4 ms·

Wikipedia has some stats comparing their current engines (probably speculative for the raptor) Raptor: Thrust 1900 KN / 330 specific impulse Merlin 1D: 480KN

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

Eridrus 8y ago
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.
- Tuna-Fish 8y ago
  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.
  - deleted 8y ago
    [deleted]
  - Gibbon1 8y ago
    > 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.
    - avmich 8y ago
      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.
  - ummonk 8y ago
    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.
- trhway 8y ago
  >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.
  - deleted 8y ago
    [deleted]
zackmorris 8y ago
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 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)#Comparison_to_other_engines 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.
- davedx 8y ago
  Methane also means they can refuel in-situ on Mars, provided the infrastructure is there.
- Tuna-Fish 8y ago
  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.

Eridrus 8y ago

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.

Tuna-Fish 8y ago
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.
- deleted 8y ago
  [deleted]
- Gibbon1 8y ago
  > 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.
  - avmich 8y ago
    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.
- ummonk 8y ago
  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.
trhway 8y ago
>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.
- deleted 8y ago
  [deleted]

zackmorris 8y ago

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 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)#Comparison_to_other_engines 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.

davedx 8y ago
Methane also means they can refuel in-situ on Mars, provided the infrastructure is there.
Tuna-Fish 8y ago
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.