Say I have an engine with 60 seconds of specific impulse, does that basically mean the same as "If I expend all of the energy that this rocket has, I will get 60 seconds of positive boost in a given direction?"
more confusing than I thought. Never realized gravity was involved in the equation. Also is there any other way to solve for Isp without using exhaust velocities and use thrust vs mass lost instead?
Gravity is used as a conversion factor so it can be communicated within the industry. It is the reason you get the odd unit of seconds in the equation but simplifies things when the US is using barbaric units. That way, the standards for comparison across different propulsion systems is simple. I believe the equation is actually accounting for mass lost already. Could you expand on what you mean?
+yrjosmiel73 There is always gravity. Also Moses is correct in that the Isp of an engine is also a function of external (ambient) pressure. The internal pressure of the propellant is determined mostly by the geometry of the exhaust nozzle. Optimum efficiency occurs where the propellant pressure is exactly equal to the ambient pressure. But since the ambient pressure drops quite rapidly with altitude, optimum efficiency for any particular nozzle geometry only occurs around a very specific altitude, and is less efficient at all other altitudes. So a nozzle geometry designed for sea level pressure will have reduced efficiency in the upper atmosphere, and conversely a nozzle designed for the upper atmosphere has reduced efficiency at lower altitudes. Modern rockets design around this by using multiple stages: first stage engines have nozzles designed for atmospheric operation, and the upper stages have nozzles designed for vacuum operation. The only other ways to get around it are by either using a variable-geometry nozzle (you see these on military jet engines), or an aerospike nozzle design (basically a normal nozzle turned inside-out, look it up its pretty neat).
One thing that gets me is everyone say aerospikes are great, efficient and multiple altitudes, smaller, lighter. So why doesn't anybody use them? And correct me if i'm wrong (no really, I'm not at all sure) its not that there is always gravity, its that gravity is used as a constant within the equation. Or is that seconds.
The reason no one uses them--aside from NASA experiments back when the X-33 was actually a thing--is they are generally heavier and much more complex compared to a normal nozzle of similar thrust/performance class. The normal nozzle design needs less material, which can be thinner and augmented by things like active cooling by routing cryogenic propellant in pipes around the nozzle, both make thermal management much easier. By 'inverting' the nozzle as in an aerospike the thin shell nozzle is replaced by a block of bulk material, which makes thermal management a lot harder. In addition, aerospikes typically require much more precise and complex pumping equipment for the propellant because maintaining the proper expansion ratio in the exhaust (and thus thrust) is almost entirely dependent on the propellant flow rate. See /watch?v=FcW9kUUTfxY for a test firing of the linear aerospike design NASA was going to use on the X-33 project, and you'll see why theoretically they make a lot more sense, but the practical and engineering challenges associated need time to get worked out. Besides they really only make sense if you're trying for a SSTO design, our current multi-stage rockets are efficient enough; don't fix what ain't broke and so forth.
The g in the formula specifically means 9.81 m/s^2, not the acceleration due to gravity the rocket is experiencing at the given time. It's just a way to get performance in a type of unit that is the same in metric and imperial systems. You could just as well use exhaust velocity to describe engine performance but then instead of 450 seconds you would get 4500 m/s or 15000 ft/s depending on your choice of units.
this usage of idiotic dark age style units (there used to be dozens of units of length specific for ropes, textiles, logs, distances, short and long... literally anything) is a true sign that it's not SCIENCE but ENGINEERING (btw, you can imagine it, using v=gt, as a time to reach the max. height when thrown upwards with the specified velocity - of course, usual reductionist idealism applied: point-like object, no air resistance, etc.)
