expansion ratio's...and other fun stuff

[AlphaHybrids]

 

[jsdemar]

 

[ClayD]

a bit more appropriate?

 

[r1dermon]

can we keep this a semi-informative topic please? this topic started off fantastic, now it's very badly degraded...

 

[AlphaHybrids]

I'll leave this thread to ClayD, the self-proclaimed expert and his theories on pressure-kinetic laws. Take his word as gospel truth and ignore all others.

Edward

 

[ClayD]

the self-proclaimed expert
Edward

I have never issued any proclimations of the sort...


I have obviously had a hoot pissing of people who must have never ever been wrong!

If you can indeed insult inteligance, proof you need to stop being so dang sensitive.....

 

[butalane]

can we keep this a semi-informative topic please? this topic started off fantastic, now it's very badly degraded...

Lower molecular weight of gases in the nozzle increases ISP.

 

[cjl]

I have obviously had a hoot pissing of people who must have never ever been wrong!

op:


(To add something informative to this otherwise worthless post, butalane is absolutely right, and it leads to the interesting result that in some cases, hydrogen/oxygen engines are actually more efficient when run fuel rich. Even though the energy released is slightly lower, the exhaust velocity goes up due to the lower molecular weight of the exhaust.

 

[ClayD]

Lower molecular weight of gases in the nozzle increases ISP.

So is this a good time to discuss what Butalane is?

 

[jsdemar]

So is this a good time to discuss what Butalane is?

Uh, a person who posts on TRF? Did I get that one right? Got another tough questions?

 

[ClayD]

Uh, a person who posts on TRF? Did I get that one right? Got another tough questions?

you absolutely did...
BUt it is also these things...

Butalane, is a french classification of propellant.
(of a high isp for particle-ular reasons) Very high velocity exauhst if i am correct...

Funny part is i remember Butalane from when i was a kid, so the user name sticks out like a sore thumb for me.. Butalane is also a short synonyym for butyl urethane in joint selants.. (so i always wondered why someone would call themselves by such a thing... BUTT if you wikipedia it , you get this....)

 

[ClayD]

What does the mass flow velocity inside the motor do when it hits the nozzle?

to rephrase, as the propellant burns along the core, the gas/particles are accellorated toward the nozzle, or is it?

Or is it accelorated to a point, and then stops.

Or, why are long motor burn throughs a good distance above the nozzle(from my observations...)

 

[cjl]

The velocity in the core is subsonic, and it goes from that to about mach 3ish (IIRC) in the nozzle.

 

[bobkrech]

Very clever.

European contraction of the most common composite propellant formulation.

Butalane = buta -: polybutadiene- l -: ammonium perchlorate NH 4 ClO 4 - ane : Aluminum

Translation: aluminized ammonium perchlorate hydroxyl termiminated polybutadiene composite propellant or (Al)AP(HTPB)CP

https://fr.wikipedia.org/wiki/Propergol_composite

Bob

 

[butalane]

Very clever.

European contraction of the most common composite propellant formulation.

Butalane = buta -: polybutadiene- l -: ammonium perchlorate NH 4 ClO 4 - ane : Aluminum

Translation: aluminized ammonium perchlorate hydroxyl termiminated polybutadiene composite propellant or (Al)AP(HTPB)CP

https://fr.wikipedia.org/wiki/Propergol_composite

Bob

You caught me Bob. The name comes from Solid Rocket Propulsion Technology by Alain Davenas in which he presents a naming standardization for solid propellant based on binder/oxidizer/fuel.

What does the mass flow velocity inside the motor do when it hits the nozzle?

to rephrase, as the propellant burns along the core, the gas/particles are accellorated toward the nozzle, or is it?

Or is it accelorated to a point, and then stops.

Or, why are long motor burn throughs a good distance above the nozzle(from my observations...)

Your observations (shocking) are correct. The highest static pressure in a rocket motor is at the end furthest from the nozzle throat. The velocity of the gas accelerates as it progresses towards the nozzle, as more gas is added as the flow approaches the nozzle; aka the mass flux increases. This accounts for erosive burning. In longer motors a lot more hot gas is added to the cores (longer length) and there is a higher mass flux, generally.


