• Have you entered ???
    LAUNCH LAB ROCKETRY GIVEAWAY

High Temperature Thermocouple for Combustion Chamber?

The Rocketry Forum

Help Support The Rocketry Forum:

Sascha

Member
Joined
Aug 12, 2014
Messages
15
Reaction score
0
Location
Brunswick, Maine
We're doing a static test of a hybrid engine and are trying to figure out a way to measure the combustion chamber temperature.

Has anyone come across a thermocouple that can be used within a combustion chamber? Or, alternatively, any other way to reasonable measure combustion chamber temps?

thanks!
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,715
Reaction score
3,578
Location
Butte, Montana
For an application like this a thermocouple if sometimes placed in a ceramic well to protect it from the chemical byproducts. Thermal shock is a consideration so you'll need the right ceramic. Having it in a well will introduce measurement lag also.
Could you measure the temperature optically (infrared actually)?
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
7,221
Reaction score
1,645
Yes to the ceramic well, yes to measuring infrared.

A hole is a near perfect emitter.
 

Nick@JET

Well-Known Member
Joined
Sep 1, 2011
Messages
1,693
Reaction score
18
Search for "thermocouple chart"
ImageUploadedByRocketry Forum1465303592.653857.jpg
Everything you want to know about them
 

Charles_McG

Ciderwright
Joined
Sep 12, 2013
Messages
2,890
Reaction score
949
Location
SE Wisconsin
Not necessarily everything.

For instance, a standard K thermocouple will drift in reading every time it goes through its annealing temperature. I forget where that is, exactly, but I've seen it on an oven running between 550C and 650C.

To get a more repeatable (over many cycles) measurement, you'd want a KS (special) or an N. We use a lot of Ns.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
Hardly. Stick a regular thermocouple inside a rocket motor and it goes away. Real fast.

Also using any temperature measuring device with protective mass will have response time issues and due to thermal conductivity will be cooler than the gas temperature.

Multi-wavelength optical pyrometry is the way to go. https://en.wikipedia.org/wiki/Pyrometer You can measure the incandescence of the particles in the combustor to determine the temperature but you must pay attention to details. If you don't have particles you have to look a the emission spectra from the gas molecules in the motor and do a lot of spectral fitting....

Been there, done that. Made a career of accurately measuring the temperature and thermal properties of very hot stuff. Best one was doing macroscopic videography of a graphite ceramic composite scramjet motor liner at 4900F and watching the CF blowing in the breeze......:dark:
 

Salvage-1

Certified
Joined
Mar 18, 2012
Messages
2,672
Reaction score
5
I may have a mate whom I can ask about this.. I think you need a MOLE in that hole. Let me have a chat with him and see if this may work.
https://www.ecd.com/
 

aerostadt

Lifetime Supporter
TRF Lifetime Supporter
Joined
Oct 27, 2009
Messages
3,435
Reaction score
327
Location
Brigham City, UT
During the space shuttle solid rocket booster program chrome-alumel thermocouples were used in the joints. As I recall they could go up to about 2300 Deg.F. and would fail within tenths of a second, perhaps a second or two at best. The data was always open to interpretation. In order to get higher temperature data in the 4000 Deg.F. range tungsten-rhenium thermocouples were used, but again failure would occur quickly and the data was open to interpretation. During joint pressurization it was believed that the temperature would not reach the adiabatic flame temperature for maybe a second or two. The flame temperature for shuttle solid rocket motors is on the order of 5400 Deg.F.
 
Last edited:

Salvage-1

Certified
Joined
Mar 18, 2012
Messages
2,672
Reaction score
5
What max temp range are we expecting with the hybrid motor??
The immediate answer the I got back was "Inconel TC with a M.O.L.E. data logger" But specific model of thermocouple would depend on the max temperature range. As aerostadt pointed out, if you go over around 4000F there may not be an option. 2500F to 4000F may get expensive.

Using the M.O.L.E that he had on hand, this is information that he pulled from a rocket mass heater that he just built.
Rocket_mass_heater.png
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
These Nanmac thermocouples are the only thermocouples designed to withstand extremely high temperatures inside a rocket motor because they are designed to erode. https://www.nanmac.com/e12.html

I never understood the design of the first field joint on the SRMs.

