SpaceX Raptor engine - General Thread 4

[tgio]

One RVac and one RSL?  That'd be weird...

Back when SpaceX was first selected for HLS, they did put out a render which showed one RVac and one RSL glowing red just before landing, so they seem to have at least considered it in the past.

[rsdavis9]

I'm trying to figure out how exhaust temperature drops with plume expansion after the plume leaves the nozzle.

Is this diagram in the ballpark?  I'm assuming that with a flow I can just divide by the two areas to get the resulting temperature ala PV=nRT but doing more of an infinitely thin slice for V.

I have no idea if 30 degrees is correct for a SL Raptor in vacuum, but eyeballing several papers it seems reasonable.  Is there a way to estimate this angle given the exhaust velocity and pressure?

I note I can't use it to explain the exhaust temperature difference between Vacuum Raptor (80:1) and SL Raptor (33:1), so I'm pretty sure I got something wrong.

EDIT:  Oh dear, I seem to have run into:

https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule%E2%80%93Thomson_(Kelvin)_coefficient

which says

n a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]

which alas rocket exhaust is at a very high temperature.

So am I correct in saying the only reason Raptor exhaust gas cools in the vacuum of space is by radiation (Stefan Boltzmann)?  That is, until it drops below the inversion temperature.

As my father used to say it's called thermogodamics.

This is my understanding. Basically with an ideal gas the cooling in the rocket nozzle is done from "work done". This is the major reason the gas cools in a rocket nozzle.

As to "free expansion"(no work done) outside the nozzle cooling happens from inter atomic forces and radiation. When dealing with a non ideal gas you have to know the van der wall forces that will cool the gas. Also as you said radiation which has to be minuscule because of the short time period and the relative transparency of the exhaust gases which will give a low emissivity.
 

[Barley]

At some point you care about energy and not temperature.  A white-hot vacuum is still a vacuum.

[wannamoonbase]

https://www.nextbigfuture.com/2023/09/beyond-the-spacex-raptor-engine-is-the-breakthrough-spacex-leet-1337-engine.html

Forgive my posting if this has been posted (I知 working on a phone). 

This all sounds fantastical and maybe reaching beyond the possible.   

[Slarty1080]

I'm trying to figure out how exhaust temperature drops with plume expansion after the plume leaves the nozzle.

Is this diagram in the ballpark?  I'm assuming that with a flow I can just divide by the two areas to get the resulting temperature ala PV=nRT but doing more of an infinitely thin slice for V.

I have no idea if 30 degrees is correct for a SL Raptor in vacuum, but eyeballing several papers it seems reasonable.  Is there a way to estimate this angle given the exhaust velocity and pressure?

I note I can't use it to explain the exhaust temperature difference between Vacuum Raptor (80:1) and SL Raptor (33:1), so I'm pretty sure I got something wrong.

EDIT:  Oh dear, I seem to have run into:

https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule%E2%80%93Thomson_(Kelvin)_coefficient

which says

n a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]

which alas rocket exhaust is at a very high temperature.

So am I correct in saying the only reason Raptor exhaust gas cools in the vacuum of space is by radiation (Stefan Boltzmann)?  That is, until it drops below the inversion temperature.

As my father used to say it's called thermogodamics.

This is my understanding. Basically with an ideal gas the cooling in the rocket nozzle is done from "work done". This is the major reason the gas cools in a rocket nozzle.

As to "free expansion"(no work done) outside the nozzle cooling happens from inter atomic forces and radiation. When dealing with a non ideal gas you have to know the van der wall forces that will cool the gas. Also as you said radiation which has to be minuscule because of the short time period and the relative transparency of the exhaust gases which will give a low emissivity.
 

Would van der Waals force have any effect in a rocket plume? I thought it was feeble and very short range ~1nm

[Ben Baley]

https://www.nextbigfuture.com/2023/09/beyond-the-spacex-raptor-engine-is-the-breakthrough-spacex-leet-1337-engine.html

Forgive my posting if this has been posted (I知 working on a phone). 

This all sounds fantastical and maybe reaching beyond the possible.

I had just run into this article as well and was like 
I hadn't heard anything about this supposed engine before which seems odd, if it's accurate it's very interesting.

