RS-25 vs. Raptor 3: one chases peak efficiency, the other aims to slash costs and fly again. We unpack the numbers, engineering trade-offs, reliability, and politics behind these very different rocket engines. Which one really comes out on top?
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👉 This channel was created in collaboration with https://www.youtube.com/@marschroniken
0:00 - The Viral Engine Comparison
2:08 - Two Engines, Two Design Missions
5:02 - Dimensions, Thrust, and Engine Design
7:30 - Propellant Efficiency and Hydrogen Tradeoffs
11:01 - Engine Costs, Reuse, and Reliability
14:04 - Politics and NASA's Dual-Engine Moon Program
16:43 - Verdict: Different Engines for Different Missions
Subscribe for more space stories!
Comment what you liked most about the video! #Space #NASA #SpaceX #Rockets
👉 This channel was created in collaboration with https://www.youtube.com/@marschroniken
0:00 - The Viral Engine Comparison
2:08 - Two Engines, Two Design Missions
5:02 - Dimensions, Thrust, and Engine Design
7:30 - Propellant Efficiency and Hydrogen Tradeoffs
11:01 - Engine Costs, Reuse, and Reliability
14:04 - Politics and NASA's Dual-Engine Moon Program
16:43 - Verdict: Different Engines for Different Missions
Category
🤖
TechTranscript
00:00In early September a comparison went viral on X that looks like a first round knockout.
00:07On the left the RS-25, the Space Shuttle's main engine, which now powers NASA's SLS moon rocket.
00:16On the right Raptor 3 from SpaceX. Raptor is 28% shorter, 46% narrower, 57% lighter
00:29and still pushes out nearly 50% more thrust at sea level. That post was viewed around 150 000 times
00:39and most people probably just nodded. I crunched the numbers. The figures check out. Roughly.
00:46And yet this comparison is unfair. In both directions actually. Because in the discipline
00:52rocket engineers love most, that roughly 50 year old engine still wins by a wide margin to this day.
01:00And when it comes to price, Raptor isn't 50% ahead, but 50 to 100 times cheaper.
01:08Which engine is good for what? Why two such different machines were created?
01:13What they really cost? And the politics behind it all. That's what we're going to clear up today.
01:19With all the numbers and without rose-tinted glasses.
01:23My name is Sirvan and this is Mars Chronicles.
01:31Before we get started, a quick personal note. About half of this video is about SpaceX. At the
01:36time of publication, I hold a position in SpaceX stock myself and explicitly do not issue any
01:42recommendation to buy or sell. This video is a journalistic analysis, not investment advice.
01:50Two design assignments from two eras. Note, we have already covered the key specs and basic
01:57information about these two engines in other videos. So today we won't be repeating the
02:03If you're missing this info, I recommend checking out the videos linked below.
02:08To understand why these two engines are so different, you have to know what they were
02:13built for. The RS-25 was developed starting in 1971 by Rocketdyne for the space shuttle.
02:22The engines sit at the rear of the orbiter, fly with it all the way into orbit and return with
02:28it to
02:28the runway. They were designed for 55 flights and 7.5 hours of burn time.
02:36Everything else follows from that. Because the engines run from the launch pad almost all the
02:42way to orbit. Around 8.5 minutes. They have to work well on the ground and in a vacuum.
02:49And because every kilogram the orbiter carries reduces the payload, the maximum must be extracted
02:55from every kilogram of fuel. That's why hydrogen is used. The most efficient chemical fuel we have.
03:01It was never cheap. Back in 1979, a package of 12 shuttle engines already cost 365.7 million dollars.
03:13Around 30 million a piece in dollars at the time. Today, that will be around 1.8 billion.
03:21On April 12, 1981, the RS-25 flew for the first time with Columbia on STS-1. A total of
03:29135 shuttle missions
03:32followed up until 2011. By the way, Raptor itself began as a hydrogen project around 2009. Back then,
03:40SpaceX sketched out an upper stage with that name. In November 2012, Elon Musk announced in London that
03:47the new engine would burn methane. Hydrogen was too difficult for him to handle. And for Mars,
03:54methane has an unbeatable advantage. Methane's easier handling is an unbeatable advantage.
