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System check went well as UTC 23:48 ... The waiting room is due to open a bit before 1:00 UTC.
7 PM Houston, 8 PM New Hampshire, mid-Morning Australia
Waiting room is open 1:00:00
(th)
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Today is Google Meeting day for NewMars.
All members (and guests) are welcome, if you can work the time into your schedule.
As a reminder, we are willing to open the meeting at an earlier time that might work better for our European members.
The existing time (midnight or 1 UTC) was convenient for the folks living in Texas and New Hampshire.
No time is ideal for everyone.
The existing time would (hopefully) work for a member living in Australia. The time there would be mid-morning ( I understand ).
That would be right in the middle of a workday morning, of course.
Special meetings can be arranged at almost any time that the participants and host(s) can agree upon.
Such a special meeting is planned for 1 PM Houston time today. kbd512 (our webmaster) and I are planning a work session to see if we can bring the NewMars image server up on the Mars Society system. The test system on the Azure account has proven itself to be reliable, and usable by NewMars members.
The regular meeting is scheduled for 1:00 UTC, 7 PM Houston, 8 PM New Hampshire, and mid-Morning New Hampshire.
I hope to have a run of extendedMerlin from time zero to 3 seconds to show to attendees. The run is in progress now, standing at 2.49 seconds.
This run will (if all went well) show operation of the new volume heating system from the start of a run for a full 3 seconds. For the first 2 seconds we slowly increase the flow of hydrogen into the heating tube, until we reach the prescribed rate of 2 kg/s. We then have a full second of operation at full flow with 40 MW of thermal energy added uniformly to the 24,000 cells in the intake, exactly as the physical heating system would supply thermal energy to the incoming hydrogen in deep space. Because of the ridiculous large nozzle throat (25 cm) we only see about a bar of pressure in the intake, but we'll see that improve when we reduce the size of the throat to better match GW Johnson's recommendation of (about) 3 cm for hydrogen.
I note that kbd512 has found documents from NASA and from other sources showing much smaller diameter throats. The Merlin engine was designed for massive flows of hydrocarbon fuel and liquid oxygen. Hydrogen would (apparently) perform better with a much smaller throat. Work on a small throat for the OpenFOAM is scheduled for next week. ChatGPT5 offers a minimum of 3 options for reducing throat size. Two are temporary methods that are implemented with adjustments to the control dictionaries. The "permanent" solution involves changing the mesh. That option is attractive because we would be building new mesh on the existing Merlin engine mesh. That seems easier to me than trying to remove cells from the mesh, as would be the case if we were trying to increase throat size. I am looking forward to trying the three options and learning about the strengths and weakness of each.
The fourth option is to create an entirely new mesh for the hydrogen engine and extended intake. While that is certainly possible, it is a ** lot ** of work, and I'm hoping one of the easier alternatives will meet our requirements. The temporary solutions involve creating a mathematical "washer" shape to be inserted into the existing mesh at run time. The resulting opening will (most likely) create turbulence because the opening will not be rounded smoothly like the existing mesh, but it should certainly build up pressure in the intake, and allow more time for hydrogen to warm up before it reaches the throat and departs for deep space. If one of the temporary solutions shows significant gains (as seems likely) we can embark upon finding a permanent solution.
Update: For those who might be curious, the CFD model should be able to compute the behavior of hydrogen in the intake if we set the throat diameter too small. The pressure should rise beyond the bursting point of any known material. The CFD model would (presumably) not break, but past the bursting point of known materials, the results are not particularly useful.
(th)
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It's not the identity of hydrogen that makes the big difference. It is merely the interplay of mass flow rate, throat area, the effective c*-velocity of the chamber conditions, and the resulting chamber pressure. Hydrogen vs something else merely changes the c*-velocity, by several-to-many percent, but not by really large factors. The temperature is higher but the molecular weight is lower. Those effects upon c* are in the same direction, though (see below).
All species, no matter how they are heated, must obey the nozzle flow rate equation: wnoz = Pt CD At gc / c* where CD is the discharge efficiency of that choked throat: effective A / geometric A, and is usually rather close to, but never exactly equal to, 1.
As for c*, there is an equation for it, too: c* = square root of (gc R Tc GF / gamma), where GF is a function of gamma, but it is always pretty close to 2.9 in numerical value. The square root reduces the effects of large changes in Tc. The molecular weight shows up in the gas constant R, which is the molar universal value divided by the molecular weight. That is actually the bigger change with non-combustion heated hydrogen, simply because the molecular weight is greatly lower than that of most other species, and the effect is only reduced some by the square root.
GW
PS: the GF is (gamma + 1)/2 raised to an exponent that is (gamma + 1)/(gamma - 1). We have modeled hydrogen's gamma as 1.4 from cryogenic to 1000 K, decreasing linearly to 1.286 as the temperature increases to 3000 K, with molecular weight constant at 2.016 throughout that temperature interval. Beyond 3000 K, dissociation changes all those numbers. It is not molecular hydrogen anymore.
Last edited by GW Johnson (2025-11-30 12:04:45)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Google Meeting for Sunday the last day of November, 2025, is under way
00:59:22
Open for business....
Post meeting notes:
This was another ** very ** productive meeting ...
GW Johnson and kbd512 are both set up on the new image server.
All NewMars members are welcome to register with the new images server. I left registration open for now. I'll close registration when the first spammer finds the site and breaks in. In the mean time registrations are welcome from NewMars members.
***
Work on the OpenFOAM study of the Optical Plane rocket engine will resume next week with three different ways of creating the effect of a smaller throat. One method involves setting a porous "wall" in front of the throat. The amount of porosity will (I gather) simulate the effect of having a smaller throat.
The next method is to mathematically create a ring or washer shaped structure dynamically at run time. This is (apparently) a bit more difficult, but it will allow us to see what an opening of 3 centimeters will "look" like to the hydrogen gas we are pumping through the system.
The third method is more challenging, and it is where I want to end up. This involves creating a mesh shaped like the ideal engine that GW Johnson has prescribed, and blending or merging it into the existing mesh. I am attracted to this option because we are shrinking the throat, and the new mesh will (if all goes well) blend seamlessly into the existing Merlin engine mesh, so that the original Merlin Mesh will be inside the new mesh. I'm told (by ChatGPT5) that this procedure will result in a model that looks to the gas like the engine GW Johnson would like to see for this application.
The distinct advantage of this approach is that all the rest of the mesh is retained and I don't have to worry about it.
This whole process is likely to take to the end of the year.
(th)
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