By definition, choked flow is achieved when the velocity of the gas in the throat is at Mach 1. It is rapidly accelerated from this velocity in the expansion cone.

 

[ClayD]

The velocity in the core is subsonic, and it goes from that to about mach 3ish (IIRC) in the nozzle.

Your observations (shocking) are correct. The highest static pressure in a rocket motor is at the end furthest from the nozzle throat. The velocity of the gas accelerates as it progresses towards the nozzle, as more gas is added as the flow approaches the nozzle; aka the mass flux increases. This accounts for erosive burning. In longer motors a lot more hot gas is added to the cores (longer length) and there is a higher mass flux, generally.

By definition, choked flow is achieved when the velocity of the gas in the throat is at Mach 1. It is rapidly accelerated from this velocity in the expansion cone.

A few questions, Cjl, if the nozzle is mach 3, how do rockets go mach 4 and mach 5. Surely some nozzles achieve mach 5 in the nozzle. (or you couldnt go that fast.) some of the other propellant types are used in this velocity category when it comes to munitions...
I think mach 3 is probably a good number for most APCP(butalane) motors in hobby rockets and "lifting" vehicles.

But, you deffiantly jumped on the grenade of my questions... How then, does a nozzless rocket motor work! Some say, they are hardly less efficient than a nozzle'd rocket, achieving 86% efficiency..


Butalane,
I have been flirting with some,, LONG-grain geometry, with some disasterous results.. what you said is in line with what was experienced. I didnt have a burn through, but the head pressure was enough to extrude everything. There were a few more "issues at play" but the cruxt of how it failed is explained with your static pressure. It actualy acted like a hydrolic ram extruding all the propellant and liner through the nozzle.
This was a large J motor....

I am going to give it another try in two weeks. I made a few more casting mandrels, and i am stepping my core up in diameter as it reaches the nozzle.
the bottom grains are about 6", and decrease in length to 2 3/8 as they hit the top grain.... after the changes it seems to jump into the K range..
Funny thing is this actualy hurts-(makes worse) the mass flux problems of erosivity.

But i am curious how you determine if you obtain "chocked flow" before hitting the nozzle (causing realy bad problems.)

 

[GregGleason]

Does anyone know how well the nozzles of commercial hobby motors (e.g., AeroTech, CTI) are optimized? My guess is that it is somewhat of a grab-bag to keep costs down.

Does anyone make a nozzle with the classic de Laval design instead of the conic nozzle?

Greg

 

[ClayD]

Does anyone know how well the nozzles of commercial hobby motors (e.g., AeroTech, CTI) are optimized? My guess is that it is somewhat of a grab-bag to keep costs down.

Does anyone make a nozzle with the classic de Laval design instead of the conic nozzle?

Greg

My Loki has a "belled" nozzle. probably not a De-laval. Still pretty much a strait cut throat.

But, it is quite short for a 98mm nozzle. and still has a 2.87 or .75"(i think) exit diameter.

 

[bobkrech]

Does anyone know how well the nozzles of commercial hobby motors (e.g., AeroTech, CTI) are optimized? My guess is that it is somewhat of a grab-bag to keep costs down.

Does anyone make a nozzle with the classic de Laval design instead of the conic nozzle?

Greg

Regardless of what a wiki say, a de Leval nozzle is simply a converging/diverging nozzle. The diverging nozzle can be either a straight conical or a bell exhaust nozzle.

Conical nozzles are just as good in expanding gas as parabolic nozzle and are simplier to make. The reason why pros go to bell nozzles is that they are shorter and lighter, but they are more expensive and the shape must be properly designed to get proper expansion and efficiency.

Bob

 

[r1dermon]

A few questions, Cjl, if the nozzle is mach 3, how do rockets go mach 4 and mach 5. Surely some nozzles achieve mach 5 in the nozzle. (or you couldnt go that fast.)

Some say, they are hardly less efficient than a nozzle'd rocket, achieving 86% efficiency..

