Rutherford Engine Nozzle Material?

[Pegabug]

I am quite confused on what they could be using for the nozzle material for the Rutherford engines. It has to withstand re-entry temperatures, which Rocket Lab says is up to 2400 C (https://www.rocketlabusa.com/updates/how-to-bring-a-rocket-back-from-space/), which that alone takes most nozzle materials out of consideration.

INCONEL Alloys: All have a melting point far lower than the temperatures RL states for re-entry. Additionally, these alloys are sensitive to heat treating, which would require a heat treatment to a suitable temperature where repeated re-heats will not change the grain structure.

Copper Alloys: A typical selection for inner wall materials to increase the heat transfer for cooling. Has a low melting point and also can be heat treated.

Stainless Steels: Again, melting point similar to INCONEL, not able to withstand the heat. Good high strength and elevated temperature performance and resists corrosion.

Niobium: Melting point around 2400 C, expensive material, and is a refractory metal. RL uses 3D printing for their engines,  which refractory metals are notoriously difficult in additive manufacturing.

None of the main materials seem to be capable of being the material they use. Yet we can see their booster stage is able to return and the nozzles remain, albeit with a bit of discoloration. How are they able to do this? Perhaps they experience a lower peak temperature than they state?

[Pipess]

SpaceX Merlins seem to survive reentry fine, I'm not sure what alloy they are made of but I believe it comes up in Tim Dodd's recent interview with Musk. Also unsure if that alloy can be 3D-printed.

[Pegabug]

The Merlin uses a retro burn however. That creates a cushion of gas to protect the engines from re-entry heating. The Electron does not do this, so the Rutherfords have to brace the full force of the atmosphere. As for Merlin materials, IIRC during the interview Elon says the chamber is copper with nickel on the outside, which is the standard. The nozzle itself may be different, but I'd doubt it. The Merlin 1D Vacuum uses Niobium alloy after the regenerative cooling section.

[Gliderflyer]

I'm pretty sure the Rutherford is Inconel. The article says the flow temperature is 2400C (~4800R), which is in the ballpark for the gas stagnation temperature for a first stage reentry: https://www.grc.nasa.gov/www/BGH/stagtmp.html

That doesn't mean the rocket gets that hot though.

[Pegabug]

I'm pretty sure the Rutherford is Inconel. The article says the flow temperature is 2400C (~4800R), which is in the ballpark for the gas stagnation temperature for a first stage reentry: https://www.grc.nasa.gov/www/BGH/stagtmp.html

That doesn't mean the rocket gets that hot though.

Could you explain why the rocket wouldn't reach those temperatures? With the engines leading on re-entry and the non-streamlined shape breaks up the airflow so the forming of slower viscous layers at lower temperatures wouldn't form like we see in the combustion chamber.

[TrevorMonty]

The Merlin uses a retro burn however. That creates a cushion of gas to protect the engines from re-entry heating. The Electron does not do this, so the Rutherfords have to brace the full force of the atmosphere. As for Merlin materials, IIRC during the interview Elon says the chamber is copper with nickel on the outside, which is the standard. The nozzle itself may be different, but I'd doubt it. The Merlin 1D Vacuum uses Niobium alloy after the regenerative cooling section.

Peak thermal loads on F9 just before reentry burn are similar to Electron at reentry. Electron doesn't need reentry burn as it is lighter so atmosphere is lot more effective at slowing it down. Comes down to kg per m2.
F9 50t over 10.75m2 =4.65tm2
Electron 1t over 1.13m2 =0.88tm2.
Neutron 50t over 38m2=1.28tm2

Which is why Neutron can avoid reentry burn.

[Gliderflyer]

I'm pretty sure the Rutherford is Inconel. The article says the flow temperature is 2400C (~4800R), which is in the ballpark for the gas stagnation temperature for a first stage reentry: https://www.grc.nasa.gov/www/BGH/stagtmp.html

That doesn't mean the rocket gets that hot though.

Could you explain why the rocket wouldn't reach those temperatures? With the engines leading on re-entry and the non-streamlined shape breaks up the airflow so the forming of slower viscous layers at lower temperatures wouldn't form like we see in the combustion chamber.

I'm not a reentry expert, but a lot of the heating goes into the bow shock from what I have read. Also, while the gas is very hot, there is not a lot of it because you are at high altitude (it's a small fraction of a psi at 100,000 feet for example). It also takes time for things to heat up, so it won't instantly get to the gas temp and you are slowing down the whole time.

I haven't looked, but there is probably some good stuff on NTRS.

[Stan-1967]

Rocketlab could also actively cool the nozzles during entry by circulating cryogenic prop residuals through the cooling channels.  When watching F9 reentry videos,  it appears SpaceX does this.

[Gliderflyer]

I doubt it (for both Electron and Falcon), it's a whole additional system and there is zero evidence of it in pictures. It wouldn't be cryo either as the engines are kerosene cooled; you would have to carry a separate LN2 tank (you are not using LOX), and the residual kerosene is a building material at cryo temps.

[Pegabug]

The Merlin uses a retro burn however. That creates a cushion of gas to protect the engines from re-entry heating. The Electron does not do this, so the Rutherfords have to brace the full force of the atmosphere. As for Merlin materials, IIRC during the interview Elon says the chamber is copper with nickel on the outside, which is the standard. The nozzle itself may be different, but I'd doubt it. The Merlin 1D Vacuum uses Niobium alloy after the regenerative cooling section.

Peak thermal loads on F9 just before reentry burn are similar to Electron at reentry. Electron doesn't need reentry burn as it is lighter so atmosphere is lot more effective at slowing it down. Comes down to kg per m2.
F9 50t over 10.75m2 =4.65tm2
Electron 1t over 1.13m2 =0.88tm2.
Neutron 50t over 38m2=1.28tm2

Which is why Neutron can avoid reentry burn.

Interesting, that is twice now while researching the Electron's re-entry I've heard it being described as having a "fluffiness" to it so it survives re-entry. I wonder how they determine the max t/m^2 to survive it; regardless aiming for the Electron's value would be safe and the Neutron as a maximum.

Rocketlab could also actively cool the nozzles during entry by circulating cryogenic prop residuals through the cooling channels.  When watching F9 reentry videos,  it appears SpaceX does this.

Building off what Gliderflyer said, RL does use room temperature RP-1 (SpaceX uses coldened RP-1 in the Falcon 9) so the cooling effects aren't as good as a cryogenic. Additionally, the Falcon 9 is quite unlikely to be able to pump during re-entry since engines that are off have their turbopump off as well. RL could still pump fuel as a coolant to the nozzle to an off engine since they use an electric turbopump. But I would like to see a source saying they do because they would have to overcome the serious problem of pumping against ullage AND vapors in a turbopump are very bad for the health of the impeller.

[Dmitry_V_home]

Could you explain why the rocket wouldn't reach those temperatures?

Among other things, the equilibrium temperature of the surface of the rocket (and engines, among other things) is much lower than the temperature of the air flow.

[edzieba]

I've found a few papers (e.g. Vázquez, Valeria. (2021). Material and Manufacturing Selection for Rocket Combustor Testing Prototype) that cite Rutherford as being printed with an unspecified "Inconel-Titanium alloy", but the "Rocketlab (2014)" citation for that is unhelpfully vague.