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JBM Ballistics question, FNG question.

T

Tyler455

Private
Full Member
Minuteman
Nov 23, 2013
64
2
AZ
I have been playing around with JBM a lot. I'm getting my dope cards figured out and making Density Altitude cards. Do most shooters make a DA dope card for each DA with increments of 1000 feet with "U cards"? If so ,what temp do you choose? Or make a " spread of DA's with a few temps, using "Trajectory Cards".

Also. I noticed something that confused me and my limited knowledge base of ballistics. When I made a singular Trajectory "U"Card for a DA of 5000 feet , I messed with the temperature. I assumed that making the temp hotter would decreased drop and decrease the amount of mils to dial. For two reasons, increased muzzle velocity and less dense, hotter air .It did the opposite. Why?
Does this have to do with the speed of sound and the creation of drag?
 
I made DA cards on an excel sheet. But I basically did temperature ranges on the cards: 0-25*, 30-65*, 70-100* (these are examples) by playing with the numbers you can determine at what temp your drop will change with a constant DA
 
If you truly used Density Altitude, temp is built into that. What you want to do is do cards for various PRESSURE altitudes and various temps. You can run the ballistics software to decide on break points based on the max error that you are comfortable with. If 1 MOA is good, then keep changing the temp until you get a 1/2 MOA change in drop, and then repeat. That way, you will have center temps for a range that will be a max of 1 MOA error (high to low or low to high).

Drag is based on air density, higher temps, less dense air. You will only see effects of increased MV (from hotter rounds) if you change the MV in your ballistics software. The safetware does not change MV based on tmep, only drag.
 
Thanks for the help so far.

So in the Trajectory Ucard and put all input the same for 168 SMK's @ 9000 foot elevation. The temp column to the left of the altitude column i put 30 degrees and the output at 800 yards was 7.6 mils 218.4 inches ( 2500 fps from 16inch) When I changed the 30 degress to 90 degrees I get 7.7 mils 221.5 inches. All the other in put the same. This seems totally backward to me. Is it the speed of sound changing?
 
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So, when Im done. I may have several sets of DA cards ranging from -2000 to 10000. I will essentially have three sets of -2000 to 10000. A set of 20 degree, a set of 60 degree , and a set of 100 degree?
 
Nuke8401 said:
New guy here,
I thought this was the case; the whole point of using DA was to just use one variable = DA. With temp, pressure and altitude all taken care of by measuring/using DA. I thought you only needed temp if you were estimating DA at a known altitude?
David E.
Click to expand...

Yes, temp is included in DA.

So if you want to calculate DA for your location, you can just do a series of cards for each DA increment.
 
Temperature
A temperature used to calculate a muzzle velocity for output. This input is also used to calculate the speed of sound to find the value of the drag function. This input is not used to calculate density.

This came from JBM and is saying what you you all are saying. I get it, but how does changing the temp to a hotter temp make my bullet drop increase. Is it because of the bold part, and if so how? Lower speed of sound higher drag function?
 
Station pressure is uncorrected pressure at your current location. Weather forecasts will give you corrected pressure for the altitude which is not station pressure. Density altitude is a combination of station pressure , temperature and to a lesser degree humidity.
 
Tyler455 said:
Station pressure is uncorrected pressure at your current location. Weather forecasts will give you corrected pressure for the altitude which is not station pressure. Density altitude is a combination of station pressure , temperature and to a lesser degree humidity.
Click to expand...

Actually you use Pressure Altitude, but you can figure pressure altitude using station pressure.
 
Tyler455 said:
Temperature
A temperature used to calculate a muzzle velocity for output. This input is also used to calculate the speed of sound to find the value of the drag function. This input is not used to calculate density.

This came from JBM and is saying what you you all are saying. I get it, but how does changing the temp to a hotter temp make my bullet drop increase. Is it because of the bold part, and if so how? Lower speed of sound higher drag function?
Click to expand...

I do not see how they can use temp to correct MV.

Yes, temp is used to calculate speed of sound. It would be nice if they showed the speed of sound in the output table, so you could see where the bullet goes trans sonic.
 
