Spacecraft Systems & Mission Design — Troubleshooting

"My propellant mass fraction from the rocket equation comes out over 100%"

Check units consistency between Δv and Isp·g0 first (both must be in the same velocity units, typically m/s) — this exact issue was flagged in this guide's own Installation section. Beyond units, also confirm Δv is a genuinely achievable requirement for the Isp being used; an extremely high Δv budget combined with a low-Isp engine can mathematically require more propellant than any reasonable mass ratio allows, which is itself a real engineering finding (this engine/Δv combination isn't feasible), not necessarily a calculation error — cross-check against Propulsion Systems' own realistic Isp ranges before assuming the math is wrong.

"My radiator area calculation seems abnormally large or small"

Check that temperature is in KELVIN, not Celsius — the Stefan-Boltzmann law's T⁴ term requires absolute temperature; using a Celsius value directly (which could even be negative, making T⁴ nonsensical for negative-base scenarios in code) produces a wildly incorrect result. Convert Celsius to Kelvin (add 273.15) before applying the formula, every time.

"Doubling waste heat load didn't double my radiator area"

This is expected, not an error — area scales linearly with P (heat load) ONLY if temperature stays fixed; if you're comparing two scenarios where BOTH heat load and temperature changed (as in Intermediate's own annotated examples), the T⁴ term's much stronger influence means the two effects don't combine in a simple, linear, intuitive way. Always isolate one variable at a time (hold T fixed, vary only P) if you specifically want to confirm the linear P relationship in isolation.

"My link budget shows a negative link margin, but I expected the link to close"

A negative margin is the calculation correctly reporting that the link, as configured, does NOT close — before assuming a calculation mistake, check each term's sign convention: FSPL should always be SUBTRACTED (it's a loss), while antenna gains should be ADDED. A common error is treating FSPL as already negative and then subtracting a negative value (which adds it back), effectively double-counting the loss in the wrong direction — or the reverse, accidentally adding path loss instead of subtracting it.

"My decibel power conversion (10·log10(Watts)) gives an unexpected value"

Confirm you're using 10·log10 for POWER quantities (like transmit power in Watts), not 20·log10 — Advanced's own material uses 20·log10 specifically for FSPL because that term involves distance and frequency in a way that corresponds to a squared relationship in the underlying physics (power spreads over a growing spherical area). Power-to-dB conversions use the 10·log10 form; mixing up which quantities use 10· versus 20· is a common source of an answer that's roughly in the right neighborhood but numerically wrong by a factor of 2.

"A halved transmit power didn't cost exactly 3 dB of link margin, like Advanced's material suggested"

The "-3 dB per halving" relationship is a close, useful rule of thumb (since 10·log10(2) ≈ 3.01 dB), but it's only EXACTLY 3 dB for an exact factor-of-2 power change — a slightly different ratio (say, reducing power by 45% instead of exactly 50%) won't produce exactly -3 dB, though it will be close. Treat "-3 dB per halving" as a fast mental-math check for sanity-checking a result, not a substitute for the actual logarithmic calculation when precision matters.