But i am curious how you determine if you obtain "chocked flow" before hitting the nozzle (causing realy bad problems.)

im very interested in these questions. Also, is your statement about nozzleless rockets pure conjecture, or is there quantifyable data which supports your statement? (in regards to 86% efficiency). also, what "sort" of efficiency are we talking about? im no chemist, but knowing what i know, and who i know, in the field of pyrotechnics, i know that certain chemical reactions may be more efficient in certain ratio's, however, a less efficient reaction sometimes results in a greater kinetic output...if that makes sense. so in that regard, in what way is a nozzle-less rocket more efficient? in creating thrust via transfer of thermal energy?

 

[AlphaHybrids]

Clay

dV = ve * ( ln (m0/m1) )


Edward

 

[butalane]

Clay,

Besides the obvious answer that Alpha provided, look at this link and pay special attention to "reference frames."
https://en.wikipedia.org/wiki/Relative_motion#Relative_motion

Nozzless motors rely on the acceleration of mass down the core to provide necessary mass flow. Their performance sucks.

 

[ClayD]

imAlso, is your statement about nozzleless rockets pure conjecture, or is there quantifyable data which supports your statement?

No, my statement doesnt meet the definition of conjecture. "Some Say" not Clayd Say... so it is hearsay. I would believe the source of the comment to be beyond conjecture, but nevertheless i dont personaly know the people or the facts.

It was merely a posed statement, made in light to open disscusion of its efficiencey..

In a later post, the builder, Fori, offers the following to address the performance of nozzleless designs.


"The simplicity,reliability and cost effectivenes due to the avoidance of a nozzle,the simple propellant configuration and the reduced insulation requirements, make the nozzleless rocket motors an attractive concept in spite of its lower specific impulse (by about 20%) compared to nozzled motors. In addition, the elimination of the nozzle assembly is used to increase the overall amount of propellant,that ,in most instances, can compensate for the reduced performance."

Timnat Y.M. ,Advanced Chemical Rocket Propulsion, Academic Press,London,1987,Chap.6

I have "heard others say" that the efficiency is down into the 60's.

Jeff Taylor of Loki has flow nozzless rockets(a rocket)- that i read about. and Ray Goodson, has some stuff out there on them on youtube and in the aerotech gallery.
Not a lot out there for facts and "data" numbers. I am sure the first guy i mentioned "knew" what he was doing, and did it for the fun of things...
Ray was a long time ago.. (i wasnt 10..) so his stuff isnt really in the internet...


Bob, how is a bell nozzle more expensive to make than a conical one????
:confused2:.. but once you have the programming, you can actualy use less material to get the same expansion as far as molds for phenolic glass, or graphite machining. (welcome to the world of CNC)... a lot of people are starting to incorporate 1.5Rt at the troat of the larger motors. (*from the nozzles ive seen*) which is just as hard to make as a bell nozzle. although, they still made a conical nozzle.

 

[AlphaHybrids]

I would expect you tell this to CTI and Aerotech that they can get more efficient nozzles by going to a bell shape. They may be interested in your wisdom.

Also, the exhaust velocity of the SRB is ~ 2600 m/s. This translates to ~5800 mph. Well over Mach 5.

Edward

 

[cjl]

A few questions, Cjl, if the nozzle is mach 3, how do rockets go mach 4 and mach 5. Surely some nozzles achieve mach 5 in the nozzle. (or you couldnt go that fast.) some of the other propellant types are used in this velocity category when it comes to munitions...
I think mach 3 is probably a good number for most APCP(butalane) motors in hobby rockets and "lifting" vehicles.

Mach 3 or so is about right for the majority of rocket nozzles. Some might make it up to mach 4-6, but they would need unusually large expansion ratios to do so (I believe the SSME exit mach is around 6, for example). However, what you aren't accounting for is the fact that the speed of sound in the exiting gas is much, much higher than in air. The extremely high temperature means that in most amateur motors, even though the exit mach number is 3 or so, the exit velocity is around 2 km/s. In typical, sea-level air, this speed corresponds to about mach 6. In the case of the shuttle main engine, the exit velocity is around 4.5 km/s, which would be around mach 13 in sea-level air (but it is only around mach 5 or 6 for the gases in the nozzle).