I made cards years ago but now I just run an ap and plug in the Da when I shoot. But my primary dope data comes from a kestrel with the ballistic ap. Balls on dead nuts winter spring summer and fall from below freezing to over 100. From negative 100 da to over 10,000.
 
Tyler455 said:
I have been playing around with JBM a lot. I'm getting my dope cards figured out and making Density Altitude cards. Do most shooters make a DA dope card for each DA with increments of 1000 feet with "U cards"? If so ,what temp do you choose? Or make a " spread of DA's with a few temps, using "Trajectory Cards".

Also. I noticed something that confused me and my limited knowledge base of ballistics. When I made a singular Trajectory "U"Card for a DA of 5000 feet , I messed with the temperature. I assumed that making the temp hotter would decreased drop and decrease the amount of mils to dial. For two reasons, increased muzzle velocity and less dense, hotter air .It did the opposite. Why?
Does this have to do with the speed of sound and the creation of drag?
Click to expand...

When you specify density altitude, you are completely defining the density, independent of temperature you entered.

Temperature is used to calculate corrected muzzle velocity usung the correction factor and zero temperature you enter.

Temperature is also used to calculate the speed of sound. This is used to find the value of the drag function as a function of the mach number of the bullet. As temperature goes up sound speed increases lowering the mach number of the bullet and putting you on a higher portion of the drag curve.

I could calculate temperature from the density altitude you entered, but why would I when you have entered the actual temperature?

The problem comes when you say for the same density altitude, if I change temperature, what happens? While mathematically it makes sense to me, it rare in my experience to see a big temperature change and a constant density altitude.

Also to say that for a density altitude if temperature goes up density goes down is not correct in this case because by specifying a density altitude you have specified the density. If I somehow "correct" it with temperature, it is no longer the density altitude.

The bottom line is that when you specify the density altitude and temperature you are telling my calculator that the density is the standard density at the entered altitude and the temperature is the entered temperature.

Brad
 
So awesome to hear from the creator. I love this forum! I guess was ,and still kinda failing to understand the effect of the Mach range and consequentially the effects of Mach with drag on the bullet. I totally get what your saying about the density altitude and temps not changing . I was failing to see that too.
 
jbmillard said:
When you specify density altitude, you are completely defining the density, independent of temperature you entered.

Temperature is used to calculate corrected muzzle velocity usung the correction factor and zero temperature you enter.

Brad
Click to expand...

Where do you input temperature for zeroing? I don't see that.

And what are you correcting? You cannot be correcting for powder-temp variability, as you don't input powder.
 
Pinecone said:
Where do you input temperature for zeroing? I don't see that.

And what are you correcting? You cannot be correcting for powder-temp variability, as you don't input powder.
Click to expand...

There are high and low temperatures and associated velocities. I wouldn't know how to begin with powder type.

When I do it, I put five or ten rounds in a ziploc bag and stick it in ice water before I drive to the range. I then shoot those five or ten in rapid succession over the chronograph. That gives me a low velocity at 32.2 (or close enough, I usually call it 32). I then shoot five or ten at ambient conditions and that gives me a high velocity. The program uses your input temperature to interpolate between these two values.

For my 308, the low velocity is 2750 and the high is 2800 at about 80 degrees (as I remember). It was more than I thought it would be...

Brad
 
jbmillard said:
There are high and low temperatures and associated velocities. I wouldn't know how to begin with powder type.

When I do it, I put five or ten rounds in a ziploc bag and stick it in ice water before I drive to the range. I then shoot those five or ten in rapid succession over the chronograph. That gives me a low velocity at 32.2 (or close enough, I usually call it 32). I then shoot five or ten at ambient conditions and that gives me a high velocity. The program uses your input temperature to interpolate between these two values.

For my 308, the low velocity is 2750 and the high is 2800 at about 80 degrees (as I remember). It was more than I thought it would be...

Brad
Click to expand...

But where do I put MY temp based data in?
 
I want to clarify with a couple of examples. Lets use Arizona. I could find myself at higher elevations in the colder months with a DA of 5000ft( physical altitude of 6000ft with temps in the 20s). I may also find myself in the summer months in the western part of the state with DA of 5000 ( physical altitude 1000ft temps at 105)

If I where to make TrajecoryU Card with a DA of 5000 foot for my 16 inch 308AR shooting 168gr SMKs at 2500 fps, zero range 100, sight height 2.5.

