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<blockquote data-quote="lichmaster" data-source="post: 9338143" data-attributes="member: 6683330">Sorry but that's not really correct.[SPOILER=&quot;Space Systems and orbital transfer engineering&quot;]Specific impulse is indeed a measure of mass efficiency of a propulsion system, but it's not directly related to deltaV. DeltaV is a measure of how much specific energy (difference in velocity) you must provide to change orbit, and this is first of all dependent of where you want to go and where from. Secondarily, deltaV is also impacted by the kind of transfer you're making: an impulsive maneuver will typically require less deltaV than a long pulsed maneuver because you're focusing the propulsion in the most effective spot instead of spreading it out.However, since energy must be supplied by an actual system and pretty much all propulsion systems we have are based on ejecting mass somehow, that deltaV is influenced by how much mass you must expel and how quickly. Highly mass efficient propulsion systems (also high specific impulse propulsion systems) eject small amounts of mass at very high velocities. Since the mass ejected is very small, these propulsion systems provide low thrust (which is also the name often given to these systems), which in turns means that they cannot provide the necessary propulsion in a very short and focused way but must do so over very long arcs, thus effectively increasing the required deltaV of the transfer. Given the low mass required however this is often advantageous from a propellent point of view since not much mass is required, and this has many benefits (lower overall spacecraft mass, smaller size, lower costs etc) and some downsides (longer transfer times, more complicated trajectories, etc).[/SPOILER]</blockquote>

[QUOTE="lichmaster, post: 9338143, member: 6683330"] Sorry but that's not really correct. [SPOILER="Space Systems and orbital transfer engineering"] Specific impulse is indeed a measure of [I]mass[/I] efficiency of a propulsion system, but it's not directly related to deltaV. DeltaV is a measure of how much specific energy (difference in velocity) you must provide to change orbit, and this is first of all dependent of where you want to go and where from. Secondarily, deltaV is also impacted by the kind of transfer you're making: an impulsive maneuver will typically require less deltaV than a long pulsed maneuver because you're focusing the propulsion in the most effective spot instead of spreading it out. However, since energy must be supplied by an actual system and pretty much all propulsion systems we have are based on ejecting mass somehow, that deltaV is influenced by how much mass you must expel and how quickly. Highly [I]mass[/I] efficient propulsion systems (also high specific impulse propulsion systems) eject small amounts of mass at very high velocities. Since the mass ejected is very small, these propulsion systems provide low thrust (which is also the name often given to these systems), which in turns means that they cannot provide the necessary propulsion in a very short and focused way but must do so over very long arcs, thus effectively increasing the required deltaV of the transfer. Given the low mass required however this is often advantageous from a propellent point of view since not much mass is required, and this has many benefits (lower overall spacecraft mass, smaller size, lower costs etc) and some downsides (longer transfer times, more complicated trajectories, etc). [/SPOILER] [/QUOTE]

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