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The Falcon Heavy rocket with 27 merlin engines

Myths Smashed By The Falcon Heavy Launch

When SpaceX’s Falcon Heavy rocket launched it smashed a number of hard to die myths about how you build a reliable space rocket.

One persistent myth on the internet among many rocket fans is that the Russian moon rocket, which competed against the American moon rocket, the Saturn V failed because it had too many engines. The N1 had 30 NK-15 rocket engines on the first stage which had to fire at the same time on launch. In contrast the Saturn V had 5 large F1 engines.

The argument goes something like this: The more engines you have, the more likely you are that one of the engines fail. If one engine fails the whole rocket has a chance of blowing up.

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First stage of Soviet N1 moon rocket, with 30 NK-15 rocket engines.

However this is not a problem if the control system is able to shut down the engine gracefully before it blows up. Even back in the 60s the N1 rocket was able to do this. In fact one of the problems the N1 rocket experienced at launch was that the control system KORD, would shut down an opposing rocket engine when one failed. However this went out of hand on occasion and created a domino effect where all engines got shut down and the whole rocket fell down and blew up on the launch pad.

N1 faced many problems but one of the biggest challenges was probably that computer control systems were very primitive back then.

Many however have focused on the number of engines in the N1 as the main culprit. If you followed debates online about the Falcon Heavy through the years, you’d notice that many predicted it would never fly because they considered 27 rocket engines used as too many. The reasoning being that one engine blowing would easily cause the whole thing to blow up. With that many engines one engine blowing up becomes much more likely.

However as we have now seen, Falcon Heavy did not suffer the fate of the N1. It managed to launch successfully on first try, unlike the N1 which blew up on all four attempts.

Is it likely that Falcon Heavy will blow up in the future due to all its engines though? I don’t think so. Falcon 9 was already considered to have too many engines. Yet it has never proved a problem. When engines have failed in the past the control system has been able to shut it down quickly enough to prevent failure.

So instead having multiple rocket engines have proven thus far to improve reliability, since the rocket can complete its mission even with multiple engine failures.

Simulations of rockets with respect to vibrations is much easier to do today, which makes using multiple engines less of a problem than it used to be. Much better computer control of engines makes it possible to shut failing engine quickly enough to avoid fatal problems. The computer control also allows sensible adjustments of the other engines to maintain the rocket on its course.

Disproving that lots of engines will doom a rocket is especially important, because it demonstrates the viability of the massive BFR rocket, which is SpaceX’s ultimate goal. It will have 31 raptor engines.

Having lots of small engines is a big benefit because:

  • Easier to mass produce. Higher volume increase reliability and reduce costs.
  • More flexibility. You can use the same engines on rocket of different size as well as on different stages on the same rocket.
  • Throttle control. Adjusting the thrust of a rocket engine is difficult and easily complicates it. It is much easier to solve this problem by simply shutting down multiple engines. To land a rocket stage you need to throttle down a lot. Hence this is a major requirement for a rocket intended for reuse.

Not everyone is following this approach however. Jeff Bezos’ rocket company, Blue Origin, is building the large New Glenn rocket will only use 7 engines in the first stage. However this will be the much bigger BE-4 engines.

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Blue Origin’s planned New Glenn, reusable rocket, using 7 BE-4 rocket engines in the first stage.

It will be interesting to see the future outcome of Blue Origin and SpaceX competing by SpaceX using the traditional Russian approach of having lots of small engines vs the traditional American approach of using fewer but bigger engines.

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