Card 1 with 20 degree temps and altitude of 5000 shows 8.1 mils and 233.4 inches of drop @ 800yards

Card 2 with 105 degree temp and altitude of 5000 shows 8.3 and 239.0 inches of drop@ 800 yards.

Is this correct?

I trust the software. This just confuses me because I still don't understand how the speed of sound increasing also increases bullet drag.

Now when I plug the same DA of 5000 and same bullet input into my Ballistic AE iPhone app AND change the temp from the 20 degrees to 105 nothing changes. My Ballistic AE actually splits the difference of output with 8.2 mils and 236.18 inches @ 800 yds. Why?
 
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Should I just leave my zero temp and current temp the same and a mild 60 degrees?
 
Tyler455 said:
I want to clarify with a couple of examples. Lets use Arizona. I could find myself at higher elevations in the colder months with a DA of 5000ft( physical altitude of 6000ft with temps in the 20s). I may also find myself in the summer months in the western part of the state with DA of 5000 ( physical altitude 1000ft temps at 105)

If I where to make TrajecoryU Card with a DA of 5000 foot for my 16 inch 308AR shooting 168gr SMKs at 2500 fps, zero range 100, sight height 2.5.

Card 1 with 20 degree temps and altitude of 5000 shows 8.1 mils and 233.4 inches of drop @ 800yards

Card 2 with 105 degree temp and altitude of 5000 shows 8.3 and 239.0 inches of drop@ 800 yards.

Is this correct?

I trust the software. This just confuses me because I still don't understand how the speed of sound increasing also increases bullet drag.

Now when I plug the same DA of 5000 and same bullet input into my Ballistic AE iPhone app AND change the temp from the 20 degrees to 105 nothing changes. My Ballistic AE actually splits the difference of output with 8.2 mils and 236.18 inches @ 800 yds. Why?
Click to expand...

Difference was explained above. Just make sure that your density altitude really is 5000 feet when the temperature goes to 105 degrees. Just because that's your altitude doesn't mean the density altitude is that.

As for what Ballistic AE is doing, no clue.

Brad
 
jbmillard said:
Which data? Velocity and temperature? In the fields provided in the trajectory card calculator.
Click to expand...

Yes, for the conditions I am shooting in. You say that you can put in the data from testing the load at different temps and have the program adjust the MV based on the temp. THAT is what I don't see.
 
Tyler455 said:
Part of what confused was this page Making Ballistic Cards Using Density Altitude shows a DA card with two temps. The card shows lower drop and mil corrections at the higher temp with an equal DA.
Click to expand...

He talks about Density Altitude, but the cards are in Pressure Altitude and Temp.

Different Temps, at the same Pressure Altitude results in different Density Altitudes.

This has been discussed before, but differences in Pressure Altitude versus Physical Altitude are seldom more than 0.1 mil.
 
When I use the this guys http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/ , I get different outputs than what it shows on his webpage. Has the software changed since these where made???


I still don't understand why, higher temps=more dope/drop on JBM ???!!!!! I've searched everywhere. Can anyone explain this?
Heres one I just made. Ive heard the speed of sound changes the drag. I just don't understand the concept and haven't found a simple enough explanation of


5000 ft DA - 168SMKs @ 2500 fps. More drop at higher temps??? Using this .http://www.jbmballistics.com/cgi-bin/jbmcard-5.1.cgi

30°F 70°F 90°F Temps with drop below

-12.4 -12.5 -12.6 mils @1000 yards
 
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If you change the temp, the DA changes. If you have the same DA, the data should be the same.
 
Tyler455 said:
...Heres one I just made. Ive heard the speed of sound changes the drag. I just don't understand the concept and haven't found a simple enough explanation of


5000 ft DA - 168SMKs @ 2500 fps. More drop at higher temps??? Using this .JBM - Calculations - Trajectory Card

30°F 70°F 90°F Temps with drop below

-12.4 -12.5 -12.6 mils @1000 yards
Click to expand...
I get a similar result. Similar as in bass-ackwards.

