New power and propulsion concepts that show promise for increased performance (such as higher Isp, higher thrust, or lower beam divergence) will have to advance up the "Technology Readiness Levels (TRL)" to the point of being ready for mission consideration (TRL ≥6). But how long will it take for the technology to advance through the technology readiness levels?
The Technology Readiness Levels are both a way to assess the readiness of a given technology as well as identifying what further steps are needed to advance that technology to mission readiness. A possible tactic is to use the TRLs as a proportional scale of advancement time, where the time to advance from TRL-n to TRL-n+1 and the time to advance from TRL-n+1 to TRL-n+2 follow similar ratios. Granted, there are significant variations that render this tactic inexact, but if used just for relative, rather than absolute, comparisons, then it is a fair starting point. A provisional proportionality of TRL advancement durations is presented in table 13.
Note that table 13 also lists "SRLs," "Scientific Readiness Levels," for the scientific advancement that precede new technology. The levels shown are adaptations from the "Applied Science Readiness Levels" outlined for the NASA Breakthrough Propulsion Physics project [13 p. 683].
For example, what if a concept is now at TRL-3, and we want to estimate how long before it reaches TRL-6. If the history of that concept shows that it took two years to go from TRL-1 to TRL-3, (for which the provisional duration increments are 1+2=3), then its TRL rate multiplier is 66%, (2 yrs actual)/(3 units). By multiplying that rate to the remaining seven duration increments gives an estimate that it will take four to five more years to reach TRL-6. To determine a representative set of these relative TRL durations, the histories of several comparable technologies will need to be examined, and then their results merged into a baseline set of TRL durations. The factors that need to be specified include the challenges of the technology itself and the history of resources devoted to the advancements. Again, these predicative models are only meant to be relative, not absolute. The key is that the difference concepts are compared with identical models.
From experience, the advocates for a particular technology tend to overestimate the readiness of their concept. To get more accurate readings on a technology’s actual readiness level, the US Air Force developed a "Technology Readiness Level (TRL) Calculator" that is accessible online [105].
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Millis 2018 Grant NNX17AE81G_for_CR.docx | Level Duration Unit | Description of Level | Comments |

| --- | --- | --- |
| TRL 7-9 | From system prototype to flight proven | Considered here to be part of the Mission Development Duration |
| (indeterminate) |  | "Valley of Death" |
| TRL-6 | System/Subsystem model or prototype demonstrated in a relevant environment | Ready for mission trades studies and mission commitment |
| 4 |  |  |
| TRL-5 | Component and/or breadboard validated in relevant environment |  |
| 2 |  |  |
| TRL-4 | Component and/or breadboard validated in laboratory |  |
| 1 |  |  |
| TRL-3 | Analytical and experimental critical function proof of concept |  |
| 2 |  |  |
| TRL-2 | Technology concept and/or application formulated |  |
| 1 |  |  |
| TRL-1 | Basic principles observed and reported |  |
| 4 |  |  |
| SRL-7 | Experimental test devised for a new power or propulsion mechanism | If found viable, ascends to TRL-1 |
| 4 |  |  |
| SRL-6 | Hypothesis devised for a new power or propulsion mechanism |  |
| 4 |  |  |
| SRL-5 | Problem statement devised to seek a new power or propulsion mechanism based on established effects |  |
| 1 |  |  |
| SRL-4 | New effect observed and reported that is relevant to power or propulsion goals |  |
| 3 |  |  |
| SRL-3 | Experimental test devised for an effect relevant to power or propulsion |  |
| 1 |  |  |
| SRL-2 | Hypothesis devised for an effect relevant to power or propulsion |  |
| 3 |  |  |
| SRL-1 | Problem statement devised for an effect relevant to power or propulsion |  |