Oh, and exit mach number does not depend on the propellant (assuming similar nozzle efficiency in all cases). It really only depends on the expansion ratio. Exit velocity depends on the propellant, because the sound speed depends on the gas composition, but the mach number is the same for the same expansion ratio, whether you are using hydrogen/oxygen, or simply compressed air.

But, you deffiantly jumped on the grenade of my questions... How then, does a nozzless rocket motor work! Some say, they are hardly less efficient than a nozzle'd rocket, achieving 86% efficiency..

They are substantially less efficient (that 86% isn't compared to ideal - it's compared to a nozzled rocket that is otherwise identical). They achieve mach 1 at the exit of the core, but at a fairly high pressure. I would imagine this means that they are getting quite a bit of pressure thrust, but quite a bit less momentum thrust than most motors, but I'm not familiar enough with them to be sure (so someone with actual knowledge may correct me on this one). They are definitely fascinating concepts though.

 

[cjl]

im very interested in these questions. Also, is your statement about nozzleless rockets pure conjecture, or is there quantifyable data which supports your statement? (in regards to 86% efficiency).

It appears to come from here (at least, they come up with the same number):

https://serge77.rocketworkshop.net/nozzleless0/nozzleless.pdf

It seems to be 86% of the performance of a motor with a nozzle and otherwise similar conditions.

 

[cjl]

I would expect you tell this to CTI and Aerotech that they can get more efficient nozzles by going to a bell shape. They may be interested in your wisdom.

Edward

I'm not positive, but some of the large (as in Pro98 xl) CTI nozzles do seem slightly belled out. Not very much though, and I'm not completely positive about whether they are at all (I'm going off of memory here, since I don't have one on hand at the moment).

 

[r1dermon]

So by hardly less efficient, in what way do you mean that? They'd be more efficient at packing propellent into the same volume tube...however their isp suffers as a result of their design. There are several variables to consider, im trying to understand what variable you're leading to when you state "hardly less efficient".

To that end, how do we quantify what "hardly less efficient" actually means? Where's the threshold separating hardly and majorly.

Be kind...im attempting to learn something, or at least arrive at an understanding. I've been flying rockets for over a decade, and id like to learn more about the physics of how they function. I appreciate your understanding. Mostly everyone in this thread has been extremely helpful. Many thanks for that.

 

[bobkrech]

V,exhaust = G, m/s/s x Isp, s = 9.8 m/s/s x Isp s = 9.8 x Isp m/s

A hobby APCP propellant with an Isp = 200 s has an exhaust velocity = 1.96 km/s which is M ~ 5.76 compared to standard atmospheric conditions sound speed = 0.34 km/s.

A LOX/LH2 SSME propellant with an Isp = 450 s has an exhaust velocity = 4.41 km/s which is M ~ 12.97 compared to standard atmospheric conditions sound speed = 0.34 km/s.

The internal Mach number referenced to the hot exhaust gas is much lower.

Bob

 

[butalane]

Butalane,
I have been flirting with some,, LONG-grain geometry, with some disasterous results.. what you said is in line with what was experienced. I didnt have a burn through, but the head pressure was enough to extrude everything. There were a few more "issues at play" but the cruxt of how it failed is explained with your static pressure. It actualy acted like a hydrolic ram extruding all the propellant and liner through the nozzle.
This was a large J motor....

I am going to give it another try in two weeks. I made a few more casting mandrels, and i am stepping my core up in diameter as it reaches the nozzle.
the bottom grains are about 6", and decrease in length to 2 3/8 as they hit the top grain.... after the changes it seems to jump into the K range..
Funny thing is this actualy hurts-(makes worse) the mass flux problems of erosivity.

I have never heard of a motor extruding everything. There are a few cases I know of in which a bottom grain was extruded, but this is very rare, and I dont believe for a second that your liner went through the nozzle. Did the grains and liner fall out of the case following a nozzle spit?

I dont think the static pressure I described is the reason for your failure.

What was the grain geometry?