04:00What's exciting is, who helped pay for it? In January 2016, the US Air Force gave SpaceX
04:07$33.6 million dollars for a Raptor prototype. SpaceX itself has to contribute at least $67.3 million.
04:17In September 2016, the first prototype fired on the test stand. In July 2019,
04:24Raptor flies under Starhopper for the first time. The mission is completely different. Raptor is
04:30meant to roll off the assembly line in large numbers. 33 of them lift a single booster and
04:38everything is supposed to fly again and again. You could summarize it roughly like this. The
04:44RS-25 was built to squeeze the absolute maximum out of every kilogram of propellant. Raptor was built to
04:51squeeze the absolute maximum out of every single dollar. Both for the individual engines themselves
04:58and for the entire Raptor program as a whole. What the numbers actually tell us. Now onto the numbers
05:05from the Bible post and we'll take a close look at them through a magnifying glass. At 109% of
05:10its
05:10original rated power, the RS-25 delivers 1860 kN at sea level. That is roughly 190 metric tons of thrust.
05:20It is 4.3 meters high, 2.4 meters wide and weighs, according to NASA, around 3.5 metric tons.
05:29According
05:29to the usual figures, Raptor 3 is around 3.1 meters high, 1.3 meters wide and weighs 1525 kilograms,
05:38according to SpaceX. The 280 tons of thrust from the post are definitely a test stand figure though.
05:45In flight operations, according to an update of their own from May 2016, SpaceX is initially
05:51running the engine at around 250 tons. That means the nearly 50 tons more thrust realistically turns
05:59into 30%. There's also a catch with the weight and ironically it was the competition that found it.
06:05When SpaceX showed the first pictures of Raptor 3 in August 2024, the then CEO of ULA, Tori Bruno,
06:14wrote that what you see there is a partially assembled engine without controls, fluid management and thrust
06:21vector control. SpaceX President Gwynne Shotwell replied with a photo from the first test firing.
06:27Still, Bruno wasn't entirely wrong. SpaceX itself mentions a second figure. If you factor in the
06:33components that are located on the vehicle instead of on the engine with Raptor 3, it comes to 1720 kilograms.
06:42With the RS-25, controllers and attachments are already included in the 3.5 tons. On a like-for-like
06:49basis,
06:50Raptor weighs about half as much and still delivers more thrust. Where does that come from?
06:55Raptor is the first full-flow staged combustion engine to ever fly. Both propellants run entirely through two
07:04preburners. One fuel rich with excess methane and one oxidizer rich with excess oxygen. That's how Raptor 3
07:12achieves 350 bar of combustion chamber pressure on the test stand. The RS-25 operates with two hydrogen-rich
07:20preburners and reaches 206 bar. More pressure means more thrust from a smaller engine. And now the metric
07:29where the tables turn. The specific impulse, which means how efficiently an engine turns its fuel into
07:35a thrust. The RS-25 achieves in a vacuum 452 seconds. With its sea level nozzle, Raptor 3 reaches around
07:46350 seconds. And that's already a vacuum value. A difference of a good 100 seconds is a whole world
07:54in rocket science. Hydrogen versus methane. Why of all things does the old engine come out ahead
08:01on this metric? The answer is chemistry. Burning hydrogen produces very light exhaust gases and
08:09lighter molecules can be expelled from the nozzle faster at the same temperature. Faster exhaust means
08:16more thrust per kilogram of propellant. Methane produces heavier exhaust gases, mainly water vapor and
08:24carbon dioxide. A calculation example. Let's take a stage that weighs eight times as much at launch as
08:30it does empty. With 452 seconds, according to the rocket equation, it gains around 9.2 kilometers per
08:37second in speed. With just under 380 seconds of the vacuum version of Raptor, it's only around 7.7. A
08:47good
08:471.5 kilometers per second difference with the exact same mass. The catch is the density. Liquid hydrogen weighs
08:56only 71 kilograms per cubic meter. Less than a tenth the density of water. Liquid methane comes in at 422.