You're right, at higher temps you should get less drop because time of flight is lower. And this calculator isn't asking for baro or humidity, which gives me to presume it must be using a standard meteoro day. So either there's some setting in this calculator that both you and I are failing to take into account, or its calculations are gefukt. You can contact JBM here, state your case and ask, "WTF?" If you do, please post the reply to this thread. I'm curious to hear WTF myself.


The reason a supersonic bullet has so much more drag is that the shockwave creates its own special kind of drag, called "wave drag," which is the result of supersonic airflow, so it can't possibly exist at subsonic velocities. And it is quite a bit more powerful than all the combined sources of drag that exist subsonically.

In the world of physics, nothing happens free. Everything that moves must pay a cost in energy. Creating and maintaining the shock wave that surrounds the supersonic bullet literally sucks Kinetic Energy out of it. This "suckage" is wave form drag.

lossy-page1-220px-Supersonic-bullet-shadowgram-Settles.tif.jpg


The bullet in this shadowgraph is shocking the air into creating three standing waves as it penetrates it, three "shock waves." The creation of each one of those waves was paid for with KE taken directly from the bullet. If the bullet were subsonic, it would not be creating those shock waves, and would not be experiencing their added drag.


The consequence of wave form drag is evident in this drag coefficient chart:

ifqyo1.png


Where the green line first begins to tip up is roughly the critical Mach number, the lower floor to the transonic region (which always occurs at less than 1.0 Mach). Where the green line peaks is the point at which airflow over the entire bullet has become supersonic. The entire bullet is enveloped in supersonic airflow, so wave form drag has maxed out. By definition, this is the ceiling of the transonic region as well as the floor of the supersonic.

In this example, the added wave form drag is more than twice as powerful as the combined effects of the two subsonic forms of drag (base drag and skin friction). Which means total drag just about triples on account of those shock waves.
 
Thank you x1000 for JBM Ballistics. I find it the most easy to use. Even if I am new to this, it is my ignorance that holds me back. I enter data for hours just comparing calibers and loads. Thanks again.
 
Yes. I too appreciate JBM. This one little issue is confusing me and apparently Im not the only one.

Brad, Could you maybe elaborate on how higher temps increase the drag on the bullet via speed of sound changes? Maybe another highly experience shooter can chime in?

Is it that with higher speed of sound, the bullet will have a lower Mach number; therefore the bullet spends more time at lower FPS at or around the speed of sound. So, the bullet enters the shockwave at a lower FPS and there is effected by drag more? Im just spitballing here...

Again, Im new the Ballistics so excuse my ignorance. My Ballistic AE app, which many regard as awesome, does not change any trajectory when a Density Altitude is given and then temps are changed.
 
Last edited:
Thanks Fred!

Fred_C_Dobbs said:
I get a similar result. Similar as in bass-ackwards.

You're right, at higher temps you should get less drop because time of flight is lower. And this calculator isn't asking for baro or humidity, which gives me to presume it must be using a standard meteoro day. So either there's some setting in this calculator that both you and I are failing to take into account, or its calculations are gefukt. You can contact JBM here, state your case and ask, "WTF?" If you do, please post the reply to this thread. I'm curious to hear WTF myself.


The reason a supersonic bullet has so much more drag is that the shockwave creates its own special kind of drag, called "wave drag," which is the result of supersonic airflow, so it can't possibly exist at subsonic velocities. And it is quite a bit more powerful than all the combined sources of drag that exist subsonically.

In the world of physics, nothing happens free. Everything that moves must pay a cost in energy. Creating and maintaining the shock wave that surrounds the supersonic bullet literally sucks Kinetic Energy out of it. This "suckage" is wave form drag.

lossy-page1-220px-Supersonic-bullet-shadowgram-Settles.tif.jpg


The bullet in this shadowgraph is shocking the air into creating three standing waves as it penetrates it, three "shock waves." The creation of each one of those waves was paid for with KE taken directly from the bullet. If the bullet were subsonic, it would not be creating those shock waves, and would not be experiencing their added drag.


The consequence of wave form drag is evident in this drag coefficient chart:

ifqyo1.png


Where the green line first begins to tip up is roughly the critical Mach number, the lower floor to the transonic region (which always occurs at less than 1.0 Mach). Where the green line peaks is the point at which airflow over the entire bullet has become supersonic. The entire bullet is enveloped in supersonic airflow, so wave form drag has maxed out. By definition, this is the ceiling of the transonic region as well as the floor of the supersonic.

