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  #41  
Old 01-12-2013, 09:40 PM
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billspad billspad is offline
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Quote:
Originally Posted by Royatl
Not sure what Jerry is quoting from, since nothing from the NAR is in imperial units, but if you're talking about the F15, the S&T report says it is black powder, and the performance appears along the lines of black powder performance (as Carl said, comparing to the old Mini-Max F16 motors. F16 was 57ns on 62.5 gms stated vs F15 50ns on 60 gms measured)


We Estes sends the motors to S & T for certification we only get a few details before they show up. In this case we knew they were F's with 60g of propellant. With that information John Lyngdal guessed that they were going to be 29mm with a total impulse of 50 Ns. So the specific impulse was exactly what was expected.
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  #42  
Old 01-13-2013, 08:51 AM
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Jerry Irvine Jerry Irvine is offline
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Sorry for getting over-excited. I guess the ISP of BP in larger motors is known and this falls well into predicted parameters. As for using imperial units I simply converted from the NAR S&T values and stated ISP in terms of lb-sec/lb rather than N-s/g. The main reason for my focus on ISP is a higher ISP motor simply has better flying characteristics. Move along, these are not the KP motors you are looking for.

I do think at whatever price these come along at they will be surprisingly popular mainly because the way you use them is essentially the same as that of a B6-4 or a D12-5. Probably the main thing Estes users will have to be reminded about is to be sure to use 1/4" x 48" rods whenever possible. Every Home Depot has them.

I heard they even use white plugs.

Jerry

Last edited by Jerry Irvine : 01-13-2013 at 09:55 AM.
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  #43  
Old 01-13-2013, 10:01 AM
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Quote:
Originally Posted by billspad
We Estes sends the motors to S & T for certification we only get a few details before they show up. In this case we knew they were F's with 60g of propellant. With that information John Lyngdal guessed that they were going to be 29mm with a total impulse of 50 Ns. So the specific impulse was exactly what was expected.
Bill,

I know it's a volunteer position and it's not like you need more to do, but I would be really interested in learning more about S & T and how you guys actually do what you do.

Would you consider posting a new thread here with some explanations of the process and some more photos?

I'd also be interested in some history of S & T, how it has evolved over the years, and any anecdotes you have to share.

S & T is absolutely vital to this hobby and yet, for most of us, it's kind of a black box. We always love seeing white smoke rising from the chimney indicating a new motor has been accepted, but that's about all most of us know about the process.

Thanks,

Steve
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  #44  
Old 01-13-2013, 12:14 PM
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Carl@Semroc Carl@Semroc is offline
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Quote:
Originally Posted by Jerry Irvine
As for using imperial units I simply converted from the NAR S&T values and stated ISP in terms of lb-sec/lb rather than N-s/g.

WARNING, BORING (You might want to skip to the last paragraph)

Specific Impulse is generally given in seconds. The earliest definition I used in the late 50's was that a given weight (at sea-level) of propellant producing the same amount of thrust (force) would have a burn time equal to its Isp in seconds. Part of the confusion from the transfer from Imperial to Metric is that the Total Impulse is given in Newton-seconds, which is force times time, and the propellant mass is given in grams (mass). To convert from the old pound(force)-sec divided by pounds(weight) to metric, the grams of mass has to be converted to ponds or grams-force. The conversion factor I have used for the last 48 or so years is 102 to go from N-sec/g to lb-sec/lb. To be more accurate, it is 453.59g/4.448222 N or 101.95g(force)/Newton.

So the Specific Impulse for the new F15 engines is 49.61 N-sec divided by 60.0 g times 102 or 84.3 seconds. The usual quality of black powder used in model rocket engines over the past 50 years has been in the range of 80 to 85 seconds. There are some exceptions, such as the B4 which has much lower specific impulse to create a softer thrust-time curve at the expense of efficiency.

Jerry is right that there are others ways of expressing Isp, but I have never been able to get my mind around velocity as a unit of Isp that you would wind up with if you expressed it in N-sec/g. In this case the F15 engines would have an "Isp" of .8268 mm/sec (whatever that means.)

I know this is boring; it is the first time I have revisited it for half a century. Just remember to divide the Total Impulse (N-sec) by grams of propellant and multiply by 100 (or 102 or 101.95, depending on how lazy you feel at the time) to get the Isp in seconds. NOW, you can compare propellant performance. One pound of black powder will produce one pound of thrust for around 83 seconds. One pound of APCP will produce one pound of thrust for about 160 seconds. One pound of LH2/LOX will produce one pound of thrust for 421 seconds!
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  #45  
Old 01-13-2013, 12:17 PM
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Quote:
Originally Posted by Gus
Bill,

I know it's a volunteer position and it's not like you need more to do, but I would be really interested in learning more about S & T and how you guys actually do what you do.

