5 ms·

Why don't they put a smaller engine in the centre for landings?

extrapickles 11y ago
It would make the rocket too heavy. To add another engine, they would have to make the rocket larger in diameter, increasing the weight by a fair amount (they currently have the 9th engine in the middle, so it would have to get moved to the ring with the rest). Reducing the power of the center engine would also reduce the amount of payload to orbit. Rockets that can reach orbit have very bad weight to payload ratios (Falcon 9 is ~2.5% takeoff weight to LEO) so there isn't much room for dead weight.
- RonaldRaygun 11y ago
  It's actually more about complexity. They could simply replace the center engine with a lower power design and still get plenty of mass to orbit. The issue then is that you have to design a completely new engine and have 2 different engines in your first stage. More complexity generally equals more risk, and historically SpaceX has sought to maximize simplicity for the sake of reliability and rapid iteration.
  - tempestn 11y ago
    And besides, it would be a waste. Landing with the engines they have is "just" a software problem. (Technically a control system problem.) Assuming they can make it work consistently, the save the added cost and inefficiency of a separate landing engine for all future launches.
  - jacquesm 11y ago
    And you suddenly introduce a single point of failure for the return part of the mission.
    - lmm 11y ago
      Can the first stage land with a failed center engine?
      - jacquesm 11y ago
        On a parachute maybe, but then it will still be damaged on landing.
        lmm 11y ago
        What I meant was I don't think a different-model center engine would be any more of a single point of failure than it already is.
        jacquesm 11y ago
        They're talking about the center engine of the first stage, not the single engine of the second stage.
        lmm 11y ago
        Yes. How would making that a different engine "introduce a single point of failure for the return part of the mission" that wasn't there already?
        jacquesm 11y ago
        Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem. But if you replace the central one with a different one that becomes critical for the return mission then you have made matters worse.
        lmm 11y ago
        > Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem. Is that true? Doesn't the landing have to use the center engine - any other single engine would be asymmetric, and multiple engines would have too high a thrust.
varjag 11y ago
You won't be able to adjust thrust vector (and stabilize the landing booster) with just one engine.
- ceejayoz 11y ago
  SpaceX's Falcon lands with only one engine. The other 8 are off.
- snaily 11y ago
  The Falcon 9 does not rely on differential thrust - the center engine (and the others, I believe) can gimbal ("rotate").
stcredzero 11y ago
The turbopumps that pump fuel into the main engines are basically mini rocket engines. They even contribute a small amount of thrust. So it's possible that the rocket could be redesigned so that the center return/hover engine could also be a fuel pump for the outer ring of main engines. However, this introduces a single point of failure for the main mission of launching payloads.

It would make the rocket too heavy. To add another engine, they would have to make the rocket larger in diameter, increasing the weight by a fair amount (they currently have the 9th engine in the middle, so it would have to get moved to the ring with the rest). Reducing the power of the center engine would also reduce the amount of payload to orbit. Rockets that can reach orbit have very bad weight to payload ratios (Falcon 9 is ~2.5% takeoff weight to LEO) so there isn't much room for dead weight.

RonaldRaygun 11y ago
It's actually more about complexity. They could simply replace the center engine with a lower power design and still get plenty of mass to orbit. The issue then is that you have to design a completely new engine and have 2 different engines in your first stage. More complexity generally equals more risk, and historically SpaceX has sought to maximize simplicity for the sake of reliability and rapid iteration.
- tempestn 11y ago
  And besides, it would be a waste. Landing with the engines they have is "just" a software problem. (Technically a control system problem.) Assuming they can make it work consistently, the save the added cost and inefficiency of a separate landing engine for all future launches.
- jacquesm 11y ago
  And you suddenly introduce a single point of failure for the return part of the mission.
  - lmm 11y ago
    Can the first stage land with a failed center engine?
    - jacquesm 11y ago
      On a parachute maybe, but then it will still be damaged on landing.
      - lmm 11y ago
        What I meant was I don't think a different-model center engine would be any more of a single point of failure than it already is.
        jacquesm 11y ago
        They're talking about the center engine of the first stage, not the single engine of the second stage.
        lmm 11y ago
        Yes. How would making that a different engine "introduce a single point of failure for the return part of the mission" that wasn't there already?
        jacquesm 11y ago
        Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem. But if you replace the central one with a different one that becomes critical for the return mission then you have made matters worse.
        lmm 11y ago
        > Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem. Is that true? Doesn't the landing have to use the center engine - any other single engine would be asymmetric, and multiple engines would have too high a thrust.

varjag 11y ago

You won't be able to adjust thrust vector (and stabilize the landing booster) with just one engine.

ceejayoz 11y ago
SpaceX's Falcon lands with only one engine. The other 8 are off.
snaily 11y ago
The Falcon 9 does not rely on differential thrust - the center engine (and the others, I believe) can gimbal ("rotate").

stcredzero 11y ago

The turbopumps that pump fuel into the main engines are basically mini rocket engines. They even contribute a small amount of thrust. So it's possible that the rocket could be redesigned so that the center return/hover engine could also be a fuel pump for the outer ring of main engines. However, this introduces a single point of failure for the main mission of launching payloads.