Wow, no kidding. The gold Merlin would be a quarter of the price![1][2][3] (At least in terms of materials.)

[1]: https://arstechnica.com/science/2020/05/nasa-will-pay-a-stag...

[2]: https://www.google.com/search?q=merlin+engine+dry+mass

[3]: https://www.wolframalpha.com/input/?i=%28630+kg+of+gold+in+U...

Keep in mind gold is a lot denser, so it'd be a miniature engine if you made it out of pure gold. If you take the volumetric space of all the materials that might lead to a different answer, but that's hard to do. (I guess you could make it out of gold foam.)

Probably closer to half the price of the RS-25 then, assuming normal Merlins are made mostly out of steel:

https://www.wolframalpha.com/input/?i=%28%28%28630+kg+*+%28d...

I LOLd but then had to do the math. Turns out that the dry weight of a SpaceX Merlin engine, in gold, costs almost exactly the same as a production RS-25.

Current spot price of gold is $1800/oz. Merlin dry weight is 1380 pounds. 1380 pounds of gold is right at 40 million dollars.

Per https://en.wikipedia.org/wiki/RS-25#Space_Shuttle_program :

"A total of 46 reusable RS-25 engines, each costing around US$40 million, were flown during the Space Shuttle program"

Beautiful.

Those numbers are out of date now, they recently restarted production and it turns out each engine is about $150M now for the new production run for the SLS. (Which about the cost of a full expendable Falcon Heavy rocket launch along with all 27 of its Merlin engines.)

That’s what happens when you decide on a product and then ask how much it costs.

They cost much more than $40M these days.

Maybe not once you consider the R&D costs of figuring out how not to melt a solid gold Merlin engine ;)

... surprisingly, this is not much of an issue. The key property for keeping a rocket engine from melting is not high melting point, but high thermal conductivity. Because of this, all the parts that touch the hot flame are made from a copper alloy, with a melting point not far from that of pure gold.

In the chamber, the hot gases are >3500K, and touching the inner walls with a melting point at ~1400K, yet the walls don't melt, because they are cooled with more power than they are heated.

> they are cooled with more power than they are heated

I'd argue that they're being cooled with equal power, otherwise they could freeze infinitely which is physically impossible.

This man (err, bot?) contracts!

There is that... ;-)