vacuum engine development and testing.

[magnemoe]

This came into focus with the BFR update. 
In short an vacuum engine has an larger engine bell to get higher vacuum ISP but they would not work at sea level even if you can make an sort of all around engine like on the space shuttle. 
Another weakness is the large bell require an large interstage and limit the number of engines you can stack.  

But the main question is, can you test fire them? SpaceX does an lot of test fires but I assume they can not test fire the upper stage engines. 

The other question is how different they are, is it more than an larger bell who looks like is pretty thin metal at the bottom. 
In short would it be a lot of development to make an raptor vacuum engine if you have an sea level one? 

[wizzlebippi]

https://en.m.wikipedia.org/wiki/Rocket_Engine_Test_Facility

Vacuum test facilities exist, just maybe not for something the size of a raptor.  They could test their scale model, and validate the flight worthy engine with a computer model.

 

[magnemoe]

Thanks, found this,  does not looks like they test the vacuum engines unless they can swap bells. 
https://en.m.wikipedia.org/wiki/SpaceX_launch_facilities#SpaceX_high-altitude_test_facility.2C_New_Mexico

now its possible that Spacex is kerbal enough to test the vacuum raptor on an BFS while doing suborbital jumps. yes it would limit them to 5 surface engines if carrying two vacuum engines so less fuel who would limit burn time but probably easier than making an vacuum chamber who can keep vacuum while firing an raptor in it if its just used for development. 

[NSEP]

I think they can test the vacuum engine without the bell in normal atmosphere.

 

[magnemoe]

3 hours ago, NSEP said:

I think they can test the vacuum engine without the bell in normal atmosphere.

 

Yes, its obvious they test it without the bell, it was one of my ideas but thought the bell was fixed as I have seen them with both normal and vacuum bell in factory. 
You only need to test in vacuum to check it the engine work well there, not to test fire the engine.