*3.1.1 Chemical propulsion OTV sizing*

In sizing the chemical propulsion OTVs, a vehicle mass scaling equation is used [14]:

$$m(dry, \text{stage}) = m(dry, coefficient) \times m(p) + a(f \text{fixed}) \tag{1}$$

where

m(dry, stage) = the stage dry mass, including residual propellant (kg). m(dry, coefficient) = the B mass coefficient (kg of tank mass/kg of usable propellant.

mass).

m(p) = usable propellant mass (kg).

a(fixed) = chemical OTV fixed mass (kg).

The chemical propulsion OTVs had a B coefficient of 0.4. The fixed mass was 500 kg. The fixed mass includes guidance systems, adapters and reaction control system masses. The Martian moon OTVs were single-stage vehicles.

### *3.1.2 NEP OTV sizing*

The NEP OTV mass and trip time were estimated based on the power system and the propulsion system design [14]. The following dry mass scaling equation was used [12]:

$$m\left(dry.stage, NEP\right) = alpha \times P + 0.05 \times m\left(p\right) + m\left(f\text{fixed}\right)\tag{2}$$
