Gradient One

Who this is for

What it is for

Two questions, one vehicle. The first is the station's own: whether people can live and work for months at 1g produced by rotation at 1.6 rpm, the operating point Aegis Station is designed around. No one has measured that in orbit. The second is the surface program's: how bone, cardiovascular, ocular and plant endpoints respond to lunar and Mars gravity over months, against a 1g control in the same vehicle, sharing the same radiation, diet, lighting and instruments.

Both questions need the same thing: several gravity levels, held for a long time, in one place, with rotation present. A fixed-rate rotor with modules at three radii is the cheapest structure that gives all of that at once. The platform also carries a rotating 0g population, so that the effect of rotation can be separated from the effect of gravity level.

Configuration

A Y-shaped rotor: three open-truss arms at 120°, each about 344 m long, with an identical habitat module hung from every tip at the 1g radius. The lunar module sits inboard on arm A and the Mars module inboard on arm B. A rotating hub module joins the three arms and houses the 0g crew: a 4.5 m node-class hull 6 m long inside the 12 m ring the arms hinge on, its upper face 3 m up the axis. A small despun node on the spin axis, its lower face at 8 m, carries the docking ports, solar arrays, reboost thrusters and antennas, and does not rotate. Between the two faces sits the crossing: an 8 m drum, 4 m long, on a 3 m bearing, with three chambers at 2.5 m from the axis, one to each arm's port.

Spin rate 1.60 rpm (ω = 0.167 rad/s). Coriolis acceleration at 1 m/s walking is 0.34 m/s², 3.4 % of g. Head-to-foot gradient over 2 m at the 1g band is 0.57 %. Overall diameter about 714 m.

Why 350 m and 1.6 rpm

The radius is not chosen for the biology. Earth is the 1g control for the biology, and a smaller, faster platform would give the partial-g bands at a fraction of the arm length. The radius is chosen because 1g at 1.6 rpm is the station's operating point, and the tolerance data AGRE produces is reported per spin rate. A platform at a different rate measures a different column of the table.

The sizing model makes the trade explicit. At 3 rpm the 1g radius is 99 m, Mars is 38 m and the Moon is 16 m. The arms shrink by about 250 m each. Because the arms are light open trusses under tension, that saves about 32 t of the 242 t vehicle, roughly 13 %, and a similar fraction of the structure cost. The crew-rated modules, the boundary, the assembly campaign and the operations are unchanged. The 3 rpm platform also carries twice the Coriolis at walking speed, 6.4 % of g, which is the effect the smaller radius was supposed to avoid measuring.

Modules

One common pressurized module, five copies: a 4.5 m diameter, 11 m hull with three 2.4 m decks and a utility deck, about 14 t dry and 17 t outfitted, in the class of the ISS laboratory and node modules. Every module is mounted radially, its long axis along the arm, so that the centripetal load runs through the hull along the same axis its launch structure already carries at several g. The 1g service load is a fraction of the launch case. Tip modules hang from the arm end; the Moon and Mars modules sit beside the arm on cradles at 0.17 g and 0.38 g respectively.

The radial orientation is also what the requirements ask for. Three decks give a measured gravity gradient between floors (AGRE-F-004). Stairs and ladders between decks are radial transit (AGRE-F-002). Floors in the 1g modules are built to the 350 m radius, as the station's rings are. The hub-facing end of every module carries the port that the transit car docks to.

Hub/rotor boundary

Rev A had a "central non-rotating hub" and said nothing about how anything crossed into it. Rev B adopts the station's own principle from the hub/ring boundary page: no continuous rotary mechanical interface carries a life-critical flow. Every crossing is cyclic, contactless, or eliminated.

The vehicle is a dual-spin spacecraft: a rotor spinning about its major axis and a despun platform on the same bearing. That arrangement flew for decades on communications satellites at far higher rates. The failure case is benign: a seized bearing turns the node into a co-rotating hub, and the two crew vehicles that stay docked for the whole campaign remain the crew's way home either way. The boundary's performance under rotation is itself a campaign objective (AGRE-F-012), and it is the station's boundary at the station's spin rate.

What the crossing costs the vehicle, by the model: a crossing there and back moves the rotor's spin by four parts in a million of itself, and the despin drive carries nothing through it but the bearing's friction. What it costs the node is the lock's engagement, which hands the node the drum's share of their remaining relative rate: at the station's lock tolerance (1e-3 rad/s) that is half the rate at which the node is called lost, so Gradient One's tolerance is a third of the station's, and the kick a fifth of the threshold. The drum's mass up the axis eases the Earth's rocking of the node a little, from 34 to 33 microradians, by lifting the mass centre toward it.