1.) The field joint expanded under pressure instead of compressing. An expanding joint increased the hot gas conductance to the o-ring.
2.) AFAIK the o-ring seal was designed as a dynamic seal. That means it needed gas pressure to effectuate the seal.
3.) The o-ring compound was chosen for its high temperature properties and the cold o-ring brittleness/flexibility was not considered. When exposed to ambient temperatures below the glass transition temperature, the o-ring did not move sufficiently upon initial hot gas pressurization. Hot gas leakage occurred on all launches below 50F, and when the temperature dropped below 32F before the Challenger launch, the quantity of hot gas bypassing the o-ring and directly impinging on the steel casing was sufficient to eventually cut a hole thru it.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
What max temp range are we expecting with the hybrid motor??
The immediate answer the I got back was "Inconel TC with a M.O.L.E. data logger" But specific model of thermocouple would depend on the max temperature range. As aerostadt pointed out, if you go over around 4000F there may not be an option. 2500F to 4000F may get expensive.

Using the M.O.L.E that he had on hand, this is information that he pulled from a rocket mass heater that he just built.
View attachment 293399
That's a low temperatures for a hybrid. You can expect gas phase temperatures up to 5600F (3400K) in a optimized nitrous/wax hybrid as calculate in ProPEP.
 

Salvage-1

Certified
Joined
Mar 18, 2012
Messages
2,672
Reaction score
5
The temperatures on that graph were for a 'rocket mass heater', which is nothing to do with actual rockets. I included the graph just to show what basic output looked like.

5600F. Will pass that on to the boss.
 

Salvage-1

Certified
Joined
Mar 18, 2012
Messages
2,672
Reaction score
5
Inconel overbraided ceramic wire type J. Rated to 13385F for up to 10 seconds, longer life at lower temps. I am told that the type J ceramic should at least last through a 10 second burn of 5600F.
 

Sascha

Member
Joined
Aug 12, 2014
Messages
15
Reaction score
0
Location
Brunswick, Maine
Yeah, the estimated combustion chamber temperature is expected to exceed 5000F for the hybrid engine. However, the static ground tests will last initially for only 5-10 seconds. Hoping we can find something that will survive and not cost a pretty penny.
 

Sascha

Member
Joined
Aug 12, 2014
Messages
15
Reaction score
0
Location
Brunswick, Maine
Hardly. Stick a regular thermocouple inside a rocket motor and it goes away. Real fast.

Also using any temperature measuring device with protective mass will have response time issues and due to thermal conductivity will be cooler than the gas temperature.

Multi-wavelength optical pyrometry is the way to go. https://en.wikipedia.org/wiki/Pyrometer You can measure the incandescence of the particles in the combustor to determine the temperature but you must pay attention to details. If you don't have particles you have to look a the emission spectra from the gas molecules in the motor and do a lot of spectral fitting....

Been there, done that. Made a career of accurately measuring the temperature and thermal properties of very hot stuff. Best one was doing macroscopic videography of a graphite ceramic composite scramjet motor liner at 4900F and watching the CF blowing in the breeze......:dark:
Is there a way to get a pyrometer into the hot combustion chamber? We're trying to keep it as simple as possible. I could see using it on the gases that are coming out directly and then trying to infer the combustion chamber temperatures from there.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
Is there a way to get a pyrometer into the hot combustion chamber? We're trying to keep it as simple as possible. I could see using it on the gases that are coming out directly and then trying to infer the combustion chamber temperatures from there.
Shasha

To obtain the correct temperature of the high temperature gas in a rocket motor combustion chamber, the measurement device must be in local thermal equilibrium with the gas. The ideal physical measurement device would have no thermal mass, or thermal conductivity, so it would be at the gas temperature. Unless you use an optical pyrometer, you can not meet that condition.

If you use a thermocouple, and it is sheathed, you will always record a lower temperature than the true gas temperature due to thermal conductivity and sheath reradiation. For gases, you want to use a unsheathed thermocouple, but the gas temperature must be lower than the melting point of the thermocouples and the thermocouples must not chemically react with the hot gases. Furthermore, you must account for the thermal conductivity of the thermocouple wires and at high temperatures, the radiative loss of the thermocouple. For example if your gas is at 2500K, the radiation losses from the thermocouple incandescence can result in a temperature measurement that is several hundred degrees cooler than the actual gas temperature. Because of these restrictions, most folks chose to perform optical pyrometry on the hot gases or surfaces, depending on which you are trying to measure.

There are many ways to perform the measurements, none of which are cheap, but some are less expensive than others. This discussion will rapidly get more technical than is appropriate for TRF. Send me a PM with you name, address, affiliation, nationality, e-mail and phone number. I will call you back and we can discuss it further. I believe you are in the Boston area. If necessary we can have a face to face since I live in Salem.
 

aerostadt

Lifetime Supporter
TRF Lifetime Supporter
Joined
Oct 27, 2009
Messages
3,435
Reaction score
327
Location
Brigham City, UT
Shasha

To obtain the correct temperature of the high temperature gas in a rocket motor combustion chamber, the measurement device must be in local thermal equilibrium with the gas. The ideal physical measurement device would have no thermal mass, or thermal conductivity, so it would be at the gas temperature. Unless you use an optical pyrometer, you can not meet that condition.

