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AGOS-Artificial Gravity Orbital Station, a possible successor of the ISS International Space Station

Authors:
  • Consulting architect
AGOS - Artificial Gravity Orbital Station,
a possible successor of the ISS International Space Station
Werner Grandl*, Clemens Böck**
* architect and civil engineer, Tulln, Austria, archigran@gmx.at
** mechanical engineer, Tulln, Austria, boeck.clemens@gmail.com
© Werner Grandl and Clemens Böck 2017
ABSTRACT
The International Space Station ISS will probably be decommissioned in the mid-2020ies. This
paper presents a design for a modular orbital station as a possible successor of ISS. The central
rotating part of the station simulates artificial gravity (AG) by centrifugal forces. AGOS is built of
cylindrical modules and structural framework. The design proposes light-weight constructions using
thin aluminum sheets and trapezoid aluminum sheeting. The station can be enlarged in stages, the
initial stage 1 has a mass of approx. 270 tons. 24 crew members can live and work in a 0.9 g
environment. To establish the initial stage 15 launches to Low Earth Orbit (LEO) with payloads of
12-22 tons will be necessary. For launching we propose esp. the reusable SPACE-X Falcon
launcher to minimize costs. By changing the rotation rate AGOS Stage 1 can be used as a testbed
for different g-levels and their influence on human health.
Contents
1. Introduction
2. Post - ISS plans
3. Artificial gravity
3.1. Former designs
3.2. The comfort box
4. Current and future launchers
5. The AGOS design
5.1. The initial stage 1
5.2. Enlargements
5.3. Structural elements and mass
5.4. The roto – joints
6. Conclusions
References
1. Introduction
Living and working in space is usually associated with lack of gravity. During the last decades
astronauts have stayed in zero-gravity-stations like Skylab, the Russian MIR and the present ISS.
On the one hand zero-gravity is an advantage for scientific research, on the other hand
weightlessness causes some danger for human health, such as bone demineralization, muscle
atrophy and orthostatic intolerance [1]. Expecting the decommissioning of ISS in the 2020ies, there
are few ideas for a new orbital station. This paper presents a possible successor for ISS, which
could provide both zero-gravity modules and rotating living quarter modules with approx. 0.9 g
artificial gravity (AG). The initial stage 1 can be used as a testbed for different g-levels by
modifying the rotation rate.
First of all we give a compact overview of international post- ISS plans. Then we discuss the
advantage of AG for future space missions. We describe various current and future launchers to lift
heavy payloads into Low Earth Orbit (LEO). We give a detailed description of our AGOS design
including possible enlargements. We present a technical solution for the joints between rotating and
non-rotating parts of the station. Last not least we propose a time frame to plan, produce and erect
AGOS, emphasizing international cooperation.
2. Post- ISS plans [2]
Due to restrictive space budgets all over the world space agencies carefully weigh up the merits of
a project against its drawbacks. NASA and ESA focus a circumlunar station at a Lagrange point or
in Lunar orbit. Europeans prefer lunar exploration with robots whilst the main target for NASA
seems to be Mars exploration. Russia´s plans have not been officially announced yet but may
provide a LEO platform [3]. China is currently developing its own orbital space station probably
functioning by 2022 [4]. The German Aerospace Center (DLR) proposes the so called Modular
Orbital-Hub, which combines hard-shell cylinders and nodes with inflatable structures, e.g. made by
Bigelow Aerospace [5]. The Orbital-Hub may be a cost-effective design which combines public and
private enterprise. But there is no actually published concept for an orbital station which provides
AG to astronauts.
3. Artificial gravity (AG)
3.1. Former designs
Early concepts of rotating space stations have been made in the 1920ies [6]. In 1926 K.
Tsiolkovsky first discussed the establishment of rotating colonies around the Earth. In 1928 H.
Potocnic (pen name: Hermann Noordung) published drawings of a wheel-shaped orbital station,
called “Weltraumrad”, which became a prototype design for many succeeding toroidal concepts [7].
In the early 1950ies W. von Braun proposed a pneumatic torus, which was the first concept to use
inflatable structures in orbit. In 1968 the famous movie 2001 a Space Odyssey showed a wheel-
shaped space station to the public. During the 1960ies several NASA designs with rotating elements
were based on the cylindrical payloads of the Saturn V launcher, but were canceled in the 1970ies
when the US space budget was cut.
