"""
This function propagates the spacecraft forward in time 1 Sim-Flanagan step (of variable length of time),
applying a thrust in the center.
"""
function prop_one(ΔV_unit::Vector{Float64},
                  state::Vector{Float64},
                  duty_cycle::Float64,
                  num_thrusters::Int,
                  max_thrust::Float64,
                  mass::Float64,
                  mass_flow_rate::Float64,
                  μ::Float64,
                  time::Float64)

  if norm(ΔV_unit) > 1.
    throw(ErrorException("ΔV input is too high"))
  end
  halfway = laguerre_conway(state, μ, time/2)
  halfway[4:6] += ΔV_unit * max_ΔV(duty_cycle, num_thrusters, max_thrust, time, 0., mass)
  return laguerre_conway(halfway, μ, time/2), mass - mass_flow_rate*norm(ΔV_unit)*time

end

"""
A convenience function for using spacecraft. Note that this function outputs a sc instead of a mass
"""
function prop_one(ΔV_unit::Vector{Float64},
                  state::Vector{Float64},
                  craft::sc,
                  μ::Float64,
                  time::Float64)
  state, mass =  prop_one(ΔV_unit, state, craft.duty_cycle, craft.num_thrusters, craft.max_thrust,
                          craft.mass, craft.mass_flow_rate, μ, time)
  return state, sc(mass, craft.mass_flow_rate, craft.max_thrust, craft.num_thrusters, craft.duty_cycle)
end