The objective is to quantify the performance of each propulsion and power concept with respect to payload mass and total mission ∆V. The challenge is that the propulsion concepts are significantly different, each with its own components and figures of merit. For example, rocket performance is described in terms of specific impulse and thrust. Laser-sail performance is described in terms of beam power, beam divergence, etc. To compare these disparate methods equitably, the strategy is to reduce each to the most fundamental physics measurements of energy, mass, time, and power. In essence, energy is the fundamental currency of all motion. Specifically, this requires converting their usual performance parameters into the following more basic measures that are used in other parts of the analysis:
• Total Propulsion Energy, Etp (J) • Spacecraft Launch Mass, Msl (kg) • Base Hardware Mass, Mb (kg) (For in-space infrastructure)
• Energy Cost of Beam, Ebhw ($, converted to J using a J/$ scaling factor, for Earth surface hardware)

From these measures, other figures of merit can be calculated. To accommodate the span of possibilities, four different analysis techniques have been developed to cover the range of the power and propulsion methods. The distinctions depend on if the concept's power is received from an external source (R) or internally (I), and if their reaction mass is onboard (O) or external (X). The variables associated with each of these distinct assessments are listed in table 6, "List of Variables," along with annotations of which variables apply to which group.

| Propulsion Type | Examples | Power Source | Reaction Mass |
| --- | --- | --- | --- |
| IP-OM | Chemical Rocket Nuclear Rocket | Internal | Onboard |
| RP-OM | Solar Thermal Rocket Laser (or solar) Electric Propulsion | Received | Onboard |
| RP-XM | Laser (or solar) Sail Particle Beam | Received | External |
| IP-XM | Plasma Magnetic Sail SpaceDrive | Internal | External |

At this stage, only some of the conversion methods have been outlined, with further refinements to occur in Stage II. Also, for this first introductory stage, the proposed equations are nonrelativistic. This is a reasonable initial assumption since relativistic changes do not become significant (> 1.0% change) until past 14% lightspeed. In Stage II the relativistic equations will be included.
It should be noted that this approach is not the only method for analyzing missions in more general terms. In particular there is the "method of equivalent lengths" for both "energy limited" (e.g., chemical, high-thrust systems) and "power limited" (e.g., low thrust ion propulsion) systems [93-97].
Another caveat is that this starting assessment assumes that the same propulsion method is used for the entire journey. Future refinements would include assessing the merit of different propulsion types at different stages of the mission. For example, consider the prospects of accelerating the craft first by laser, then later by rocket, and then braking perhaps with a magnetic sail. It has been postulated that rockets might make better second stages, while laser propulsion would make better first stages. This is because the laser performance decreases as the spacecraft increases speed, and the ∆V of a rocket is independent of its velocity at ignition. Further refinements are necessary to evaluate such trades.