Estes C11 vs D12

[Larry Curcio]

Here's a comparison of the thrust curves of the C11 and D12:

https://www.rocketreviews.com/compare-estes-d12-to-estes-c11.html

-- Roger

Using the specs here as Gospel…

The *overall* effective exhaust velocities (Ve) for the D12 and C11 are 802 m/s and 733 mps respectively. The Ve in the peak is normally higher than the Ve in the plateau, because of extra thermodynamic efficiency at high pressures. The higher peak raises the overall Ve (and hence Isp) of the D12.

The difference in Ve is likely by design, because the C11 impulse at the higher Ve would come out to 9.6 ns, which is probably too close to the 10 ns limit for a BP motor.

At the same time, the plateau thrust of the C11 seems higher than that of the D12. If this height difference isn’t just random variation, it suggests the following:

1) The propellant perforation in the D12 is slightly longer than that of the C11. This creates a higher peak and overall Ve for the D12. (I grant you the perforations are small, but they have an effect. Note that a peak occurs even when no perforation pertains because of a quasi-hemispherical burning pattern.)

2) The nozzle throat diameter of the C11 may actually be slightly smaller than that of the D12. This would yield a higher plateau for the C11. Of course the plateau is shorter in duration, because there is less propellant.

And this makes vague sense on its face, as longer propellant perforations are generally associated with wider nozzle throat diameters.

Remark: I've been doing spot landing experiments using simulations to determine launch angle. Real world accuracy is much better for short-burning motors, and the C11 is a good spot landing choice in the C impulse class.


Oops! I see that Bob K. said essentially the same thing earlier. My bad!

 

[georgegassaway]

I have one problem with that, and that is the classification of the D12 as a "68%" D. That sounds rather wimpy indeed, until you consider that, with that scale, a full C would be considered a 0% D, which is really kinda silly. A full C is 50% of the power of a full D, so the D12 should be about a 84% D, since 16.8 is 84% of 20.

It seems like a really weird convention to me, but apparently that's the standard.

Not any "standard" I'm familiar with. Around 1970, Semroc had "3/4 C" engines,which were C's of about 7.5 N-sec (and IIRC 3/4 B engines, of about 3.75 N-sec). And Cox had 15 N-sec D8's that some referred to as "3/4 D's" (although it later turned out the D8's were more like 11 to 11.5 N-sec or so, but a whole different issue).

That makes more sense. A D is up to 20 N-sec. If it's 15 N-sec, then that's 3/4 of that N-sec range. If it's 10 N-sec that is 50% of a D N-sec range, and no longer called a D, it's a C. If a D really is 11 N-sec, it's not 1/10th of a D, it's a D with 55% of the N-sec that a D could have.

And, THAT is the standard that was established about 45 years ago. Not one person with a webpage playing loose with reinventing an old "standard" the wrong way or whatever.

As for the comparison of D11 and D12 curves, well, try another comparison. C11 and E9. The thrust curve looks a LOT like the same shape, as though it is an E9 nozzle and core, with less black powder so it is only a C that burns out a lot sooner (and has a D12 sized case). The C11 spike is a bit different, but close and the sustainer curve is almost dead on. So, I am suspecting that the C11 is more like a short burn E9 in a shorter case, than a short burn D12.

Slight variations in how much clay is loaded in for compacting into the nozzle could account for the spike difference between the C11 and E9 that were test fired. If the clay nozzle ends up as say 1/16" less tall for a given C11 than for a given E9, then that C11 would have 1/16" effectively deeper core depth in the propellant (The spindle mandrel shape and length that creates the nozzle and core depth is fixed so if the point where the nozzle should end and propellant shifts due to a different volume of clay poured and rammed, it shifts the effective core depth in the propellant) and therefore produce a bit higher thrust until it transitions to and endburning sustainer burn (but also would burn out a little quicker than a C11 that didn't have 1/16" less clay nozzle height. Assuming that otherwise everything else was equal).

- George Gassaway

 

[tbonerocketeer]

The letter ratings are a nice guide, but it's best to just look at total impulse and not an arbitrary scale. You can have J and K motors separated by less than 20N/s.

The scale is good for a general guide, but best not to entirely rely on it.

That is true, but there is a bunch of difference between a 1% N and a 100% N. By the same token, a J420 and a J415 are close to the same average thrust, however the J415 has a bunch more total impulse....I have a friend that learned this with trash bag recovery from a nice shred. He flew J420s all the time in the rocket, the J415 ate it

 

[Handeman]

Not any "standard" I'm familiar with. Around 1970, Semroc had "3/4 C" engines,which were C's of about 7.5 N-sec (and IIRC 3/4 B engines, of about 3.75 N-sec). And Cox had 15 N-sec D8's that some referred to as "3/4 D's" (although it later turned out the D8's were more like 11 to 11.5 N-sec or so, but a whole different issue).

That makes more sense. A D is up to 20 N-sec. If it's 15 N-sec, then that's 3/4 of that N-sec range. If it's 10 N-sec that is 50% of a D N-sec range, and no longer called a D, it's a C. If a D really is 11 N-sec, it's not 1/10th of a D, it's a D with 55% of the N-sec that a D could have.

