SpaceX Raptor engine - General Thread 4

[Slarty1080]

Raptor SN56 was loaded into the Raptor van and taken away.

The top image from Mary shows one of the raptors and I wonder about the welds in the top right part, the big methane inlet pipe. These welds look very low fidelity, like someone filled the gap between two pipes with metal paste and melted it. I guess thats normal since they are the professionals and I merely want to understand how things work. Would these welds be cleaned on the inside? Or are there perturbances, small cavities and bulges like on the outside? Im just wondering how stuff like this is done. Also, wondering, would it be better to bend the pipe instead of piecing it together with angled sections?

I know very little about Raptor, but I will stick my neck out and guess that its best to piece the pipes together with angled sections. I will also guess that the welds on the inside are either minimally cleaned or not cleaned at all because it isn't necessary.

Keep in mind what SpaceX are focused on: making humanity a multiplanetary species, they will do whatever it takes to achieve that goal, everything else follows from that. It means, they optimise for both performance and cost. A part needs to be as light as possible and capable of doing what it needs to do, but it also needs to be made as cheaply as possible and be as easy as possible to assemble. Appearances count for nothing. 

[Tangilinear Interjar]

That's pretty much standard method of assembling precise and complex routing with tubing. The inside will be very smooth (and of course thoroughly cleaned) but that outside looks great. Worrying about polishing the outside is just for looks. I would expect down the road when the engine design and plumbing routing is finalized they will use some mandrel bending to reduce the amount of welds.

Pic is from Burns Stainless and is an example of a race usage where performance is paramount but looks are secondary.

[Herb Schaltegger]

That's pretty much standard method of assembling precise and complex routing with tubing. The inside will be very smooth (and of course thoroughly cleaned) but that outside looks great. Worrying about polishing the outside is just for looks. I would expect down the road when the engine design and plumbing routing is finalized they will use some mandrel bending to reduce the amount of welds.

Pic is from Burns Stainless and is an example of a race usage where performance is paramount but looks are secondary.

Bending is fine for reasonable operating pressures. Once you’re talking about high pressure though, it can be problematic. Bending causes localized stress concentrations and can thin the material around the periphery of the bend. For low pressure it’s not an issue. With some alloys, bending may be usable at higher pressures than others, and the radius and design mass flow rate matters greatly too. Part of the magic of engine design for quantity production is determining where this trade lies and making the right fabrication choice(s) for cost and reliability.

[Remes]

Pic is from Burns Stainless and is an example of a race usage where performance is paramount but looks are secondary.

Is that exhaust pipe finished? It looks like at the exhaust flange at the motor the pipe is just tacked. There is some work left. Among that cleaning of the welds (if it's not used for just one race).

The discolorations are oxyde. And no weld with stainless is accepted, where the oxyde has not been cleaned. (Yes, stainless can oxydize and it can rust, google it ).

The inside might nevertheless look perfectly clean: When welding stainless pipes, the inside is typically purged with an inert gas. So that there is no chance of oxydation. Especially with stainless an effect called "sugaring" happens.

In Aerospace many times things are welded in an argon box. When the parts are bigger, e.g. submariness, the part is placed in a room, purged with argon, and the welder goes in with oxygen and a suit.

So for high quality, for engines which are reused for n-times, which are stored close to the see, etc. the welds would be cleaned of oxydation and passivated. In the current situation I guess the lifetime of raptors is so short, that it doesn't matter.

Does anyone know what this oxydation would do when it comes into contact with lox?

[schuttle89]

Pic is from Burns Stainless and is an example of a race usage where performance is paramount but looks are secondary.

Is that exhaust pipe finished? It looks like at the exhaust flange at the motor the pipe is just tacked. There is some work left. Among that cleaning of the welds (if it's not used for just one race).

The discolorations are oxyde. And no weld with stainless is accepted, where the oxyde has not been cleaned. (Yes, stainless can oxydize and it can rust, google it ).

The inside might nevertheless look perfectly clean: When welding stainless pipes, the inside is typically purged with an inert gas. So that there is no chance of oxydation. Especially with stainless an effect called "sugaring" happens.

In Aerospace many times things are welded in an argon box. When the parts are bigger, e.g. submariness, the part is placed in a room, purged with argon, and the welder goes in with oxygen and a suit.

So for high quality, for engines which are reused for n-times, which are stored close to the see, etc. the welds would be cleaned of oxydation and passivated. In the current situation I guess the lifetime of raptors is so short, that it doesn't matter.

Does anyone know what this oxydation would do when it comes into contact with lox?

If it's on the outside of the engine only then it *should* never come into contact with LOx.

[OTV Booster]

Raptor SN56 was loaded into the Raptor van and taken away.