In this example, the added wave form drag is more than twice as powerful as the combined effects of the two subsonic forms of drag (base drag and skin friction). Which means total drag just about triples on account of those shock waves.
Click to expand...
 
Tyler455 said:
Yes. I too appreciate JBM. This one little issue is confusing me and apparently Im not the only one.

Brad, Could you maybe elaborate on how higher temps increase the drag on the bullet via speed of sound changes? Maybe another highly experience shooter can chime in?

Is it that with higher speed of sound, the bullet will have a lower Mach number; therefore the bullet spends more time at lower FPS at or around the speed of sound. So, the bullet enters the shockwave at a lower FPS and there is effected by drag more? Im just spitballing here...

Again, Im new the Ballistics so excuse my ignorance. My Ballistic AE app, which many regard as awesome, does not change any trajectory when a Density Altitude is given and then temps are changed.
Click to expand...

The drag function models I use (given to me by Bob McCoy) are a function of mach number. (See the G1 here http://www.jbmballistics.com/ballistics/downloads/text/mcg1.txt.) Note that it increases as the mach number decreases for supersonic flight.

The drag is proportional to the velocity squared, density and CD of the bullet. If the density is constant and the velocity is the same, then when the temperature changes, the mach number changes, which changes where you are on the drag function. For supersonic flight, that lowers the mach number, raising the value of the drag function. [You may ask why the density doesn't change when the temperature changes -- that's because you have specified a density altitude and it's constant.]

There are some tables that give the drag functions as a function of velocity -- but I believe that it is at standard conditions only, as all the coefficients I've seen published are as a function of mach number (for example, the values Lapua publishes for CD for some of their bullets).

As for what Ballistic AE does, I have no idea. I've never seen the code. Maybe the author would post here and tell us.

Brad
 
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Awesome thanks! Im wrapping my head around this. Im starting to get it.

Fred,
My example , which Brad just described was a constant Density altitude of 5000 ft. In AZ I might find this in the lower deserts at 105 degree temps or in higher, cooler country .Same DA, two totally different environments. So I entered the 5000 ft DA. When I changed the temp number the Density altitude was that same because of the aforementioned shooting environments, but with two very different temperatures. Just to clarify Density Altitude already takes into account temperature. What I was failing to understand was why the higher temp, even at a constant Density Altitude had higher drop. The mach thing still isn't totally understood for me but I understand the theory now.

Heres how wikepedia puts it.

Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on the fluid temperature around it. Mach number is useful because the fluid behaves in a similar way at the same Mach number. So, an aircraft traveling at Mach 1 at 20°C or 68°F, at sea level, will experience shock waves in much the same manner as when it is traveling at Mach 1 at 11,000 m (36,000 ft) at −50°C or −58F, even though it is traveling at only 86% of its speed at higher temperature like 20°C or 68°F.[6]

So the way Im starting to understand it is: Since he speed of sound increase at higher temps and the bullet leaves the muzzle a its fastest speed, the bullet will enter the " shock waves" sooner at higher temps; therefore causing drag sooner. I hope thats right. Feel free to correct me.

jbmillard said:
The drag function models I use (given to me by Bob McCoy) are a function of mach number. (See the G1 here http://www.jbmballistics.com/ballistics/downloads/text/mcg1.txt.) Note that it increases as the mach number decreases for supersonic flight.

The drag is proportional to the velocity squared, density and CD of the bullet. If the density is constant and the velocity is the same, then when the temperature changes, the mach number changes, which changes where you are on the drag function. For supersonic flight, that lowers the mach number, raising the value of the drag function. [You may ask why the density doesn't change when the temperature changes -- that's because you have specified a density altitude and it's constant.]

There are some tables that give the drag functions as a function of velocity -- but I believe that it is at standard conditions only, as all the coefficients I've seen published are as a function of mach number (for example, the values Lapua publishes for CD for some of their bullets).

As for what Ballistic AE does, I have no idea. I've never seen the code. Maybe the author would post here and tell us.

Brad
Click to expand...
 
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