SpaceX Raptor engine (Super Heavy/Starship Propulsion) - General Thread 1

[Davidthefat]

I wouldn't be surprised if they film cooled in the chamber near the injector.

[DJPledger]

I wonder how fast can SpaceX iterate a design? Could they have tested version 1.0 then incorporate design changes into the next one and be testing version 2.0 the next week or two?

Super allow foundry allows for rapid iteration of Raptor. So it is entirely possible that SpaceX have made changes to Raptor SN2 based on data from SN1 firings. Modern CAD/CAM + 3D printing of mold patterns for cast parts can allow for very rapid iteration of designs.

EM said he was worried about Raptor meeting it's thrust/cost goal. Perhaps SpaceX could scale up Raptor to increase it's thrust/cost ratio. Larger engines should be cheaper per unit of thrust than smaller ones with everything else being constant. 19 larger Raptors for SH should be cheaper than 31 smaller Raptors for same installed thrust.

[Tuna-Fish]

EM said he was worried about Raptor meeting it's thrust/cost goal. Perhaps SpaceX could scale up Raptor to increase it's thrust/cost ratio. Larger engines should be cheaper per unit of thrust than smaller ones with everything else being constant. 19 larger Raptors for SH should be cheaper than 31 smaller Raptors for same installed thrust.

They can't do that unless they also make the upper stage a lot heavier. The upper limit on Raptor size is set by the requirements to be able to hover and to be able to safely land even if they lose 2 engines during landing. This means that the thrust of the raptor at min safe throttle at sea level cannot be more than 1/3rd of the mass of the upper stage when near-empty.

They might relax those requirements a bit for the initial, unmanned versions, but I really don't see them making raptors bigger than the current design without also going to a larger diameter craft. Besides, Merlins are the champions of thrust/cost already, even with Raptors being a little worse than that, so long as they are reusable enough times, they should be fine.

[ZachF]

EM said he was worried about Raptor meeting it's thrust/cost goal. Perhaps SpaceX could scale up Raptor to increase it's thrust/cost ratio. Larger engines should be cheaper per unit of thrust than smaller ones with everything else being constant. 19 larger Raptors for SH should be cheaper than 31 smaller Raptors for same installed thrust.

They can't do that unless they also make the upper stage a lot heavier. The upper limit on Raptor size is set by the requirements to be able to hover and to be able to safely land even if they lose 2 engines during landing. This means that the thrust of the raptor at min safe throttle at sea level cannot be more than 1/3rd of the mass of the upper stage when near-empty.

They might relax those requirements a bit for the initial, unmanned versions, but I really don't see them making raptors bigger than the current design without also going to a larger diameter craft. Besides, Merlins are the champions of thrust/cost already, even with Raptors being a little worse than that, so long as they are reusable enough times, they should be fine.

Elons wording also seems to suggest that it will still be close though.

A 300 bar FFSC engine with 330+ sl ISP and and 'only' the second best TWR and cost/thrust ratio is still an engine that is so much better than all of the competition it's almost crazy... Merlins are insanely cheap by industry standards (under $1m).

If I had to guess Raptor is probably ~1200kg and costs a little over $3m each... I bet they still have a good chance of beating the TWR figures with the "full thrust" Raptor. Other LV providers would probably kill to have such an engine at a little over $3m.

Cost/thrust is probably out of reach just for the FFSC architecture.

The Raptor engine really is almost a work of art...

[punder]

A few days ago it was all "OMG look how fast they're going!!" and now it's all doom-n-gloom.

This is the very first full-scale, flight-worthy Raptor, in its very first series of firings, correct?

A quick review of previous engine development efforts might provide some perspective.

Okay off my high horse. Just sayin.   

Mr. Obvious

WOW my 500th post! An extra Glenmorangie tonight.

[DJPledger]

EM said he was worried about Raptor meeting it's thrust/cost goal. Perhaps SpaceX could scale up Raptor to increase it's thrust/cost ratio. Larger engines should be cheaper per unit of thrust than smaller ones with everything else being constant. 19 larger Raptors for SH should be cheaper than 31 smaller Raptors for same installed thrust.

They can't do that unless they also make the upper stage a lot heavier. The upper limit on Raptor size is set by the requirements to be able to hover and to be able to safely land even if they lose 2 engines during landing. This means that the thrust of the raptor at min safe throttle at sea level cannot be more than 1/3rd of the mass of the upper stage when near-empty.

They might relax those requirements a bit for the initial, unmanned versions, but I really don't see them making raptors bigger than the current design without also going to a larger diameter craft. Besides, Merlins are the champions of thrust/cost already, even with Raptors being a little worse than that, so long as they are reusable enough times, they should be fine.

