[TaylorAlexander]

> What really matters at the end of the day for actual rocket building is thrust per area of nozzle size.

What? I am a casual observer but I believe specific impulse is most important - how much thrust do you get per unit of fuel. The amount of fuel you have to carry has a huge impact on how much. Of course things like ion thrusters have high ISP but not enough thrust for launch - but once you get enough thrust to launch a rocket, you want a high ISP. High ISP means way less fuel required which means even with less thrust you will be okay. So thrust per nozzle area isn't strictly that important if the engine is so inefficient you need to carry twice as much fuel.

[marcosdumay]

There are quite a few important numbers here.

Weight/thrust is important; thrust/area is important; ISP is important; thrust/dollar is important...

[lamontcg]

> thrust/dollar is important.

something more like that is probably the most important variable.

all the rest of them impress space nerds, the ones ending in per-dollar are the ones that impress accountants and finance people.

[marcosdumay]

Accountants and finance people are not the almighty rulers of projects.

If you can not get enough thrust under your rocket sectional area, your capsule will simply not leave the ground and your astronauts won't make it to the Moon, independently on the accountants opinion on the matter.

All of those things are important.

[panick21_]

If you have a high ISP engine that is low thrust, your rocket will either not get of the ground or if it does get off the ground it will suffer from a very high amount of gravity loses. High ISP on the engine doesn't matter when your rocket spends a long time just fighting gravity.

High thrust to weight ratio is important for a rocket. The higher your TWR the less gravity loses your rocket suffers.

There is a reason most rockets main liftoff comes from lowers ISP RP-1 engines or solids rocket. High ISP hydrogen engines just result in losses.

SpaceX Raptor actually deliberately reduced its ISP to increase its thrust.

[dotnet00]

ISP isn't everything, even in space, as you don't have infinite time to get places, transfer burns etc are more efficient when closer to being instantaneous (in terms of dV) and capture burns need to happen within a fixed timeframe.

Also I think what they meant is that the engine's thrust dictates the height above it that it can lift. So lots of larger low thrust engines = short and stubby rocket, which adds all sorts of limitations (construction, transportation, aerodynamics).

[pfdietz]

High Isp is actually a bad thing in a first stage. That stage is done almost immediately, so high Isp doesn't buy much, but low propellant density (which tends to go with high Isp) makes the stage larger and more expensive. This is why launchers tend to use hydrocarbons, not hydrogen, as fuel in the first stage. Low propellant density also makes the pumps larger and so reduces the thrust/mass ratio of the engines.