5 ms·
I've always pretended to myself that I understand what impulse means but today you've helped me actually understand it. Thank you!
by Taek 5y ago
I've always pretended to myself that I understand what impulse means but today you've helped me actually understand it. Thank you!
- fouronnes3 5y agoGP is a nice reverse engineered explanation, but the actual origin of using seconds as a unit is to avoid having to compare metric and imperial units of speed. Isp is just exit velocity divided by g and it's conveniently in seconds, a pretty universal unit. And "one pound of thrust" only makes sense when you're in Earth's gravity field with gravity losses. Otherwise it's more precise to use Newtons for thrust.
- jcims 5y agoI use it because even after being an Estes nerd as a kid it was the first explanation that actually made sense to me.
- kbenson 5y ago> conveniently in seconds, a pretty universal unit. Thankfully! Imagine if systems/cultures commonly differed in fundamental measures of time, just as they sometimes do for volume, distance, weight, etc.
- taneq 5y ago> And "one pound of thrust" only makes sense when you're in Earth's gravity field with gravity losses. Ah, but now we're talking about "pounds of force", not 'pounds of mass': https://en.wikipedia.org/wiki/Pound_(force) https://en.wikipedia.org/wiki/Pound_(force)
- deleted 5y ago[deleted]
- HPsquared 5y agoFor this kind of thing it's helpful to think about the dimensions and units involved. In mechanics, "impulse" is a word for a unit of momentum. Momentum has dimensions (Mass * Speed), or (force * time), and can be stated in units of Newton-seconds. "Specific" impulse is simply Impulse per unit mass of fuel (i.e. Newton-seconds per kilogram). The convention of stating it in units of seconds is based on Earth gravity (Newtons per kilogram). If a system has specific impulse of 100 seconds, 1 kilogram of fuel would be able to accelerate a mass of 1kg at an acceleration of 1 G, for 100 seconds.
- jcims 5y agoIf I'm thinking about this correctly, the potential energy in the fuel isn't changing the net kinetic energy of the total mass, it's just re-distributing it between the vehicle and the exhaust.
- HPsquared 5y agoTotal momentum is conserved, but distributed: the individual momentum of the vehicle and the exhaust jet will be of equal magnitude and opposite direction (assuming a straight line), so if the rocket started in free space at rest (zero momentum), after it's run for a while the exhaust jet and vehicle will have equal and opposite momentum vectors. Energy is a different thing, but related to specific impulse. In a perfectly efficient system (not what really happens), the decrease in stored energy within the fuel tanks would be equal to the increase in total kinetic energy (i.e. the sum of vehicle and exhaust kinetic energy).
- jcims 5y agoThank you, momentum is definitely the term I should have used.
- Someone 5y agoNitpick: I think that should be 1 g (https://en.wikipedia.org/wiki/Gravity_of_Earth https://en.wikipedia.org/wiki/Gravity_of_Earth), not 1 G (https://en.wikipedia.org/wiki/Gravitational_constant https://en.wikipedia.org/wiki/Gravitational_constant)
- deleted 5y ago[deleted]