And, THAT is the standard that was established about 45 years ago.

- George Gassaway

I was unaware of the motors being measured that way. That means you couldn't have anything less then a 1/2 D because anything less than that would make it a C or smaller motor?

Would I be correct if I said, any impulse designation that uses a fractional part of the Impulse letter i.e. 3/4 D would indicate a fraction of the impulse range from 0 to the max of the letter. The 3/4 D = 3/4 of 20 Ns or 15 Ns.
When the impulse designation is given as a percentage of the designation, such as 50% D, then it is calculated only over the Ns range of that letter designation. A 50% D would be 50% of the range from 10 Ns to 20 Ns, or 15 Ns.

So a 3/4 D would have the same Ns as a 50% D?

 

[shreadvector]

C11 nozzle is smaller than D12.

C11 centerbore is shorter than D12.

When BP motors ignite, the centerbore area burns and spreads outward and forward until peak thrust - when the dome is at it's largest. Then, as the edges of the dome burn away, you are left with the end burning sustaining phase, BUT the shape of the end burning phase can be a nearly flat surface or a moving dome. If the NAT S&T official thrust curves show you a slightly higher or lower sustaining thrust than a similar motor, you can be assured that the sustaining burning face is different - and you also need to factor in the different nozzle sizes.

 

[rcktnut]

I was unaware of the motors being measured that way. That means you couldn't have anything less then a 1/2 D because anything less than that would make it a C or smaller motor?

Would I be correct if I said, any impulse designation that uses a fractional part of the Impulse letter i.e. 3/4 D would indicate a fraction of the impulse range from 0 to the max of the letter. The 3/4 D = 3/4 of 20 Ns or 15 Ns.
When the impulse designation is given as a percentage of the designation, such as 50% D, then it is calculated only over the Ns range of that letter designation. A 50% D would be 50% of the range from 10 Ns to 20 Ns, or 15 Ns.

So a 3/4 D would have the same Ns as a 50% D?

You know the right answer to the question!! :wink: On the 3/4 calc they are reverting back to 0. With our motor designations having a range like you said, ( D motor) 10 to 20Ns the math has to be done equally. The D is starting at 10 Ns not 0. That is a range of 10 not 20. Math for both calcs has to be based on the range of 10. Doing the math correctly and adding "full" in front of the motor letter designation is simple enough. 50% full D, full E, full M, etc. would be the correct way to describe the power range of our motors, which it actually is!! Leaving out the full could be confusing especially for a novice.

 

[RoyAtl]

I was unaware of the motors being measured that way. That means you couldn't have anything less then a 1/2 D because anything less than that would make it a C or smaller motor?

Would I be correct if I said, any impulse designation that uses a fractional part of the Impulse letter i.e. 3/4 D would indicate a fraction of the impulse range from 0 to the max of the letter. The 3/4 D = 3/4 of 20 Ns or 15 Ns.
When the impulse designation is given as a percentage of the designation, such as 50% D, then it is calculated only over the Ns range of that letter designation. A 50% D would be 50% of the range from 10 Ns to 20 Ns, or 15 Ns.

So a 3/4 D would have the same Ns as a 50% D?

The first part would be correct. but the second part should be discarded, BECAUSE IT ISN'T A *****50%***** D. IT IS A 75% D. 75% = 3/4 .

We don't call 1/2A motors 0% A motors, or 1/4A motors -50% A motors. The naming standard for less-than-A motors is the standard that you should carry up to larger motors. As George pointed out, that has been the 'standard' since Vern started making 1/2A motors in 1961.

 

[rcktnut]

50% full D = 15Ns. 75% full D = 17.5 Ns.

 

[ThirstyBarbarian]

The first part would be correct. but the second part should be discarded, BECAUSE IT ISN'T A *****50%***** D. IT IS A 75% D. 75% = 3/4 .

We don't call 1/2A motors 0% A motors, or 1/4A motors -50% A motors. The naming standard for less-than-A motors is the standard that you should carry up to larger motors. As George pointed out, that has been the 'standard' since Vern started making 1/2A motors in 1961.

I think the 1/2A and 1/4A motors are just an extension of the same alphabetic motor class system below the A class. They don't tell you the actual impulse --- they are a range, just like the other classes. A motor designated as 1/2A does not have exactly 1.25 Ns impulse --- it has an impulse somewhere between .625 and 1.25. So what would a 1.0 Ns motor be? A 4/5 1/2A? It makes no sense.

Face it, no fraction or percentage system is going to make sense with the letter scale system. The letter system is a system of ranges, and the scale increases geometrically. You're trying to use a linear system of fractions or percentages to describe where a particular motor lies within a range on a geometrically increasing scale. It won't work. It seems more useful to me to just use term like "baby", "mid-range", "full", etc. when trying to describe where the motor is in its motor class. Then, when you need precision, abandon the motor class altogether, and use Ns.

 

[rcktnut]

Yes, especially with the big motors. The higher up the class the bigger the spread of 1% is. At .1 Ns. per percentage point for a D it is 204.8 Ns for an N