Edit: just look at the quoted specifications

[TheRadicalModerate]

Remember that we have no clue how they're going to visualize the landing site.

You visualize the landing spot from orbit a few days or years before.  It's not like there are herds of wildebeest roaming the lunar surface.

But you ask "what happens if a big meteor hits the landing site between reconnaissance and landing?", the same thing that happens if any meteor hits the landing site between landing and takeoff.

I'd guess that there will be all sorts of contingencies that will require, if not a Mark I eyeball, at least a Mark I eyeball looking through one or more cameras.  Top of the list:  lighting conditions that didn't exactly match the pre-landing survey, and failure to pattern-match the landing spot due to exahaust impingement from a few hundreds meters up.  NASA's gonna want a human in the loop, at least to approve the land/no-land condition.

[OTV Booster]

AIUI, which might be totally flawed, the lunar lander would be virtually horizontal and not too high above the surface when it's main engine(s) start their retro burn. Maybe only a kilometer up. The ship will slow and drop but the attitude need not change until the main engine(s) gimbal up, pitching the attitude towards the vertical.

According to the NASA blurb that went with the cold-soak RVac video, it was a 281s-long firing, which is less than half the burn time of an Apollo-style PDI, but it's still a lot more than a burn that starts only a kilometer up.  (Apollo 11 started PDI at about 50,000ft = 15.2km.)

If we need 170t of prop for ascent, 95t dry, and 15t crew module/cargo, and LOI to LS is 2060m/s, then we need about 215t of prop for landing.  that's about 770kg/s of mass flow, which is... kind of a funny number.  Lots of ways to divide that number, but two center RSLs running at 50% throttle comes closest to meeting a 281s burn.  But that can't be right, because they did the test with an RVac.

One RVac and one RSL?  That'd be weird...

What height this happens at depends on how 'sporty' the approach is. The sportier the approach, the closer the lander will be to the surface when impingement becomes an issue, and the more fuel stingy the landing, assuming the landing engines are roughly comparable in performance to the main engines. They probably won't be.

A sportier approach implies a higher throttle, which implies more impingement.  But the 281s number doesn't imply huge amounts of sportiness.

Remember that we have no clue how they're going to visualize the landing site.  It could be that the plume will be almost transparent, in which case an aft-pointing camera (or set of stereo cameras) would be fine.  But if somebody or something needs to peek outside the bulk of the plume, then coming to a dead stop over the site and then powering straight down may be the best way to see what's going on.

I don't recall any small engine tests at MacGregor. Bad memory?

I seem to recall a hot gas methox test of something, but it was a while ago.

The early renders actually show one SL and one Vac, but that can change at the drop of a hat - and probably will already has. Ref: the hot staging thread.


I thought I remembered a small engine test of some sort but chalked it up to wishful thinking.


Good point on scoping out landing spot. Maybe a camera in the engine bay for that last minute fine adjustment. I'm trying to figure out how to translate on the landing engines without at least a small pitch over, and coming up blank. Would cold gas translation engines be enough?

[rsdavis9]

I'm trying to figure out how exhaust temperature drops with plume expansion after the plume leaves the nozzle.

Is this diagram in the ballpark?  I'm assuming that with a flow I can just divide by the two areas to get the resulting temperature ala PV=nRT but doing more of an infinitely thin slice for V.

I have no idea if 30 degrees is correct for a SL Raptor in vacuum, but eyeballing several papers it seems reasonable.  Is there a way to estimate this angle given the exhaust velocity and pressure?

I note I can't use it to explain the exhaust temperature difference between Vacuum Raptor (80:1) and SL Raptor (33:1), so I'm pretty sure I got something wrong.

EDIT:  Oh dear, I seem to have run into:

https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule%E2%80%93Thomson_(Kelvin)_coefficient

which says

n a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]

which alas rocket exhaust is at a very high temperature.

So am I correct in saying the only reason Raptor exhaust gas cools in the vacuum of space is by radiation (Stefan Boltzmann)?  That is, until it drops below the inversion temperature.

As my father used to say it's called thermogodamics.

This is my understanding. Basically with an ideal gas the cooling in the rocket nozzle is done from "work done". This is the major reason the gas cools in a rocket nozzle.