Would you consider posting a new thread here with some explanations of the process and some more photos?

I'd also be interested in some history of S & T, how it has evolved over the years, and any anecdotes you have to share.

S & T is absolutely vital to this hobby and yet, for most of us, it's kind of a black box. We always love seeing white smoke rising from the chimney indicating a new motor has been accepted, but that's about all most of us know about the process.

Thanks,

Steve



Also, Chas Russell, a member here, was on S&T back in the early 70s. He could probably share some interesting stories of his days in that capacity, if he is willing. And, since he is not currently serving in that capacity, he may feel a bit more free in sharing how things work, or at least worked, back in those days.

Earl
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  #46  
Old 01-13-2013, 12:30 PM
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Jerry Irvine Jerry Irvine is offline
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Quote:
Originally Posted by Carl@Semroc
WARNING, BORING (You might want to skip to the last paragraph)
Specific Impulse is generally given in seconds.

Jerry is right that there are others ways of expressing Isp, but I have never been able to get my mind around velocity as a unit of Isp that you would wind up with if you expressed it in N-sec/g. In this case the F15 engines would have an "Isp" of .8268 mm/sec (whatever that means.)
A lot of my commercial rocket buddies like to calculate things in SI units then to be compatible with the world around them convert the answers to (ewww) metric.

I see from the NAR S&T thrust curve for the Estes F15 that my observation from John's video that the casing is eroding is verified by the curve. About a second and a half in the curve becomes noisy as material is burned off the inside of the tube and ejected out the nozzle. Free massflow!

Using meters per second is more suitable to liquids since everything is a flow and they are often throttled. Exhaust velocity is more applicable to vacuum operation. Since all motors are massflow devices, the speed at which you flow it matters. It is influenced by the mass of the effluent. Low mass effluent (H2-O2) is more easily accelerated due to low inertia.

When we fire throttleable hybrids we measure all fuel and oxidizer supplies before and after to get as close as possible to a true ISP, but even that does not account well for the start-up process, so lowers the measured ISP as compared to the momentary ISP during the main portion of the run.

We are firing hybrids with measured SL ISP of 220-240, which I understand to be exciting. Our regression rates are comparable to solids which I know to be exciting.

Jerry

Last edited by Jerry Irvine : 01-13-2013 at 01:32 PM.
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  #47  
Old 01-13-2013, 02:06 PM
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Ez2cDave Ez2cDave is offline
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Quote:
Originally Posted by Carl@Semroc
[B]One pound of black powder will produce one pound of thrust for around 83 seconds. One pound of APCP will produce one pound of thrust for about 160 seconds. One pound of LH2/LOX will produce one pound of thrust for 421 seconds!


So, one pound of Vulcanite EB-75 will produce one pound of thrust for . . . ???
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  #48  
Old 01-13-2013, 02:13 PM
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Royatl Royatl is offline
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Quote:
Originally Posted by Ez2cDave
So, one pound of Vulcanite EB-75 will produce one pound of thrust for . . . ???


Referring to the chart in the patent, from 120-140 seconds.
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  #49  
Old 01-13-2013, 02:22 PM
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Quote:
Originally Posted by Gus
Bill,

I know it's a volunteer position and it's not like you need more to do, but I would be really interested in learning more about S & T and how you guys actually do what you do.

Would you consider posting a new thread here with some explanations of the process and some more photos?

I'd also be interested in some history of S & T, how it has evolved over the years, and any anecdotes you have to share.

S & T is absolutely vital to this hobby and yet, for most of us, it's kind of a black box. We always love seeing white smoke rising from the chimney indicating a new motor has been accepted, but that's about all most of us know about the process.

Thanks,

Steve



I've got a PowerPoint presentation that I did at a couple of NARCONs that would answer a lot of your questions. Anybody know how to turn it into something I can post here? It's got a lot of pictures and videos.

The white smoke comment made me laugh. The testing done on the East Coast is done in a test cell at MIT. I assume it wasn't designed by anyone who graduated from there (or anywhere else for that matter) because it dumps the smoke into a passage way between two buildings instead of a chimney. We've been visited by the Cambridge fire department a couple of times.
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  #50  
Old 01-13-2013, 02:27 PM
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Quote:
Originally Posted by Earl
Also, Chas Russell, a member here, was on S&T back in the early 70s. He could probably share some interesting stories of his days in that capacity, if he is willing. And, since he is not currently serving in that capacity, he may feel a bit more free in sharing how things work, or at least worked, back in those days.

Earl


I'd like to hear the stories too. My knowledge of S & T only goes back a little more than 20 years.

Sharing how things work now is not a problem. We even put it in writing . It's giving out specific details of motors that are currently being certified that's forbidden.
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