Space Systems — Ethan Sheehan

01

STS-74

Second-year Space Systems coursework, October to November 2024. The subject is STS-74, the Atlantis flight of November 1995 that carried the Docking Module up to Mir. The brief was the standard mission-design chain done once, end to end: mission aims, payload, ten student-written requirements each traced to an aim, a CONOPS with off-nominal branches, launcher and launch delta-v, orbit selection in GMAT, transfer delta-v, a configuration trade and power-system sizing, then a reflection against what actually flew. Two requirements drove everything: rendezvous with Mir fixed the orbit at 51.6 degrees and about 354 km, and interfacing with both Shuttle and Mir drove the configuration. Twenty slides, submitted 26 November 2024.

The Mir space station photographed from Atlantis against black space, modules and solar arrays extended — Mir from Atlantis.NASA

The Docking Module raised on the Orbiter Docking System in Atlantis' open payload bay with the Earth's limb behind — The Docking Module raised on the ODS in Atlantis' bay.NASA

Press-kit line drawings of the Docking Module's starboard and port interior layouts with the equipment labelled — Docking Module interior layouts, STS-74 press kit.NASA

02

The orbit

Mir's orbit set ours: 354 by 374 km, inclination 51.6 degrees, eccentricity 0.0015. I modelled Atlantis in GMAT at the real launch epoch, 12 November 1995 07:30:43 UTC, with RAAN 80 degrees for a Kennedy launch, a JGM-2 4x4 gravity field and an RK89 integrator, with ECI, ECEF and ground-track views. Treated as circular at 354 km the period is 91.5 minutes with 36 minutes of eclipse per orbit, the two numbers the power sizing hangs on. The ground track is the same elements from the script propagated two-body over three orbits.

GMAT Earth-centred inertial view of Atlantis' orbit track drawn around a textured Earth on a black background — GMAT ECI view (EarthMJ2000Eq) at the launch epoch.

Ground track of three orbits over a world map at 51.6 degrees inclination, starting at the launch epoch — Three orbits from the GMAT elements at the launch epoch, two-body plus Earth rotation.

03

Hohmann, 200 to 354 km

First, could the launcher do it. A two-stage rocket-equation estimate from the press-kit masses gives 9.75 km/s available against roughly 9.3 km/s needed, taking the SSME Isp at its sea-level 3750 m/s so the margin is conservative rather than flattering. Then the transfer: a Hohmann from a 200 km parking orbit up to 354 km. The two burns are about 45 m/s each, 0.090 km/s in total, and the coast between them takes 45 minutes. The report had the split as 48 and 42 m/s from a transfer semi-major axis of 6653 km; the correct axis is 6648 km, which evens the burns and leaves the total unchanged.

To-scale diagram of a Hohmann transfer ellipse between a 200 km and a 354 km circular orbit with both burns labelled — Hohmann 200 to 354 km: 45 + 45 m/s, 45 min.

04

Power

The Docking Module's budget is 235 W: GLO-4 50 W, PASDE 100 W, APDS 10 W, thermal control 50 W, camera 5 W, backup ECLS 20 W. Carrying that through a 36-minute eclipse and recharging in daylight, with path efficiencies of 0.6 and 0.8, means the array must source 551 W. Solar flux at altitude is 1369.9 W/m². With 15 percent cells, 0.88 packing and the report's degradation factor, the area equation was evaluated for the 235 W load and gives 13 m²; against 551 W it gives 30.5 m², and slide 18 of the report conflates the two. One battery at 20 percent depth of discharge and 0.9 efficiency needs 789 Wh. The sweep below runs the same method from 200 to 1000 km.

Eclipse time, array area and battery capacity plotted against circular orbit altitude from 200 to 1000 km with the 354 km design point marked — Eclipse, array area and battery against altitude, report method. Design point 36 min, 13 m², 789 Wh.

05

Configuration

Requirement 004, interface with both the Shuttle and Mir, comes down to where the two docking ports sit relative to the solar arrays. The first layout put the Mir port and the Shuttle port at 90 degrees. It fails twice: Atlantis' approach path crosses the arrays, and the arrays cannot track the sun while both ports stay aligned. The selected layout opposes the ports 180 degrees apart with the arrays on top. The approach corridor is clear and the arrays are free to track. Both renders are mine, annotated with body axes: X along the flight direction, Y north, Z nadir.