As to "free expansion"(no work done) outside the nozzle cooling happens from inter atomic forces and radiation. When dealing with a non ideal gas you have to know the van der wall forces that will cool the gas. Also as you said radiation which has to be minuscule because of the short time period and the relative transparency of the exhaust gases which will give a low emissivity.
 

Would van der Waals force have any effect in a rocket plume? I thought it was feeble and very short range ~1nm

More generally any inter atomic forces affect the expansion cooling effect.

https://en.wikipedia.org/wiki/Intermolecular_force
Van der waals are mentioned in the first section.

[OTV Booster]

Remember that we have no clue how they're going to visualize the landing site.

You visualize the landing spot from orbit a few days or years before.  It's not like there are herds of wildebeest roaming the lunar surface.

But you ask "what happens if a big meteor hits the landing site between reconnaissance and landing?", the same thing that happens if any meteor hits the landing site between landing and takeoff.

I'd guess that there will be all sorts of contingencies that will require, if not a Mark I eyeball, at least a Mark I eyeball looking through one or more cameras.  Top of the list:  lighting conditions that didn't exactly match the pre-landing survey, and failure to pattern-match the landing spot due to exahaust impingement from a few hundreds meters up.  NASA's gonna want a human in the loop, at least to approve the land/no-land condition.

LiDAR? Radar?


Assuming the lander is never vertical and near the surface (1km?) with the mains burning, there should be no impingement at the exact point of touchdown. With LiDAR or radar, the lighting shouldn't be an issue. Still, ya gotta expect the expected. A set of Mark I's wouldn't hurt, but I'd figure them as a backup.

[TheRadicalModerate]

The early renders actually show one SL and one Vac, but that can change at the drop of a hat - and probably will already has. Ref: the hot staging thread.

If a single RSL fails during final approach with only one RVac running, things are gonna get interesting real fast.  ISTM that two RSLs, one RVac might be the minimum needed.

Good point on scoping out landing spot. Maybe a camera in the engine bay for that last minute fine adjustment. I'm trying to figure out how to translate on the landing engines without at least a small pitch over, and coming up blank. Would cold gas translation engines be enough?

If the landing engines are clustered around the LCH4 dome (which is what I expect), then throttling them selectively might be enough to do small translations.  And if you need a large translation by then, you're aborting.

Even more off-topic:  Non-lunar Starships (i.e., with TPS) are also going to need maneuvering engines, for descent initiation, phasing maneuvers, etc.--the stuff for which D2 uses Dracos.  Continuing my "it's a SuperDraco in spirit but not reality" motif, you need to keep the pods out of the TPS in this case.  So you probably have 2 or 3 pairs instead of four pairs.

That begs the question:  Can you just fix their angles and manage attitude control with a combination of throttling and cold-gas RCS, or do they need to gimbal?  The former would obviously be better.

[ETurner]

I'm trying to figure out how exhaust temperature drops with plume expansion after the plume leaves the nozzle.

Is this diagram in the ballpark?  I'm assuming that with a flow I can just divide by the two areas to get the resulting temperature ala PV=nRT but doing more of an infinitely thin slice for V.

I have no idea if 30 degrees is correct for a SL Raptor in vacuum, but eyeballing several papers it seems reasonable.  Is there a way to estimate this angle given the exhaust velocity and pressure?

I note I can't use it to explain the exhaust temperature difference between Vacuum Raptor (80:1) and SL Raptor (33:1), so I'm pretty sure I got something wrong.

EDIT:  Oh dear, I seem to have run into:

https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule%E2%80%93Thomson_(Kelvin)_coefficient

which says

n a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]

which alas rocket exhaust is at a very high temperature.

So am I correct in saying the only reason Raptor exhaust gas cools in the vacuum of space is by radiation (Stefan Boltzmann)?  That is, until it drops below the inversion temperature.

As my father used to say it's called thermogodamics.

This is my understanding. Basically with an ideal gas the cooling in the rocket nozzle is done from "work done". This is the major reason the gas cools in a rocket nozzle.

As to "free expansion"(no work done) outside the nozzle cooling happens from inter atomic forces and radiation. When dealing with a non ideal gas you have to know the van der wall forces that will cool the gas. Also as you said radiation which has to be minuscule because of the short time period and the relative transparency of the exhaust gases which will give a low emissivity.
 

