# Is this equation for NEP thrust correct?

I have bolded the part that I am most interested in checking, because I am not sure if the formula is valid.

I have been using Excel to find the thrust, power, etc for a NEP mission. I have been using a delta-V map I found online which states that for low thrust, I should multiply by 1.5. So, in SI units:

Required deltaV = 1.5x stated on chart Required thrust time = Hohmann transfer time Wet mass (kg) is payload+reactor+structure+propellant Propellant (kg) is determined by Isp and rocket equation

Thrust in N is determined as (wet mass + dry mass)/2 x delta V / thrust time

Power in kW is thrust x Isp / 125 (based on VASIMR needing 200kW for 5N, 5000s) Structure (kg) = 1/10 x propellant Reactor mass (kg) = 1500 + 12.5 x power Solver in Excel optimizes Isp for maximum payload and wet mass is 8300 kg based on Falcon 9 to GTO. (I am calling GTO and Earth Escape close enough to the same.)

So am I on track or am I completely off?

• I see several problems: 1. Nuclear thermal propulsion parameters are vastly different from VASIMR, 2. You're much better off taking the Falcon 9 launcher to LEO and packing more fuel for the probe to use with its efficient propulsion for escape. 3. thrust time is definitely not the whole Hohmann transfer time. – SF. Sep 3 '17 at 23:26
• I was actually thinking NEP vs NTR. I was assuming that the power requirements of all electric thrusters are about the same for equal thrust and ISP. Also, since they are low thrust, long lifetime devices, the most useful use of heavy power supplies is when the thrust is as low as possible without extending the mission. Is this not correct? – M Weiss Sep 5 '17 at 4:36
• OK, confused NTR vs NEP. That would be correct if you had a full spectrum of specific impulses and thrusts, which isn't exactly the case. You pick a propulsion that exists and fits your mission profile best, instead of building one that fits it perfectly. Also, reactors are heavy; for these thrusts - NEP currently is sci-fi. (you'd need prohibitively expensive large array of thrusters and vast acreage of radiators.) Point 2 stands; so does point 3, Hohmann transfer assumes point burns. Plus 4. Oberth effect: lower ISp burn deep in gravity well can be better than high ISp out of it. – SF. Sep 5 '17 at 6:58
• ...unless you meant RTG for power source... which still provides little power (albeit over very long time) and weighs quite a bit, meaning excessively long mission duration. (also, the x1.5 multiplier is a rough ballpark, not the actual solid value.) – SF. Sep 5 '17 at 7:09