09:06If you factor in the oxygen, the same mass of fuel with hydrogen takes about 2.3 times as much
09:14tank volume
09:15as with methane. The core stage of the SLS carries around 2 million liters of liquid hydrogen and they weigh
09:24just
09:25144 tons. Larger tanks mean more structure, more insulation and more aerodynamic drag. Plus, hydrogen is extremely
09:35cold at minus 253 degrees, creeps through every seal and makes some metals brittle over time. Artemis 1 showed
09:45just how tricky that is. The second launch attempt on September 3rd, 2022 was halted by a hydrogen leak
09:52at a quick disconnect. And the low density explains why hydrogen rockets almost always need help at liftoff.
10:01The SLS's four Ares 25 engines couldn't even lift a third of its liftoff weight. More than three quarters
10:07of the launch thrust comes from the two solid rocket boosters in the first two minutes. Super Heavy,
10:12on the other hand, lifts off using only its 33 Raptor 3 engines with a solid 8000 tons of thrust.
10:20That's exactly what Raptor is made for. High thrust in a tight space, on the ground, with dense propellant.
10:27Methane has two more advantages. It barely coax, meaning it doesn't leave residues in lines and
10:33cooling channels like kerosene. And that helps with reusability. And on Mars, it can be produced
10:40from atmospheric carbon dioxide and water ice using the Sabatier process, which has been known for over
10:47100 years. Anyone who wants to fly back refuels on site. By the way, we calculated what a hydrogen-powered
10:54starship could look like in our Hydra episode, which came out a few days ago. I linked it for
10:59you down below as well. What an engine really costs. Now for the money. And this is where it gets
11:05uncomfortable for NASA. For the new Ares 25Es, which are scheduled to fly starting with Artemis 5,
11:12NASA signed contracts totaling nearly $3 billion in 2015 and 2020. For 24 engines, including the restart of
11:23production. In 2023, the NASA Inspector General already put the contract value at $3.6 billion.
11:33As a benchmark, NASA cites $104.5 million in manufacturing costs for an engine from the shuttle
11:40era. The new ones are supposed to cost $70.5 million. That is, however, without $2.3 billion in overhead
11:48costs
11:49and restarting the production line. And in October 2023, the same Inspector General wrote that the
11:55unit costs would likely even exceed the $104.5 million. For the Raptor, there are no invoices.
12:02Only statements from the manufacturer. Elon Musk wrote in 2019 that the first version was well under $1
12:09million. The goal was to be under $250,000. Back then, Everyday Astronaut estimated the Raptor 1 to be
12:18closer to around $2 million. None of these figures have been independently verified. Even with generous
12:24calculations, a new RS-25E still costs somewhere between 50 and 100 times as much as a Raptor. And the
12:36SLS, all four engines crash into the ocean after every launch. Starting with Artemis 5, that's roughly
12:42$300 to $400 million for new engines per flight, just for propulsion. A starship has 39 Raptors, 33 on the
12:52booster and 6 on the ship. Even at a million apiece, that would be $39 million. And they're supposed to
13:01fly again. With Raptor 2, SpaceX has already shown that. On the 9th flight in May 2025, a reuse booster
13:09carried 29 flight-proven engines. With Raptor 3, that proof is still pending. To be fair, reliability
13:18also has to be factored in. The RS-25 has locked 405 engine flights aboard the shuttle and had to
13:26shut
13:26down in flight just once. In 1985, on STS-51F because of faulty temperature sensors. The mission
13:37still made it to orbit. Raptor 3 has only flown 3 times so far. And on every flight, engines failed.