Future iterations of Raptor could have even deeper throttle capability which could allow the no. of Raptors on SS to be reduced from 7 to 5 which would allow Raptor to be sized for 19 on SH. Also 19 is one of the nos. that gives max. packing density on SH while 31 is not. OTOH, SpaceX could make Raptor smaller for 9 on SS and 37 on SH which would also give max. packing density on SH. Still think that smaller Raptor may cost more than larger one for entire SS/SH propulsion system.

[RoboGoofers]

EM said he was worried about Raptor meeting it's thrust/cost goal. Perhaps SpaceX could scale up Raptor to increase it's thrust/cost ratio. Larger engines should be cheaper per unit of thrust than smaller ones with everything else being constant. 19 larger Raptors for SH should be cheaper than 31 smaller Raptors for same installed thrust.

he is saying here that Raptor might not be a record-breaker, not that it won't meet its design goals.
https://twitter.com/elonmusk/status/1098613993176850432

[livingjw]

I think the concern about having to dial up film cooling is that it means you're dumping more unburned fuel out of your nozzle and this is diametral to maximizing ISP.

No unburned fuel is dumped.

Fuel film cooling is typically very small fraction  of total flow and is introduced around the main chamber at the injector plane. It is also used at the throat. The cycle is fuel rich, so of course unburnt fuel is expelled, but mostly in the form of H2 and CO.

[spacenut]

Even if it didn't meet it's thrust/weight and thrust/cost goals, being reusable, will save money in the long run.  Raptor also will not have to be cleaned or rebuilt as often as Merlin. 

[RobLynn]

Superheavy would work fine with Merlin, or a modified Merlin burning Methalox if that turned out to be more cost effective (and at this stage who knows, it might, as per-flight engine depreciation is quite possibly the dominant cost for the whole SS/SH system).

Regarding film cooling: methane has low molecular mass and high specific heat.  At point it enters the chamber it is already at 700-800K and even it it only stays at that temperature without combusting with LOX will expand and be ejected with a velocity of nearly 2500m/s (Isp 250s).  In reality it will be heated further by radiation, so that no-oxygen thin film region next to the wall costs very little in reduced average Isp.

Hydrogen film cooling is even better- it can potentially have higher Isp than the main combusted flow.

[Coastal Ron]

Superheavy would work fine with Merlin...

Something LIKE the CH4/LOX powered could be built that uses the RP-1/LOX Merlin engines, but I don't know why SpaceX would want to make such a radical change at this point.

...or a modified Merlin burning Methalox if that turned out to be more cost effective...

I'm not a rocket engineer, but from what I do understand you can't simply swap fuels on rocket engines because they are optimized for a specific fuel.

I don't see any need for SpaceX to change away from Raptor and methane at this point...

[Nilof]

I'm mostly curious to see whether the Raptor will see continued improvements over the year like the Merlin did.

Let's imagine the case where they managed to uprate the chamber pressure by some absolutely crazy factor to say, 35-40 MPa. What would the most likely change to the design be? A tank stretch like on the Falcon to increase payload, or maybe reducing the number of engines to match the increased thrust, and using the extra space to increase the expansion ratio to squeeze out more Isp?

[CorvusCorax]

You do have to wonder if the unstated part of “(as expected)” is “but not as hoped.”

It’s doubtful they actually already intended to test to destruction after so few runs, especially when they need three tested to operational readiness for BFH in just a couple months.

Elon does appear to be worried that performance for Raptor may not improve at fast enough rate to overcome any weight budget challenges Raptor or Starship has getting built.

Maybe. But there is a reason to find out what it can't do early on, rather than taking a lot of time characterizing what it can do: if it ultimately falls short, you've wasted that time.

My guess would be they learned something important on the first runs that immediately got incorparated into sn2 and would make further testing of that part of the design less useful so they decided to stress some other parts hard since sn2 is coming soon

That!  I think, as seen in the first videos posted by Elon, the engine had a flaw. Maybe a hotspot at the throat or in the chamber with insufficient regen cooling. Sonething they could fix for SN2, like a a minor cooling channel redesign or a slight change in geometry.

In the interim, they worked around the issue with excessive "film" cooling. To the point you could see the flaming flow of burning Methane in the exhaust. This amount likely eats into their ISP and wouldnt be practical for a flight engine, but it allowed them to continue the test campaign and rattle the engine to see what else needs fixing.

Apoarently nothing else came loose, so they rattled harder. Got a neat chamber pressure miledtone, rattled even harder ( see L2 ) until they found something that breaks, which then gets incorporated into the next design change.

You wouldn't stop testing when you find the first small issue with the first full size prototype. If you did, iteration would take far too long. You rather work around it and try to learn as much as you possibly can from the thing.