The crossing

Between the rotating hub and the despun node there is no rotary seal and no slip ring. Crew cross the boundary in a drum of three chambers that belongs to one frame or the other and is never free by design: it idles locked to the rotor with its chambers at the three arms' ports on the hub's upper face, and to cross, the chambers retract and seal, the rotor lock releases, the drum's own drive brings it to rest against the node, the node lock engages, and the chambers push a metre to the node's ports and mate. It is the station's crossing at three chambers instead of four, at the station's spin rate with a seventieth of its inertia, and every rule of the station's mechanism holds here unchanged.

A crossing, rotor to node

1. Every chamber retracts and seals: 1 m at 20 mm/s, 50 s. Nothing moves with a chamber out or a hatch open.
2. The rotor lock releases, under 10 N·m sensed on its dogs.
3. The drive ramps the drum to rest against the node, 120 s inside its 200 N·m; the torque reacts into the rotor.
4. A loop aligns the chambers to the node's ports, then the node lock engages inside 10 mm and 3 × 10^-4 rad/s, three tries before a fault.
5. The chambers push a metre: soft capture at 0.9 m, hard mate, the tunnel equalized, a 60 s leak check.
6. The hatches open. The way back is the same sentence toward the rotor.

On the flight software's image

The crossing is in the simulation since Rev C: the drum and its berths in the plant, two applications and the executive's two words on the cFS image, two Limit Checker rows and two stored sequences, seven scenarios in the fault library (a crossing both ways, a stalled drive, a lock that will not engage, a push toward a frame that does not hold the drum, a leak, a hatch out of sequence, and the node lost mid-crossing). On the image the drum goes to the node in 182 s of drive and back in 207 s, at its 200 N·m most of the way; three chambers are ready at a side's ports in 216 s; the node is held to 7.8 × 10^-5 rad/s through it, and the spin stays where it was. The console draws the crossing as the station's does: the drum from the node with the chambers at their phase to the arms' ports and the node's ports at theirs, the strokes beside it.

Figures

The sizing is option C of the crossing sizing in the systems model (the hub's dimensions as they are, the chambers at 2.5 m), November 2026; the drawings are generated from it. The flight software's figures are ADR-0015 of the Gradient One flight software. Open: the leak budget per cycle, the chambers' outfit, a brake toward the rotor as the station has, and the trim's bill for a tip load with the drum up the axis.

Dynamics and balance

The vehicle in simulation

Since Rev B the vehicle has a dynamics model (the hub, three arms hinged for flap and lag, the despun node on its bearing, the cars and their weights, the water between the tanks, the pods, the Earth's pull across it) and a flight software built on it: nine applications on a NASA cFS image (the crossing's drum and berths among them since Rev C), a C plant held to the model, an operator console, a fault library of nineteen scenarios flown on the image, and the vehicle's own telemetry alphabet with every scenario kept as a tape. The figures in the cells above are the model's. The clip is a spin-up from rest on the flight software's image, cut to a couple of minutes.

Radial transit and radial plumbing

Crew move between bands in a pressurized two-seat car on a rail along each arm, docking to the hub-facing port of each module. The alternative, a pressurized spoke tube along each arm, is a walkable path and is what the station will need, but at about 90 kg per metre it adds about 60 t to this vehicle. The car gives the transition measurements AGRE asks for and leaves the spoke to the station's own design.

The water and gas lines along the arms are where the rotating-frame fluids requirement gets tested. The static pressure head from the hub to the tip is 1.8 MPa, about 18 atmospheres, and the station's rings at 390 m see 1.9 MPa. Pumps at the tip work against it; a line draining toward the hub has it for free. Coriolis asymmetry in radial flow, two-phase behaviour and separator performance at each gravity level are measured on the lines and in the lab module (AGRE-F-011).

Systems

Crew and campaigns

The requirements page asks for at least six months of exposure per subject (AGRE-F-007) and for the study to be powered before it starts (AGRE-F-014). Rev A's 30–90 day campaigns with 2–4 crew did not meet either. Rev B baselines 180-day campaigns with ten crew: four at 1g, two at Mars gravity, two at lunar gravity, two in the rotating hub. The first campaign is a 60-day shakedown.

That is still two subjects per partial-gravity band per campaign. The human dose-response result is a program result, not a campaign result: five campaigns over about three years give ten subjects per partial-g band and twenty at 1g. The plant, microbial, fluids and mechanical results are complete within a campaign. The page says so rather than implying a single mission settles the human question.

Mass budget

Rev A did not carry a mass budget. Its "does not assume" list excluded a 250–500 t vehicle; Rev B landed under that, and Rev C still does with the crossing in, because the arms are light and the modules are few. The dynamics model carries 289 t: this budget with the crew, the two docked crew vehicles, the balance weights and the mass placed to make the arms even.