13:44On flight 13, only 8 of 13 fired during the booster landing. On flight 14, at the end of September,
13:53the booster and ship together lost. Three engines. That is, the price of a philosophy that learns in
13:59flight. Before humans fly on Raptor, this rate still has to improve significantly. Whoever pays,
14:06chooses the engine. That leaves the question of why NASA, knowing all this, is still flying with
14:11shuttle engines in 2026. The answer is in the law. The NASA Authorization Act of October 2010 mandates
14:21that the new heavy-lift rocket utilize existing contracts, investments, workforce and industrial
14:28base from the shuttle program, to the extent practicable. The RS-25 was thus politically locked
14:35in. Built in California. Tested in Mississippi. The core stage is built in Louisiana. The program
14:42is managed out of Alabama. In aerospace jargon, that is why the SLS is still called the Senate launch
14:49system to this day. We told the whole story about that in the ARIS-25 video. That is also linked
14:54down
14:54below. And politics isn't done yet. The White House wanted to phase out the SLS after Artemis 3 in the
15:02budget draft for 2026. In the summer of 2025, Congress legally locked in funding for Artemis 4 and fee. The
15:12draft for 2027 keeps the SLS through Artemis 5. After that, commercial solutions are supposed to take
15:21over. Nothing is decided yet. NASA is currently operating on temporary funding until December.
15:27NASA stands behind Raptor. In a different way. Namely, as a customer. In April 2021, SpaceX received
15:34$2.89 billion for the Starship HLS Lunar Lander. Short for Human Landing System. And that flies with
15:43Raptor engines. That means NASA's moon program depends on both engines at the same time. Following
15:50the reorganization in February 2026, Artemis 3 is scheduled to test in Earth orbit in the summer of
15:562027. How the Orion capsule docks with a lunar lander. To Starship? To Blue Origin's Blue Moon? Or to both?
16:04Orion will reach orbit atop the SLS with four RS-25 engines. If all goes well, technology from 1981
16:13and from 2026 will work together for the first time on the same mission. Quick pit stop before the verdict.
16:20If you're liking this comparison so far, give this video a thumbs up and hype it, please. It's just a
16:25single click under the video and helps the channel tremendously. Because the algorithm will then
16:31show the video to significantly more people. And if you want to go one step further, as a channel
16:36member you get to watch every video before anyone else. Thank you and now on to the verdict.
16:43Conclusion and assessment. So which engine is better? My honest take? Obviously it's labeled as an
16:50opinion. The question is framed wrong. Both are the best answers to two different questions.
16:55The RS-25 answers to the question from 1971. How do I get the absolute maximum out of every
17:03kilogram of propellant, no matter what it costs? To this day, there is hardly a better answer to that.
17:10Raptor answers the question that the industry is asking itself today. What does a ton in orbit cost and
17:17how often can I fly? For boosters that return, for ships that are supposed to refuel on mass,
17:25and for production volumes in the hundreds, methane is the better choice. Even if it costs efficiency.
17:33Where does hydrogen still have its place today? In upper stages, where every second of specific
17:38impulse counts and the large volume hurts less. That is where its descendants continue to live.
17:44Such as the RL-10 in the Centaur upper stage or the Vinci on Ariane 6. From a purely technical
17:52perspective, no one would probably build a new RS-25 for a core stage today. NASA is doing it anyway.
18:00And that's a political decision. And Raptor still has to prove that it can become as reliable as the
18:06engine the viral post made fun of. 405 flights with a single failure. That's the benchmark.
18:13If Raptor 3 pulls that off at a hundredth of the cost, this comparison is truly settled. Until then,
18:22will the fight go the full distance? Let me know in the comments. Which engine would you have built in
18:271971
18:28and which one today? And don't forget, if you want to grab the floating hover pen, my discount code
18:34MARS-1 gives you 15% off for the first 72 hours, then 10% off after that. Link and
18:41code are in the
18:42description and here's the QR code once more. My name is Sirwan and this was Mars Chronicles.
18:48Thanks for tuning in. Per aspera, ad astra.