From a controls perspective, two engines is nice, as it's the minimum required for roll control.

A single engine would give you just pitch and yaw control, so you'd need a separate system just for roll control.

Like the cold gas thrusters on stage 2 of Falcon 9. They seem cheap, and that's a good reason to prefer them over a second engine when you're disposing of the whole stage.

This leaves me wondering: Why pay for two engines when a single engine + cheap cold gas thrusters would do? Or is there something about New Glenn that just demands a second engine?

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    $\begingroup$ I'm not sure this is a fair comparison. When designing a launch vehicle, there would be a required thrust for each stage, assuming that the stage weight is already known, and because of this, it would likely be a choice between two engines or one engine that has twice the thrust. I'd imagine this would have been quite a close decision, as the cost of both options would be so similar, with hundreds of much more inconsequential variables to contend with. $\endgroup$ May 4 at 6:53
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    $\begingroup$ I doubt that you will get a definitive answer to this question. The people who could provide a definitive answer either currently work for or formerly worked for Blue Origin, and they have all signed NDAs that prevent them from answering questions like this. Jeff Bezos has very deep pockets and has access to the very best lawyers. Those who can answer this question know this, and thus will not answer this question. $\endgroup$ May 4 at 10:36
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    $\begingroup$ Redundancy, my dear user39728. $\endgroup$
    – Vikki
    May 5 at 0:45
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    $\begingroup$ Falcon 9 does just fine with a single engine, my dear Vikki. $\endgroup$
    – user39728
    May 5 at 2:19
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    $\begingroup$ The merlin engine used on falcon 9 is very reliable and(partly because of its pintle injector and relatively simple power cycle) and so it was probably deemed sufficiently reliable. this may not have been the case for new glenn. $\endgroup$ May 5 at 6:58

In 2015-2016, the New Glenn design was expected to use a single BE-4 methane-LOX engine on the second stage (about 2400kN thrust), and a single BE-3 (the same as the suborbital New Shepard's single engine, and likely in the 500-600kN range for the upper stage version) on an optional third stage. There wasn't any need for an engine in between the power of the BE-3 and BE-4, so Blue Origin didn't develop one.

By 2018, the design had changed, and the second stage apparently no longer needed the thrust of a BE-4; instead, a pair of BE-3Us would be used, producing together about 1400kN thrust. The BE-3U is an expander-cycle version of the BE-3, and presumably it could be developed much more quickly and cheaply than a new engine optimized for the New Glenn second stage.

If and when the New Glenn gets ordered in large numbers, Blue Origin might find it worthwhile to design a single cheap engine for the second stage, but that kind of thing hardly ever happens in rocket development -- the production runs are so small that the effective cost of each successive unit (taking into account the development cost amortized over the number of units) drops very rapidly.

As a rule of thumb, research and development costs for aerospace hardware run around 10x as much as the production of the first unit; the first unit costs around 5x as much as the 1000th unit.

Assuming that the hypothetical right-sized engine costs 1.5x as much as a BE-3U, and using the above rule of thumb for R&D cost, using the two smaller engines is cheaper through the first 30 flights, at which point the savings on a single big engine pay off the cost of R&D.

At this point, however, Blue Origin's biggest concern is getting the rocket flying as soon as possible so they can start competing for customers with SpaceX. Developing a new engine is a long process that would set New Glenn back by years. The incremental cost of expended engines is a small concern, and Bezos's greatest successes to date have been achieved by throwing away vast amounts of investor money in order to strangle the competition.

  • $\begingroup$ Yeah but when you’re trashing two outrageously expensive engines when you could trash just one, things get way more expensive. With the money you throw in the sea when stage 2 comes burning through the atmosphere, you could finance at least some of the R & D needed to develop the less powerful version of a BE-4, if in fact you need that less powerful version. $\endgroup$
    – user39728
    May 5 at 2:24
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    $\begingroup$ 1. A modern engine designed by the same company that intends to buy it is not outrageously expensive. 2. Two small engines are not 2x the cost of a single larger engine with 2x the thrust. 3. Reread my last graf and do the math. 4. Time is also money; a small modification of an existing engine is faster to develop than a new engine. $\endgroup$ May 5 at 2:32
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    $\begingroup$ @user39728_i_said_user_39728_i_ Develop the proper engine over the years and te competitors will leave yo in the dirt. It's often more profitable to provide n unoptimal, but working solution ASAP, and start making some money, gain customers and trust, than to spend years struggling for perfection, and finally arrive to a market that is already saturated, and without economy of scale to back up your "perfect" solution, being unable to turn profit. $\endgroup$
    – SF.
    May 5 at 7:03
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    $\begingroup$ "Better is the enemy of good." Two disposed engines is worse than one, but better than no rocket at all, stuck in development hell. $\endgroup$
    – SF.
    May 5 at 7:04
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    $\begingroup$ @user39728_i_said_user_39728_i_ Just re-read the last paragraph of the answer. Blue Origin is running out of time. If SpaceX manages to get Starship flying with the kind of a launch cadence they're proposing, they are going to pretty much vacuum up the whole LEO market. "Investing" a few years into making New Glenn a little cheaper would likely mean that by the time New Glenn is ready to fly, SpaceX already has a flight-proven launch system with a much lower cost per launch. You'd have a hard time convincing customers to switch to your untested yet more expensive offering. $\endgroup$
    – TooTea
    May 5 at 7:27

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