Would van der Waals force have any effect in a rocket plume? I thought it was feeble and very short range ~1nm

Yes, van der Waals attraction forces have a negligible effect when gas molecules are well separated. At liquid (or liquid-like supercritical fluid) densities, molecules have close neighbors, and during expansion, energy will be absorbed to pull them apart. After expansion to low density, few molecules near neighbors, and the work has been done.

[Slarty1080]

I'm trying to figure out how exhaust temperature drops with plume expansion after the plume leaves the nozzle.

Is this diagram in the ballpark?  I'm assuming that with a flow I can just divide by the two areas to get the resulting temperature ala PV=nRT but doing more of an infinitely thin slice for V.

I have no idea if 30 degrees is correct for a SL Raptor in vacuum, but eyeballing several papers it seems reasonable.  Is there a way to estimate this angle given the exhaust velocity and pressure?

I note I can't use it to explain the exhaust temperature difference between Vacuum Raptor (80:1) and SL Raptor (33:1), so I'm pretty sure I got something wrong.

EDIT:  Oh dear, I seem to have run into:

https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule%E2%80%93Thomson_(Kelvin)_coefficient

which says

n a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]

which alas rocket exhaust is at a very high temperature.

So am I correct in saying the only reason Raptor exhaust gas cools in the vacuum of space is by radiation (Stefan Boltzmann)?  That is, until it drops below the inversion temperature.

As my father used to say it's called thermogodamics.

This is my understanding. Basically with an ideal gas the cooling in the rocket nozzle is done from "work done". This is the major reason the gas cools in a rocket nozzle.

As to "free expansion"(no work done) outside the nozzle cooling happens from inter atomic forces and radiation. When dealing with a non ideal gas you have to know the van der wall forces that will cool the gas. Also as you said radiation which has to be minuscule because of the short time period and the relative transparency of the exhaust gases which will give a low emissivity.
 

Would van der Waals force have any effect in a rocket plume? I thought it was feeble and very short range ~1nm

Yes, van der Waals attraction forces have a negligible effect when gas molecules are well separated. At liquid (or liquid-like supercritical fluid) densities, molecules have close neighbors, and during expansion, energy will be absorbed to pull them apart. After expansion to low density, few molecules near neighbors, and the work has been done.

So presumably just about all that work occurs between the combustion chamber and the end of the nozzle, rather than in the exhaust plume?

[InterestedEngineer]

So presumably just about all that work occurs between the combustion chamber and the end of the nozzle, rather than in the exhaust plume?

I believe Raptor Vacuum converts about 60% of the energy in the MCC to kinetic energy, leaving exhaust at a lower temperature than the MCC temperature of ~3600K.  (~1600-1900K depending on fuel mixture, or so I heard).

But my question was about what happens *after* the plume exits the engine bell, and thus no work is being performed.

I think the answer is the temperature of the individual molecules stays rather toasty, but they are so spread apart that energy flux dissipates with square of the area, with roughly this equation:

area = (((bellDiameter + 2 * distance * tan(angleOfExhaust))/2)^2 * 3.14

So for example a 1.3m diameter bell with expansion angle of 20 degrees has some area values over distance:

0 - 1.33 m²
3 - 9.53
6 - 25.22
9 - 48.39
12 - 79.06

Thus at 12m past the bell, the expansion ratio is 79.06/1.33 or 59.44, and thus if we start with 10bar at the bell exit we end with .17 bar out at 10m, and thus the energy flux is also 1/59 that at the bell

It  would appear that the exhaust isn't going to cool off for quite a while.  Here's what happens to the solar wind temperature:

https://news.wisc.edu/new-research-helps-explain-why-the-solar-wind-is-hotter-than-expected/

[RobLynn]

High combustion chamber pressure rockets are ridiculously efficient (well above 90%) at converting combustor heat energy into exhaust velocity, and the exhausts are cold in their reference frame - to the point that water will start to condense and freeze at near to nozzle exit of high expansion ratio vac engines.