Cost class

Rev A gave $600 M to $1.4 B. That is below the cost of a single crew-rated module program with its launch and does not survive contact with the analogs. Rev B states a rough order of magnitude by element, using ISS-module, Gateway-element and commercial-station analogs, with 30 % for program management and reserves. It is a cost class, not an estimate.

Development timeline of 8–10 years to first campaign is retained from Rev A; it is consistent with the Gateway analog. The number that moved is the price. The crossing drum sits inside the boundary-hardware line; its cost has not been estimated separately, and the Rev C mass moves the launch line by less than its own spread.

What it is made of

Flown hardware, by element. Nothing on this vehicle needs inventing: each row names the flown article the G1 element descends from and the firm that built it. Three rows have no analog at the scale G1 needs and say so; they are the open items. Firms are named as builders of record, nothing more. No relationship with any of them is implied, and a row is a hardware-class match, not a proposal they have seen. The engagement page says what a partner position means on this site.

Rows name flown hardware only. Announced programs that have not flown are left out, whoever is building them. Where a class has several builders with flown articles, the row lists the ones with ISS or Gateway hardware; corrections to any attribution go to contact@aegisstation.com.

What it validates

Plant growth and bioregenerative systems (AGRE‑F‑010)

Growth racks sit in the 1g laboratory module and in the Mars and Moon modules, and growth in every band is measured against the 1g lab. Every band shares the same radiation, diet, lighting and instruments. The racks draw 11 kW of the 55 kW load. The plant result is complete within a campaign; it does not wait for the five-campaign human program.

Fluids and two-phase behaviour (AGRE‑F‑011)

The water and gas lines along the arms and the lab module are the radial fluids test bed. The static pressure head from the hub to the tip is 1.8 MPa, about 18 atmospheres; the station's rings at 390 m see 1.9 MPa. Coriolis asymmetry in radial flow, two-phase behaviour and separator performance at each gravity level are measured on the lines and in the lab module. Water recovery and oxygen generation run in the 1g habitats, where gravity phase separation applies, and are themselves a validation item. The fluids result is complete within a campaign.

What changed from Rev A

Open items

• Coupled attitude simulation. Done in the model (November 2026): the axis rides about 9° off the orbit normal and keeps station with the orbit's plane for nothing, by altitude and inclination; holding the normal by thrust would cost tens of tonnes a year. Array shadowing and the node's own pointing remain.
• The crossing. Taken in Rev C: the station's crossing at three chambers, 13.8 t, the drive reacting into the rotor; see the boundary table. What remains: the leak budget per cycle, the chambers' outfit, the lock's tolerance on rate (3e-4 rad/s is the suggestion, to be set with the bearing's real friction and the lock's real engagement), and a drum that cannot be powered down: its bearing is frictionless in the model and a drum left free coasts, so a lost drive leaves it floating between the frames until the drive is back.
• The trim's law on the Rev C vehicle. The drum's mass up the axis joins the node's in what a tip load tilts the principal axis against. A tonne set down at a tip now wobbles the vehicle at 5.5e-5 rad/s against Rev B's 3.0e-5, and the pods across the plane fire 80 s to take it out against 9 s. The law is right to fire; the cost is propellant. The wobble floor and the law's gain were sized on Rev B and are to be resized.
• Arm first modes. The model finds the first in-plane rate near 0.42 rpm, passed on every spin change with a swing of about 4 m at the tips on placeholder stiffness and damping; the car's Coriolis load is carried by the weight that moves against it. The truss's real hinge stiffness and damping, and the ramp's pace through the band, are the item.
• The balance weight's sizing. As sized it balances a 2.8 t car over the whole rail, and the car is 2.4 t empty: 400 kg of payload for a tip-to-hub transit. A heavier weight, a longer weight rail, or the payload written at 400 kg.
• The despun node's offset. Its mass 10 m up the axis is the lever for the Earth's twice-a-turn rocking of the axis; on the rotor's plane, or balanced across it, the rocking goes. The drum at 5.5 m adds a sixth to the mass up the axis and eases the rocking a little by lifting the mass centre toward the node; the lever is still the node's. The bearing's friction, a placeholder, sets how fast the node is lost when its drive fails (11 s as modelled).
• Module thermal at 1g. Natural convection inside the 1g and partial-g modules changes cabin air handling and condensation behaviour from the ISS design basis; a benefit, but an unmodelled one.
• Assembly sequence. Arm erection and module hanging before spin-up, with a robotic and EVA crew campaign, is costed but not sequenced.

Technical reference

The Rev C design dossier carries the sizing set above with its bases, the mass and cost tables, the band and deck geometry, the crossing's sizing, and the requirement closure map.

Design Dossier (PDF), Rev C
Rev B dossier (archived, September 2026) · Rev A dossier (archived, September 2026)

Reference concepts: Artificial Gravity Requirements Evaluation, which defines the dataset; Hub/Ring Boundary, whose principle the despun node adopts.