The top image from Mary shows one of the raptors and I wonder about the welds in the top right part, the big methane inlet pipe. These welds look very low fidelity, like someone filled the gap between two pipes with metal paste and melted it. I guess thats normal since they are the professionals and I merely want to understand how things work. Would these welds be cleaned on the inside? Or are there perturbances, small cavities and bulges like on the outside? Im just wondering how stuff like this is done. Also, wondering, would it be better to bend the pipe instead of piecing it together with angled sections?

A day late and a dollar short. My experience with tubing benders is a reduction in cross section through the bend. Extrapolating what I know about fluid flow there would be an increase in fluid velocity through the bend and a drop in pressure. This would all even out past the bend. This is not based on direct measurement and other than the downstream turbulence expected from any curve in a flow path, I can not see any downstream issues from a slightly pinched bend.


I suspect there are ways of making a bend that does not pinch if that causes issues and it might be a design waiting for the machine that makes the machine.

On the welds, I can only venture a guess based on limited experience with MIG and TIG on mild steel. I think an experienced welder could get a reasonably
smooth surface on the underside and do a bit of buildup on the top side for strength.


Edit: umph!  Day late - yup

[launchwatcher]

I suspect there are ways of making a bend that does not pinch if that causes issues and it might be a design waiting for the machine that makes the machine.

One approach used in musical instrument making involves using ice to support the tube walls during the bend, then fixing the bore after the bend by pushing a series of balls of appropriate diameter through the bore while the outside of the tube is constrained by a die.   



(from around 2:45 to 3:45 covers drawing the tube, bending it, and then cleaning up the bore; this is a 180-degree bend for a trombone tuning slide).

[JoeWakefield]

Nice video of a Raptor test out at McGregor yesterday:

[Hann]

Does the Raptor have booster pumps?  It is common for Soviet engines to have booster pumps.  Judging by the photo, the Raptor does not have booster pumps, but how is the engine's non-cavitation operation ensured?

[Slarty1080]

Does the Raptor have booster pumps?  It is common for Soviet engines to have booster pumps.  Judging by the photo, the Raptor does not have booster pumps, but how is the engine's non-cavitation operation ensured?

I believe the tanks themselves are pressurised to several atmospheres in part to help prevent this

[Alberto-Girardi]

Does the Raptor have booster pumps?  It is common for Soviet engines to have booster pumps.  Judging by the photo, the Raptor does not have booster pumps, but how is the engine's non-cavitation operation ensured?

I believe the tanks themselves are pressurised to several atmospheres in part to help prevent this

So presurized tanks to several atmposphere isn't usually used?

[rsdavis9]

I don't know what other rockets use for pressure.
The raptor is designed for 6 bar.
I think spacex killed 2 birds with one stone here.

6 bar provides significant strength to their stringerless construction almost balloon tanks.
6 bar gets rid of the booster pumps.

win/win.

[OTV Booster]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

[Doom2pro]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

I remember Musk saying 7-8 bar for Flight...

[livingjw]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

I remember Musk saying 7-8 bar for Flight...

7-8 bar is too much pressure for 4 mm tanks. Engine inlet pressure probably 5-6 bar.

John

[baldusi]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

I remember Musk saying 7-8 bar for Flight...

I think that was the proof pressure and flight was around 6.4 or so. Can't remember.

[niwax]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

I remember Musk saying 7-8 bar for Flight...

I think that was the proof pressure and flight was around 6.4 or so. Can't remember.

8.4 bar is 6 bar * 1.4 safety factor. It's what they pop tested earlier tanks to.

[Nomadd]

 Where are all these bars everyone is referring to? The top of the tank? Is there some sort of regulator to the engine inlet to keep gas pressure in the tank plus head pressure from getting too high?

[rsdavis9]

Where are all these bars everyone is referring to? The top of the tank? Is there some sort of regulator to the engine inlet to keep gas pressure in the tank plus head pressure from getting too high?

You got the vertical dimensions of the 4 tanks handy?
for the SH LOX I have:
(1.25t/m^3)*g*1.5*36m =6.6bar head pressure.
1.25t/m^3 for sub-cooled LOX
36m vertical
1.5g at takeoff
As the booster goes up it gets to 3g but it is probably only half full at that point.

[OTV Booster]

Picking a nit. At launch the tanks are pressed significantly below 6bar. The combined ullage pressure and propellant weight at the bottom of the tank is targeted for 6bar. As propellant level goes down Autogenous pressure increases - except acceleration is adding to the force at the tank bottom. It's a delicate dance.

I remember Musk saying 7-8 bar for Flight...

I think that was the proof pressure and flight was around 6.4 or so. Can't remember.

I think they took a test unit to 7bar+, but did not press to destruction. I think. I got CRS too.