To quote Elon Musk ( in the context of grasshopper, but it applies here )  " If it didn't blow up, you did not test hard enoigh"

[ZachF]

Superheavy would work fine with Merlin, or a modified Merlin burning Methalox if that turned out to be more cost effective (and at this stage who knows, it might, as per-flight engine depreciation is quite possibly the dominant cost for the whole SS/SH system).

Regarding film cooling: methane has low molecular mass and high specific heat.  At point it enters the chamber it is already at 700-800K and even it it only stays at that temperature without combusting with LOX will expand and be ejected with a velocity of nearly 2500m/s (Isp 250s).  In reality it will be heated further by radiation, so that no-oxygen thin film region next to the wall costs very little in reduced average Isp.

Hydrogen film cooling is even better- it can potentially have higher Isp than the main combusted flow.

It would "work" but there is much more than just Thrust per $.

Raptor's potent combo of TWR, T/$, ISP, Impulse density, and cheap fuel cost is what makes it the best engine out there, IMHO.

Let's do a little thought exercise:
------------------------------------------------------------------------------

Suppose you want to switch out Raptors for GG "Methamerlin" engines, How would that impact the system as a whole?

A gas generator Merlin-style engine could probably get an ISP of 300/320 on a sea-level version, and ~358 in a vacuum version. So lets plug these in to see what we'd need to match the current design, but first a few points on how these numbers were derived:

(See attachment)

-The landing dVs were derived from the original 2016 ITS slides. The original required 7% of it's fuel, so I calculated the dV given from that and set it as the landing load. Landing fuel for the upper stage was taken from measurements of the landing tanks from the 2016 iteration an dV calculated.

-Dry masses are taken from the 2016 and 2017 proposals. On a percentage basis. I included a little mass increase on the upper stage as well.

-Lower stage was stretched some in latest proposal. Estimated here.

These numbers are not perfect but a good enough for the purposes of showing the comparison

The top three are, left to right:
-First iteration with all SL Raptors
-GG "Methamerlin" lower stage with SL Raptor upper stage... doesn't make much sense, but I put it there for comparison's sake.
-GG Methamerlins on both the upper and lower stage

Bottom four have vacuum engines and a larger payload, left to right:
-Raptors with Raptor vacs
-GG Methamerlins with Raptor vacs
-GG Methamerlins with Methamerlin vac upper stage
-Kerolox Merlin lower stage with vacuum raptors on upper stage


You'll notice in the non-Raptor iterations mass increases massively. Lower ISP also means a larger fraction of the fuel must be held as a landing reserve. 

Some findings:
------------------------------------

-Switching out Raptors with GG Methamerlins on the lower stage increases the mass of the lower stage by 37%. It's likely that switching to GG engines would require the mixture ratio to be dropped from ~3.7:1 to ~3:1, reducing fuel density by ~5% and increasing volume by a larger factor than mass. Mass of the stack as a whole increases by 28%, thus you need 1.28x thrust if you wanted to switch out Raptors for GG Methalox engines. If you kept Raptor Vacs for the upper stage but used Methamerlins on the lower stage you'd no longer have common engines and the cost for both would rise, probably by at least 20%. So Raptor could cost >50% more in Thrust/$ and still come out on top in this scenario... But that does not factor in the >37% increase in the lower stage mass which would increase the fabrication costs, with those Raptor could cost much much more and still come out on top.

-Switching out Raptors for GG engines on both the upper and lower stages increases the mass of the stack as a whole by 60% when comparing the vacuum-engined versions. Raptor could thus cost >60% more than the GG Methamerlin and come out on top. Again, this does not factor in the increased cost from dry mass increasing by over 60%. So Raptor could again cost much more in thrust/$ and still win out big time on a systemic basis. Also, in this version, because of the poorer ISP in the vacuum Methamerlins, it now takes 30% more tanker flights to refuel in orbit.

-Switching out a Raptor powered lower stage with a Kerolox Merlin-powered lower stage increases the size of the lower stage by over 50%. Dry mass would increase by a similar amount. So again, Raptor could again cost much more in thrust/$ and still win out big time on a systemic basis. Also, on this proposal, fuel costs per launch go from about $500k for the Methane, to $2+ million for 5,140 tonnes of kerolox fuel.


Long story short, Raptor can cost much more on a thrust/$ basis but it will still win on a systemic basis. If it is even close to Merlin it is a no-brainer to go with Raptor.

(EDIT- noticed I used vacuum figures for the SL Methamerlins, so the numbers are actually worse...)

[Chris Bergin]

As promised, thread 2 now live:

https://forum.nasaspaceflight.com/index.php?topic=47506.0