If you use a thermocouple, and it is sheathed, you will always record a lower temperature than the true gas temperature due to thermal conductivity and sheath reradiation. For gases, you want to use a unsheathed thermocouple, but the gas temperature must be lower than the melting point of the thermocouples and the thermocouples must not chemically react with the hot gases. Furthermore, you must account for the thermal conductivity of the thermocouple wires and at high temperatures, the radiative loss of the thermocouple. For example if your gas is at 2500K, the radiation losses from the thermocouple incandescence can result in a temperature measurement that is several hundred degrees cooler than the actual gas temperature. Because of these restrictions, most folks chose to perform optical pyrometry on the hot gases or surfaces, depending on which you are trying to measure.
Bob,
You are right about thermal inertia or lag time of a thermocouple, especially, if the researcher wants to capture temperature information about the ignition transient. As I remember someone calculated that the lag time for the thermocouples in the RSRM field joint tests was on the order of 0.3 seconds, which provided room for interpretation of the data. Calculating the lag time involved things like calculating the Biot number, etc.
 

aerostadt

Lifetime Supporter
TRF Lifetime Supporter
Joined
Oct 27, 2009
Messages
3,435
Reaction score
327
Location
Brigham City, UT
I never understood the design of the first field joint on the SRMs.

1.) The field joint expanded under pressure instead of compressing. An expanding joint increased the hot gas conductance to the o-ring.
2.) AFAIK the o-ring seal was designed as a dynamic seal. That means it needed gas pressure to effectuate the seal.
3.) The o-ring compound was chosen for its high temperature properties and the cold o-ring brittleness/flexibility was not considered. When exposed to ambient temperatures below the glass transition temperature, the o-ring did not move sufficiently upon initial hot gas pressurization. Hot gas leakage occurred on all launches below 50F, and when the temperature dropped below 32F before the Challenger launch, the quantity of hot gas bypassing the o-ring and directly impinging on the steel casing was sufficient to eventually cut a hole thru it.
Bob,
In the original design there was a gap between the insulating rubber on the mating surfaces between the 2 segments. This was filled with polysulfide putty prior to the mating of the 2 adjacent surfaces. The idea was that the polysulfide would completely fill the gap upon mating the 2 segments. In practice there were occasional blow holes in the putty that acted like blow torch paths for high temperature gas impingement on the oring. So, blow holes were just as bad a problem as the oring material itself, which was made famous by Feynman, but Feynman didn't emphasize the blow hole problem.

In a sense the joints themselves were stiffer than the rest of the either case walls away from the joint, simply because there was more metal there. Thus, the walls tended to rotate about the joint and during the Challenger recovery effort a gap as much as 40 mils could open between the bottom of the oring and the steel case. As you know if the oring did not have the cold temperature dynamic seal capability response to seal this gap, there could be blow-by erosion. In the re-design effort the Seals group evaluated different oring materials and the gave the properties different point values, so that Viton won out over the old material for its cold temperature response.

The role of the polysulfide in the original design may not have been thought out completely, because afterwards some people felt that the polysulfide would push down into the joint compressing the trapped air ahead of the oring and moving the oring into position. However, some tests during the Re-design effort showed that the polysulfide putty could actually hold back pressure.
 
Last edited:

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,353
Reaction score
37
I am sure ATK did a fine job on the original and redesigned SRMs. I'm also certain the system was sufficiently engineered and understood, and performed exactly as required and specified before it was shipped and delivered to the customer.

It is a fact of engineering life, that once a system is delivered to the buyer, the operation and control of that system is out of the engineers control. There was a massive book on shuttle ops, and multiple checklists. We know the low launch temperature was flagged and the fact is that someone in NASA made a conscious decision to launch under conditions that were out of bounds for the SRM system. That's not an engineering failure, it's a management failure and a character failure of that individual.
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,715
Reaction score
3,578
Location
Butte, Montana
I am sure ATK did a fine job on the original and redesigned SRMs. I'm also certain the system was sufficiently engineered and understood, and performed exactly as required and specified before it was shipped and delivered to the customer.

It is a fact of engineering life, that once a system is delivered to the buyer, the operation and control of that system is out of the engineers control. There was a massive book on shuttle ops, and multiple checklists. We know the low launch temperature was flagged and the fact is that someone in NASA made a conscious decision to launch under conditions that were out of bounds for the SRM system. That's not an engineering failure, it's a management failure and a character failure of that individual.
Well said!


[emoji1010] Steve Shannon [emoji1010]
 
Top