On March 30, 2017, SpaceX made history.  

"Lighting the candle" on a Falcon 9 first stage rocket, which had previously made history -- SpaceX does this a lot -- by becoming the first rocket to land on a ship at sea, SpaceX re-launched said rocket to launch a Luxembourgian communications satellite to orbit. The rocket then landed, natch, on a ship at sea... again.

Since that flight, launchings, landings, and relaunchings of "flight-proven rockets," as SpaceX calls them, has become business as usual at Elon Musk's space upstart. Including the March 30 flight, SpaceX has now conducted a total of 259 successful rocket launches, 221 successful rocket landings, and 193 successful reflights of previously flown rockets, a feat unmatched by any other space company in the world.  

But one company is trying to match it.

Rocket Lab reuses a single, solitary rocket engine

In a feat that tracks the concept of "baby steps," on Aug. 23, SpaceX rival Rocket Lab (RKLB 3.29%), the second most frequent launcher of rockets in the United States, launched to orbit an Electron rocket equipped with eight brand new Rutherford rocket engines... and one previously used Rutherford engine.

The rocket performed flawlessly, even though one of its engines enjoyed a dip in the Pacific Ocean on its most recent flight.

And then, in what Rocket Lab hopes will become less of a first and more of a routine, the Electron's first stage parachuted back to Earth and splashed down in the ocean -- soon to be recovered for further reuse.

Counting from one to nine

So what's next for Rocket Lab? Well, encouraged by the performance of one reused engine on one flight, CEO Peter Beck now says Rocket Lab is ready to try flying an Electron entirely powered by nine reused engines. Potentially, the company may try to refly an entire previously flown first-stage rocket.

Already, last month's successful launch places Rocket Lab in a select group of organizations, alongside SpaceX with its Falcon fleet, and NASA with its Space Shuttles, to have figured out a way to reuse rocket engines. But this is about more than just bragging rights.  

Will savings add up... or down?

As space reporter PayloadSpace points out, engines make up half the cost of a rocket booster. So reusing engines has the potential to cut Rocket Lab's launch costs significantly. This could permit Rocket Lab to cut its launch prices to underprice the competition, or expand its profit margins and reach operating profitability sooner -- or both.  

There are caveats to that equation, however.

Ark Invest, the high-tech investing company run by Cathie Wood, has estimated that SpaceX spends $1 million to refurbish each landed Falcon 9 rocket to ready it for reuse. But when SpaceX first started out reusing rockets, this cost was closer to $13 million per rocket. So investors should be aware that Rocket Lab's savings from engine reuse may not be a lot at first.  

Also, refurbishment costs may not be directly comparable. Falcon 9 is a much bigger rocket, and its Merlin engines are much bigger than Rocket Lab's Rutherfords. And Falcon 9 rockets land "dry," either on land or on a drone ship. Rocket Lab's engines parachute down into the ocean where they are -- briefly -- exposed to the damaging effects of salt water before being recovered.

All these caveats aside, it makes sense to assume that Rocket Lab will enjoy at least some cost savings from reusing an engine, and that the more engines it reuses, the more money the company will save. But investors in Rocket Lab still shouldn't expect cost savings to have a transformative effect upon Rocket Lab's income statement right away. It will take time for the savings to add up, and time even to confirm that the savings from reuse justify the cost of refurbishing reused engines.

Meanwhile, analysts polled by S&P Global Market Intelligence don't expect to see Rocket Lab turn its first profit before 2025 -- and expect the company's fully reusable, larger Neutron rocket to have a bigger effect upon profitability than any reuse of engines on the smaller Electron.

But every little bit helps.