

Again, you can build a liquid fuel rocket engine without knowing where this formula comes from. It would be prudent, however, to make connections with the faculty in the physics and/or engineering programs before you start buying materials.
As far as the whole SI/imperial confusion we were having, you just clearly expressed what I was trying and failing to say. We are quite a ways away from actually building any liquid bipropellant engine, but I want to be ready when the times comes, and that means being knowledgable and working on designs on paper before hand. I'm glad that we will be able to build our engine by plugging and chugging, and I don't doubt that that is how we will successfully complete our first engine. I have talked to multiple professors in our chemistry and physics departments who have given us their word that they will assist however they can. The physics department has also agreed to allow us to use their fabrication shop (I do not currently know what equipment they have. I will find out). Nevertheless, I would very much like to develop an understanding of the relevant fluid mechanics and thermodynamics so that it isn't all just words. The catch is I would like to do as much of this learning as possible outside of the classroom since they don't in any way intersect with my major or minor course work, and I will be taking a leave of absence for the Spring of 2017 to finish my airline transport training. I don't think I can fit 300 and 400 level thermodynamic classes and their prerequisites into my schedule given my circumstances. I'll definitely look at picking up a DQ class though. I'm definitely willing to read textbooks cover to cover if that is what it takes.

#### mrblack859

##### Member
I think I was busy typing while you were editing...wish I'd waited.

You are asking if you can teach yourself rocket science in your spare time while pursuing a degree in a completely unrelated field. There is a name for what you are proposing. The shorter answer is that you can teach yourself enough to duplicate what others have done. It is not practical, or necessary, to try to teach yourself to the level of expertise required to understand why others have done as they have done. You can build and fly a liquid fuel rocket. You will not know everything that informed the design of that rocket.
Haha, I like your references. I appreciate your honesty and guidance. I guess my English minor may have to walk the plank! I will continue my planning doing as much as I can with plug and chug. I'll go scroll through the physics major course requirements. More to come in Fall 2017...