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Merlin and RD-180 engines

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Peter B:
I got into a little discussion on YT about the Axiom 1 launch.

One commenter said that Merlin engines were just re-branded NK-33 engines. I asked what the similarities were, and his blustery response boiled down to "high efficiency", although he also repeated the claim the design was copied. When I called him on this, in his next reply he switched over to comparing the Merlin to the RD-180.

He made five claims, which I think I know the answers to, but I thought I'd run them past the experts here for correction where necessary:

1. In 2019 Musk said the RD-180 was the finest rocket engine ever made. Sort of true - he said "excellent engineering"
2. Both engines use RP-1 and LOX. True but irrelevant - many other engines do too
3. Both engines have a duel nozzle design. False - the Merlin is a single nozzle
4. The RD-180 has a dual combustion chamber design. True but irrelevant - the Merlin doesn't
5. Reusable first stages are "showmanship without benefits", according to Russians. False - the reusability of the Falcon 9 first stage and the payload shrouds has reduced launch costs considerably, which is a benefit. (I couldn't find any reference to the "showmanship without benefits" quote but didn't look far.)

I have a suspicion he's now comparing the RD-180 to the Raptor engine, but I'm not sure, and I'm also unsure how similar they are if he is (of course, if he is, it's irrelevant to the Falcon 9...).

Any help would be appreciated. Thank you!

molesworth:
I was about to go into how the Raptor is nothing like the RD-180, so there's not really any basis for comparison, when by coincidence a new video from Tim Dodd (Everyday Astronaut) popped up on my YouTube notifications.  I've not finished watching yet, but it looks like it's a good comparison of the different engine types, flows, fuels etc.  Hopefully it'll answer all/most of your correspondent's questions or claims.

smartcooky:

--- Quote from: Peter B on April 28, 2022, 10:14:51 AM ---I got into a little discussion on YT about the Axiom 1 launch.

One commenter said that Merlin engines were just re-branded NK-33 engines. I asked what the similarities were, and his blustery response boiled down to "high efficiency", although he also repeated the claim the design was copied. When I called him on this, in his next reply he switched over to comparing the Merlin to the RD-180.

He made five claims, which I think I know the answers to, but I thought I'd run them past the experts here for correction where necessary:

1. In 2019 Musk said the RD-180 was the finest rocket engine ever made. Sort of true - he said "excellent engineering"
2. Both engines use RP-1 and LOX. True but irrelevant - many other engines do too
3. Both engines have a duel nozzle design. False - the Merlin is a single nozzle
4. The RD-180 has a dual combustion chamber design. True but irrelevant - the Merlin doesn't
5. Reusable first stages are "showmanship without benefits", according to Russians. False - the reusability of the Falcon 9 first stage and the payload shrouds has reduced launch costs considerably, which is a benefit. (I couldn't find any reference to the "showmanship without benefits" quote but didn't look far.)

I have a suspicion he's now comparing the RD-180 to the Raptor engine, but I'm not sure, and I'm also unsure how similar they are if he is (of course, if he is, it's irrelevant to the Falcon 9...).

Any help would be appreciated. Thank you!

--- End quote ---

RD-180
Fuel: Kerolox
Config: Dual combustion chamber
Cycle type: Closed Cycle (Staged combustion)
Length: 3.56 m
Diameter: 3.15 m
Dry weight: 5,480 kg

Merlin
Fuel: Kerolox
Config:Single combustion chamber
Cycle type: Open cycle (Gas generator)
Length: 2.92 m
Diameter: Sea level 0.92, Vacuum 3.3 m
Dry weight   630 kg

Raptor
Fuel: Methalox
Config: Single combustion chamber
Cycle: Closed cycle (Full flow staged combustion)
Length: 3.1 m
Diameter: 1.3 m
Dry weight: 1,500 kg

Identical in every way ::)

Your Russian troll correspondent is an idiot

Peter B:

--- Quote from: smartcooky on April 28, 2022, 08:18:24 PM ---...Identical in every way ::)

Your Russian troll correspondent is an idiot

--- End quote ---

LOL! Thought so.

smartcooky:

--- Quote from: Peter B on April 29, 2022, 04:42:30 AM ---
--- Quote from: smartcooky on April 28, 2022, 08:18:24 PM ---...Identical in every way ::)

Your Russian troll correspondent is an idiot

--- End quote ---

LOL! Thought so.

--- End quote ---


Yup, I also take issue with his characterization that "Reusable first stages are "showmanship without benefits". This is just demonstrably rubbish. As you correctly point out, the reuse of the first stage and the payload fairings has dramatically reduced launch costs. While SpaceX have not directly said what the difference is between building a first stage and  reusing one, there are some things we do know, and Musk has made some hints.

1. The full fuel load is about $200K for both stages (about $550K for Falcon Heavy) so it is not really a significant part of the cost.

2. SpaceX charges $62M for a launch with a new booster, and $50M with a reused one - that is 20% cheaper, a significant cost reduction for customers.

3. In the past, Musk has dropped a few hints about his costs. Firstly, he has indicated that manufacturing the booster is 60% of the total cost of a new rocket, while the second stage (including payload fairings) is about 20%, and he has stated that a second stage costs around $10M to make, so we can easily work out that a booster should cost around $30 million to manufacture. Secondly, Musk has said the cost of refurbishing the recovered booster is only $250,000. It appears this does not require much manpower or much in the way of hardware replacements or complex inspections. I am unsure if those costs include recovery costs such as operating the three ASDS landing platforms.

4. One drawback is that reusability reduces the maximum payload because it needs to reserve fuel for boostback (sometimes), entry and landing. There is also the additional mass of the landing legs, grid fins etc. However, Musk reckons that only lowers the maximum payload capacity by less than 40% (so far only 26 of their 94 Block 5 launches have carried a max payload). After two launches, the total cost and combined payload capacity are roughly comparable to one launch of an expendable rocket, so if the booster flies at least three times they're on a win.

5. Finally, after originally insisting that all their missions were to be on new boosters, NASA are now opting for refurbished boosters, not just for the cost, but because of their "flight proven" status....

CRS-21 was launched on B1058.4 (the ".4" means it was its 4th flight)
CRS-23 was on B1061.4
DAT was on B1063.3
IXPE was on B1061.5 (the same booster that launched CRS-23)

This has even extended to NASA crewed flights...
Crew-2 was on B1061.2 (the same booster that launched Crew-1, and which went on to launch CRS-23 and IXPE)
Crew-3 was on B1067.2
Crew-4 was on the same booster as Crew-3 (now B1067.4 because in between those, it launched Turksat 5B)

And further extended to  the initially skeptical USSF...
GPS111-05 was on B1062.2
NROL-85 was on B1071.2

I thing the most telling stat though is this one.... They have 13 Block 5 boosters in their current fleet, two have flown 12 times, one flown 11 times and one flown 10 times, so those four boosters have launched 45 of their 94 Block 5 launches, almost half of them!! 

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