Annotated 3D render of the Docking Module with the Shuttle and Mir ports opposed 180 degrees and the solar arrays on top, body axes marked — Selected: ports opposed, arrays on top free to track the sun.

Annotated 3D render of the rejected Docking Module layout with the Mir and Shuttle ports at 90 degrees and the arrays in the approach path — Rejected: ports at 90 degrees, arrays in the approach path.

06

Reflection

The array that flew was not 13 m². The Cooperative Solar Array on STS-74 was 48.6 m², because it also powered Mir and NASA wanted redundancy on a novel array. My sizing answered the requirement I wrote, not the mission NASA flew, and the report says so with the numbers rather than around them. Same treatment for the launcher: using the SSME's sea-level Isp of 3750 m/s gives 9.8 km/s; the vacuum figure of about 4500 m/s gives roughly 11 km/s. Reading the design against the flown hardware is where the study earned its marks.

07

Report

The submitted report is the twenty-slide PDF: aims, requirements table, CONOPS, launcher, orbit, transfer, configuration, power and reflection, with 31 references. Behind it sit two pieces of code. spacepower.m is the MATLAB sizing script: orbit period, eclipse time, solar flux, total power, array area and battery capacity as six short functions, with the design point and load set at the top. The GMAT script models Atlantis at the launch epoch with a JGM-2 4x4 force model and an RK89 integrator and drives the ECI, ECEF and ground-track views.

STS-74 mission studyThe submitted report, 20 slides, 26 Nov 2024.20 pp

08

Slides

The report as rendered, one page each. Three pages are left out because they carry third-party imagery: the payload table with the docking-module drawings, the CONOPS timeline built from stock cutouts, and the Hohmann method page with its borrowed diagram. Everything that remains is text, my own renders and calculations, or NASA photographs.

Title slide of the STS-74 mission study over the NASA photograph of the Docking Module in Atlantis' payload bay — 1. Title.

Mission summary slide listing the five mission aims A1 to A5 — 2. Mission summary and aims.

Requirements table: ten student-written requirements with rationale and traceability to the mission aims — 4. Requirements.

Launcher slide with the Atlantis specification table beside the JSC launch photograph — 6. Launcher.

Launcher slide describing the three SSMEs and two SRBs, with a parameter table for the boosters, orbiter and external tank — 7. Shuttle stack.

Launch calculations slide: the rocket equation and a table deriving mass flows, effective Isp and stage masses from the press-kit figures — 8. Launch delta-v method.

Launch calculations results table: stage 0 and stage 1 Isp, masses and delta-v summing to 9,800 m/s against 9.3 km/s required — 9. Launch delta-v result.

Orbit selection slide: the orbit dictated by Mir, with a table of periapsis 354 km, apoapsis 374 km, inclination 51.6 degrees and eccentricity 0.0015 — 10. Orbit selection.

GMAT Earth-centred inertial orbit view of Atlantis at the launch epoch — 11. GMAT orbit view.

Delta-V slide: Hohmann transfer definitions and a two-burn table totalling 0.09 km/s from 200 to 354 km — 13. Hohmann transfer.

Configuration alternative candidate: render with the Mir and Shuttle docking ports perpendicular and the reasons it was rejected — 14. Configuration, alternative.

Configuration selected: render with the docking ports opposed 180 degrees and the arrays on top, with the rationale — 15. Configuration, selected.

Power budget table for the Docking Module payloads totalling 235 W — 16. Power budget.

Power slide with the orbit period, eclipse time and power-required equations beside a constants table: 92 minutes, 36 minutes of eclipse, 551 W — 17. Period, eclipse and power required.

Power slide with the array area, solar flux and battery capacity equations beside the parameter table: 13 m² and 783 Wh — 18. Array and battery sizing.

Reflection slide comparing the study against the flown STS-74 hardware — 19. Reflection.

References slide listing 31 sources — 20. References.