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buboard

Wikipedia has some stats comparing their current engines (probably speculative for the raptor)

Raptor: Thrust 1900 KN / 330 specific impulse

Merlin 1D: 480KN / 275 s.i.

i can't find the weight of the Raptor. Merlin is 630Kg

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To put this in perspective, the F-1 engine of the Saturn V (one of the biggest rocket engines ever made) only put out about 3.5 times more thrust (6.7 MN or 1.5 Mlbf) than the Raptor (1.9 MN or 0.43 Mlbf).

Also some people might not realize how much liquid is being pumped by rocket engines (the Saturn V F-1 could drain a 30,000 gallon swimming pool in 10 seconds):

Each second, a single F-1 burned 5,683 pounds (2,578 kg) of oxidizer and fuel: 3,945 lb (1,789 kg) of liquid oxygen and 1,738 lb (788 kg) of RP-1, generating 1,500,000 lbf (6.7 MN; 680 tf) of thrust. This equated to a flow rate of 671.4 US gal (2,542 l) per second; 413.5 US gal (1,565 l) of LOX and 257.9 US gal (976 l) of RP-1. During their two and a half minutes of operation, the five F-1s propelled the Saturn V vehicle to a height of 42 miles (222,000 ft; 68 km) and a speed of 6,164 mph (9,920 km/h). The combined flow rate of the five F-1s in the Saturn V was 3,357 US gal (12,710 l) per second,[4] or 28,415 lb (12,890 kg). Each F-1 engine had more thrust than three Space Shuttle Main Engines combined.[5]

https://en.wikipedia.org/wiki/Rocketdyne_F-1#Design

So the methane-oxygen Raptor at 380 SI is almost as powerful as the hydrogen-oxygen Space Shuttle SLS at 453 SI, if I have the math right:

https://en.wikipedia.org/wiki/Raptor_(rocket_engine_family)#...

The tradeoff is reduced price and improved safety with the choice of methane over hydrogen, but with a reduced specific impulse.

Another tradeoff is size. You can pack about 4 raptors in the same area you can fit a single SSME. (Or, 8 in the space you can fit a single F-1). This is relevant because to lift the shuttle on 3 SSMEs, or the SLS on 4, they need to pack in massive amount of thrust in SRBs.

Instead of doing that with all it's cost and safety implications, SpaceX intends to put 31 raptors on the bottom of their rocket. You could not fit the same thrust in other major US engines without making the rocket comically large.

And, with 31 engines instead of 4, you get a massive increase in redundancy. If you lose one SLS SSME, you can push the others extra hard (they won't be reused anyway) and still make it to orbit (or land in the Atlantic). You'd need to lose 7 Raptors to be in the same bad position.

I wonder how nobody talks about ill fate of N-1 with 30 engines on the first stage with the idea that the number of engines was the reason for breakdowns.

N-1 steered by differential thrust, meaning every engine points directly backwards and losing one on the rim means you have to turn off another on the opposite side to compensate, and now you've lost all the margin there is in the design and quite a lot of control authority too. All SpaceX rockets steer by gimbaling engines, meaning you lose one on the rim and you just adjust the thrust vectors to compensate.

Apparently the N-1 never actually fired all 30 engines simultaneously before launch, so there was no way to detect plumbing issues.

Additionally, their control scheme was designed to shut down the engine opposite to any failed engine, meaning that they had very little engine-out margin in the inevitability of an engine failure.

All in all, we can (and have, with the Falcon Heavy) do much better.

> meaning that they had very little engine-out margin

N-1 could complete the mission with up to 4 engines turned off from the start. And I guess it it's not from the start, you could turn off even more engines and still have a successful flight.

Even though Saturn-V is heavier, N-1 has more lift-off thrust. Partially to have this kind of redundancy.

The relevant tech - computer control, metallurgy, etc. - has come a ways since the mid 1960s.

SpaceX's lots-of-engines approach has been well proven on the Falcon 9 thus far.

That's fine, but it means you can't blame the sheer number of engines on N-1 to its poor performance. There is no law which forbids launching, e.g. Soyuz with 32 working from the start chambers, or Energiya with 20 chambers, or Falcon Heavy with 27 chambers successfully. It's other reasons - maybe computer control (KORD was a complex system to create), maybe metallurgy (even though NK-33, manufactured in ~1973, manage to fly after 40+ years in storage), maybe something else.

This is just to compare optimism for heavily multi-engined BFR with suggestions that N-1 was doomed just because of the number of engines.

As I recall, nobody could stop talking about it in the weeks before the Falcon Heavy demo.

Methane also means they can refuel in-situ on Mars, provided the infrastructure is there.

So, for the folks like me who don't know anything about rockets, it seems like "specific impulse" is the measure of how much "impulse" is generated per unit of fuel.

So 330/275 = 20% better.

Most of that is actually because of the difference in fuel -- Raptor runs on Methane, which produces more H2O and less CO2 than RP-1, and H2O is a smaller molecule and therefore more efficient.

Yep. The actual physically relevant number is exhaust velocity, which is the mean velocity of the particles in the rocket exhaust.

Isp = Specific Impulse = exhaust velocity/(9.81ms^-2).

9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. The unit of Isp has no physical significance, the convention just arose because early rocketry was a collaboration of people who all used different units, and they wanted to be able to compare numbers. Since the unit they all shared was a second, and they all agreed (roughly) on the real value of g, they just divided exhaust velocity by their g to get a number that could be compared. The convention stuck.

Note that when using different fuel or oxidizer, specific impulse isn’t the only thing that matters. Density matters too due to its effect on non-payload mass ratio.

> 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor.

I'd look it up but my textbooks[1] are in boxes, but the units everyone uses are wrong. Actually inappropriately simplified.

[1] I have one and exactly one textbook that has the full units for gc.

Isp often defined as "thrust per unit of mass flow". That is, how many Newtons of thrust engine gets from each kilogram per second of propellant spent. Since N = kg * m / s^2 , and mass flow is in kg / s , Isp becomes (kg * m * s) / (s^2 * kg) = m/s , i.e. Isp has units of speed. In vacuum Isp is equal to the speed of gases flying from the engine.

>So 330/275 = 20% better.

due to the exponential nature of the rocket equation, 100 tons of fuel will result in 2 tons to LEO in the former case and just 1 ton in the later.

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