Spacecraft Engineering

Draft

Earlier sections covered where a spacecraft goes and how it gets there. This one covers the vehicle itself: the subsystems that keep it powered, pointed, cool, in contact and alive, and the discipline that stops them from contradicting each other.

Reading order

Systems engineering comes first because it defines the budgets every other page draws on. Power and thermal are coupled and best read together. Fault tolerance comes last because it draws on all of them.

The numbers this section turns on

Solar array output

P = S\,\eta\,A\cos\theta\,K_p\,K_d

At 30% cell efficiency, roughly 2.9 m² of array per kilowatt at Earth, and the array is sized for end of life, not beginning.

Radiator area

A = \frac{Q}{\varepsilon\sigma\left(T^4 - T_{\text{sink}}^4\right)}

Roughly 2.6 m² of radiator per kilowatt at room temperature. The fourth power means raising rejection temperature from 250 K to 350 K cuts the area almost fourfold.

Carrier to noise density

\frac{C}{N_0} = \text{EIRP} + \frac{G}{T} - L_{fs} - L_a - k

The link budget. Path loss is 150 dB from low Earth orbit and 274 dB from Mars at one astronomical unit, which is what the Deep Space Network exists to close.

Exponential reliability

R(t) = e^{-\lambda t}

Ninety per cent reliability over fifteen years implies a mean time between failures of 142 years, which is reachable only through redundancy, not through better parts.

Four things worth knowing

The end-of-life case sizes everything. Arrays are sized for their degraded output, radiators for surfaces that have darkened with age, batteries for their worn capacity. A spacecraft is oversized on the day it launches, and that surplus is itself a design problem.

Batteries are sized by cycle count, not by energy. A LEO satellite eclipses 87,000 times over fifteen years and a GEO satellite 1,350 times. The LEO satellite has the shorter eclipse and needs the larger battery, because it can only use a fifth of it each time.

Knowledge is often worth more than accuracy. An imaging satellite that points 100 arcseconds off but knows where it pointed to 2 arcseconds can geolocate correctly. The reverse cannot, and no ground processing will fix it.

Redundancy does not protect against design faults. Ariane 5 flight 501 lost both inertial reference units to the same unhandled overflow within milliseconds. Identical units given identical inputs fail identically.

Pages

01Spacecraft systems engineering

Requirements flowdown and the four verification methods, the budgets that hold a design together and how much they grow, TRL used honestly, review gates, and the integration and test flow.

02Power systems

Array sizing and degradation, why battery selection is driven by eclipse cycling rather than energy density, radioisotope generators, bus architecture, and the charge-rate constraint that is often missed.

03Thermal control

Radiation as the only way out, radiator sizing and fourth-power scaling, passive and active hardware, hot and cold cases, and cryogenic cooling.

04Avionics and onboard computing

Why flight processors are three orders of magnitude behind consumer parts, redundancy architectures, data buses, memory scrubbing, and the flight software frameworks and constraints.

05Communications subsystem

The link budget term by term with worked path losses and antenna gains, band selection, coding against the Shannon limit, and the ground networks.

06Guidance, navigation and control

Attitude sensors and their blind spots, why quaternions win, wheels against control moment gyros, the disturbance torques that saturate them, and the three-part pointing budget.

07Onboard propulsion

Pressurisation architecture, propellant management when there is no bottom to the tank, gauging uncertainty at end of life, and plume impingement.

08Structures and mechanisms

Launch loads and which components each one drives, stiffness as the binding requirement, materials and sandwich construction, and deployables that must work exactly once.

09Life support

Metabolic requirements and the 26 tonnes a Mars crew would otherwise carry, the ISS air and water loops, atmosphere selection, and the EVA prebreathe problem.

10Fault tolerance and reliability

The reliability arithmetic, FMEA and single-point failures, why common cause defeats redundancy, safe mode design, and what four real failures actually teach.