What is Zero?

pattern of coloration of the plot, solar powered designs are restricted to a maximum lifetime. Thus the number of samples that can be collected and analyzed is limited to a little over thirty. A more refined lifetime range (to define the trade space) results in a higher number of samples, but only about ten more.

There are three trends worth noting. First, as wheel diameter increases, the rover mass increases without more samples. Second, for solar missions, as the lifetime increases, the number of samples increases without much increase in mass. For RTG missions there is no dependency of mass to lifetime as expected. Third, for the two different types of communication selected, the result is an increase in mass for the DTE-LMO without any science benefits.

Figure 4.2.1: MER-like rover trade space; Number of samples versus rover mass

Highlighted in Figure 4.2.1 by numbers 1 – 4 in black, are the four non-dominated designs that constitute the Pareto front. The designs to the immediate left and below the design labeled 1 were not considered because the mass different was negligible and the difference in the designs was not considered

significant. In multi-objective optimization a design is dominated if it is possible to find another design that would increase an objective without decreasing at least one of the others. Before showing the specific Design Vectors for these designs, two apparent traits are seen from the plot itself. There is a pair of solar powered designs and one RTG powered design. Each pair has one design at A1 and the

other at A3 for long distance autonomy while short distance autonomy is at A3 for both. It was expected that the best designs would have the best possible autonomy (A3/A3) however, A1/A3 autonomy fairs better than A1/A3 autonomy with respect to mass.

Careful analysis of the trade space reveals that the extra mass for A3/A1 autonomy comes from a larger battery supply. A possible explanation is that a rover equipped with A1 long distance traversal autonomy stops more often and thus can charge its batteries more often, while with A3 autonomy it does not have enough time to charge and thus needs more batteries.

Table 4.2.2 shows the Design Vectors for the four Pareto front rover designs. All four designs have a DTE communication system and all are equipped with A3 short distance navigation autonomy as well as autonomous acquisition. Aside from designs 1 and 3 having A1/A3 long/short distance navigation autonomy, there are some other interesting results to note about the rover designs. Nighttime

processing penalizes solar powered rovers because it entails a larger battery requirement. However, the best RTG designs have nighttime processing resulting in higher science return. Also, with the increase from A1/A3 to A3/A3 navigation autonomy in both the solar and RTG designs there is an increase in the number of on-board computers (from one to two).

Design #1 Design #2 Design #3 Design #4

Mass [kg] ~124.5 ~129.5 ~150.5 ~171.5

Samples 23 33 78 117

Lifetime [sols] 200 200 700 700

Wheel Diam. [m] 0.25 0.25 0.25 0.25

Power Solar Solar RTG RTG

Communication DTE DTE DTE DTE

Long Dist. Aut. A1 A3 A1 A3

Short Dist. Aut. A3 A3 A3 A3

Acquisition Aut. Yes Yes Yes Yes

Night Processing No No Yes Yes

Computers 1 2 1 2

Table 4.2.2: Design Vectors for rover designs on the Pareto front

In Figure 4.2.2 the highlighted designs lie along lines of isoperformance; rovers along these lines have the same performance defined as the ratio of number of samples over mass. The blue dotted lines overlaid on the plot represent the ratio of one sample per every 7kg of rover mass. As ratios increase (2:7, 3:7, etc), they are shown with a greater slope. Along the line 5:7, two RTG designs are highlighted whose lifetimes differ by 100 sols. The Design Vectors for these rovers are shown in Table 4.2.3. The main difference between these two designs is that the design with a shorter lifetime has two computers instead of just one, implying that the extra computer saves 100 sols.

Figure 4.2.2: MER-like trade space; Isometric lines were used to compare designs with similar productivity ratios.

Table 4.2.3: Isoperformance line 5:7 Design Vectors Left Design Right Design

Mass [kg] 175 200

Samples ~95 117

Lifetime [sols] 600 700

Wheel Dia. [m] 0.25 0.30

Power RTG RTG

Communication DTE DTE

Long Dist. Aut. A3 A3

Short Dist. Aut. A3 A3

Acquisition Aut. Yes Yes

Night Processing Yes Yes

Computers 2 1

Along the isometric line 3:7, two RTG designs have been highlighted; their Design Vectors are listed in Table 4.2.4. They are similar except that one is a 500 sols mission with long distance autonomy at A3

while the other is a 700 sols mission with long distance autonomy at A1. Both rovers have the same performance (samples over mass) but the smarter one needs 200 less sols, which does not impact mass but does impact operations costs. Reducing the mission lifetime by 200 sols results in roughly $60 million saved on operations costs. This estimate can then be used as a cap on development costs required for A3.

Table 4.2.4: Isoperformance line 3:7 Design Vectors Left Design Right Design

Mass [kg] 220 235

Samples 77 ~82

Lifetime [sols] 700 500

Wheel Dia. [m] 0.35 0.35

Power RTG RTG

Communication DTE DTE

Long Dist. Aut. A1 A3

Short Dist. Aut. A3 A3

Acquisition Aut. Yes Yes

Night Processing Yes Yes

Computers 1 1

Finally, along the isometric line 2:7, designs with different power systems were compared and are listed in Table 4.2.5. Unfortunately, there are too many variables in the Design Vectors that are different, so the comparisons are not as compelling as some of the other observed differences. However, it is still interesting to note that across the same line of performance, using RTGs as a power source allows for longer lifetimes.

Table 4.2.5: Isoperformance line 2:7 Design Vectors

Left Design Middle Design Right Design

Mass [kg] 145 175 245

Samples 33 ~37 ~57

Lifetime [sols] 200 300 600

Wheel Dia. [m] 0.25 0.25 0.35

Power Solar RTG RTG

Communication DTE DTE DTE

Long Dist. Aut. A3 A3 A3

Short Dist. Aut. A3 A3 A1

Acquisition Aut. Yes No No

Night Processing No Yes Yes

Computers 2 2 2