[InterestedEngineer]

High combustion chamber pressure rockets are ridiculously efficient (well above 90%) at converting combustor heat energy into exhaust velocity, and the exhausts are cold in their reference frame - to the point that water will start to condense and freeze at near to nozzle exit of high expansion ratio vac engines.

Raptor consumes 140kg of methane per second with a specific energy of 50MJ/kg, which is 7GW.

650 kg/sec at 3677 m/sec (Raptor-Vac) is 4.4GW of kinetic energy

An efficiency of 63%.

Which leaves you with 2.6GW of thermal energy contained in the exhaust molecules H₂O and CO₂.

Getting back to "what happens to that energy in a vacuum" question I asked, the thermal energy slowly dissipates via Stefan Boltzmann radiation, no energy is dispersed via e.g. Van der Waals forces because the molecules are too far apart, and the velocity doesn't slow down until something is hit.

So like the solar wind, it gets "thinner" farther away (the flux decreasing as function of the square of the distance), but retains all kinetic and a good portion of its thermal energy.

(this has implications for landing on the moon, for hot-staging, and for the OLM pad, but I'm just keeping it to Raptor-specific issues here on this thread)

Is that the correct answer to my question "what happens to the exhaust energy of a Raptor in a vacuum"?

[Brigantine]

I'm no expert on this by any means, but a few thoughts:

1) In the combustion chamber & nozzle, the specific heat ratio is determined by the degrees of freedom thermal energy can occupy - some which are relative motion between molecules, some of which are internal to a molecule (vibration, electron states, quantum weirdness)

2) Under high pressures, the energy can freely move between these different DoFs. In your scenario with low density and short time increments, it may or may not make sense to consider them separately.
(they are reported separately in lab experiments with liquid-helium temperatures at 1 bar)

3) The plume will be expanding at slightly more than sｲ, since as long as dP/ds < 0, dv/ds > 0

4) P*dV as a function of density is basically ⁻dT ∝ f(ρ) ∝ (ρT)* ⁻1/ρｲ dρ, as ρ gets very small, when does that become negligible?

5) What fraction of the heat SH experiences is from local adiabatic heating, as opposed to residual heat in the plume's own reference frame?

[EDIT: 6) if #4 seems counter-intuitive, consider that it may not be a physical change, but rather a statistical change as the population of nearby molecules self-selects to have more and more similar velocities. #1-2 are definitely relevant at this point. In the extreme case of ρ=0, there is no bulk fluid to speak of, therefore no thermal energy in DoFs defined relative to bulk fluid motion.
(which at the molecular level were secretly kinetic energy all along - all that changed was that the mask dropped)

dT/T = B*dρ/ρ
ln(T) = B*ln(ρ)+C
T = Aρ^B
where A is a constant from initial conditions, B is a constant derived from gas constant, molecular weight, relevant heat capacity once the DoFs are completely isolated

Stefan Boltzmann (and other) radiation will slowly deplete only the other DoFs]

[steveleach]

https://www.nextbigfuture.com/2023/09/beyond-the-spacex-raptor-engine-is-the-breakthrough-spacex-leet-1337-engine.html

Forgive my posting if this has been posted (I知 working on a phone). 

This all sounds fantastical and maybe reaching beyond the possible.

It looks like his source is the Musk biography. Has anyone got a copy of that yet?

[sferrin]

https://www.nextbigfuture.com/2023/09/beyond-the-spacex-raptor-engine-is-the-breakthrough-spacex-leet-1337-engine.html

Forgive my posting if this has been posted (I知 working on a phone). 

This all sounds fantastical and maybe reaching beyond the possible.

It looks like his source is the Musk biography. Has anyone got a copy of that yet?

Got it and have started reading it. 

[sferrin]

https://www.nextbigfuture.com/2023/09/beyond-the-spacex-raptor-engine-is-the-breakthrough-spacex-leet-1337-engine.html

Forgive my posting if this has been posted (I知 working on a phone). 

This all sounds fantastical and maybe reaching beyond the possible.

I had just run into this article as well and was like 
I hadn't heard anything about this supposed engine before which seems odd, if it's accurate it's very interesting.

Edit: just look at the quoted specifications

I did see him say, recently, about Super Heavy getting to "three Saturn Vs".  Only way to get there, with 33 engines, is with that LEET 1337 engine.