Meanwhile some ambitious designs for advanced space colonies with AG emerged from
academia. The Stanford Torus of 1975 was a toroidal habitat largely made of lunar material. It
should have a diameter of 1.6 kilometers and was considered to have a population of 10,000 people
[8]. The most amazing designs for future space colonies were made by G.K. O´Neill, Space Studies
Institute, Princeton, in 1975. Giant rotating cylinders entirely made of extra-terrestrial material, the
biggest one 36 kilometers long and 6.5 kilometers in diameter, with a population of several hundred
thousand inhabitants should be located in the Lagrange points L4 and L5 [9]. Inspired by O´Neill´s
ideas A. Germano and W. Grandl published a detailed design of big space colonies in 1993,
considering feasibility and safety and emphasizing structural engineering [6,10]. Maybe some of
these utopian concepts will be realized in the 22nd century or later.
3.2. The comfort box
It is evident, that the simulation of gravity by the use of centrifugal forces will provide a more
comfortable habitat for humans in space than a zero-gravity environment. The bigger the radius the
better the conditions. At large radii and low rotation rates the Coriolis acceleration, which may
disturb the vestibular sense, can be neglected. In 1987 NASA engineer J. von Puttkamer published
the so called “comfort box” (Fig.1), which indicates acceptable living conditions in a rotating space
station [11].
Fig.1 Comfort box defined by rotation rate and radius of a space station
According to Fig.1 a rotating space station should have a minimum radius of 30 meters. Shorter
radius centrifugation generates AG levels that are different throughout the body; i.e., smaller at the
head and larger at the feet. Also, inside a rotating vehicle, the AG level is constantly being distorted
as the astronauts move about within the space station, except when they move along an axis that is
parallel to the axis of rotation [12]. To simulate an AG of e.g. 0.9 g we may choose 40 m radius and
a rotation rate of 4.2 rpm.
A future challenge for space architects and engineers should be the design of space habitats and
interplanetary spaceships with a rotating device to provide an AG up to 1 g.
4. Current and future launchers
There are just a few launch vehicles either now available or planned to be put into commission
until 2030 with payload capacities we need to erect AGOS. To build a modular orbital station in
LEO we assume typical payloads between 12 and 22 tons. Several providers in the USA, Europe
and Russia could offer launching vehicles for this purpose (Table 1):
Table 1
Launch vehicles with payloads 15-70 tons to LEO (approx. 450-500 km altitude)
Provider Launch vehicle Payload to LEO (tons) Availability
Roscosmos Proton M 21 2001
ULA United Launch Alliance Delta IV Heavy 23 2004
ESA European Space Agency Ariane 5ES 20.25 2008
ILS International Launch Services Angara 24 2014
SpaceX Falcon 9 Full Thrust 22.8 2015
SpaceX Falcon Heavy 54.4 2017 ?
Reaction Engines Ltd. Skylon (HOTOL) 15 2022 ?
NASA SLS Block 1 70 2022 ?
The SpaceX Falcon rockets have a fully reusable first stage. Thus payload costs to LEO may be
reduced to $ 2700 per kg [13]. The Skylon vehicle is a HOTOL(Horizontal Take Off and Landing)
spaceplane. It is propulsed by a SABRE engine (Synergistic Air Breathing Rocket Engine). At
Mach 5.5 and 25 kilometers altitude the engine transitions to its rocket engine mode, using liquid
oxygen stored on board. Sklyon is designed to achieve 200 flights to LEO and to reduce costs to
approx. € 800 per kg [14]. The payload bay is just 4.6 m in diameter and 12.3 m long, so the size of
modular payloads is limited.
5. The AGOS design
5.1. The initial stage
The fairing for payloads on top of launching vehicles is usually cylindrical or conical. A manned
spaceship or a space station is a pressure vessel filled with air or oxygen. For these reasons we
prefer cylindrical-shaped modules to build an orbital station. Although there is done much research
on inflating structures, e.g. by Bigelow Aerospace, we propose hard-shell aluminum structures for
the AGOS modules. By using prefabricated hard-shell modules with 7 m diameter and 14-18 m
length we can reduce the fairing of the launcher to a small conic top. Metal-frame hard-shell
modules can be lifted into orbit with their entire furniture and equipment, air locks, etc., whereas
pneumatic structures are empty after inflation[15]. The AGOS station would be assembled by
astronauts and assisting robots. The present ISS should be used as a “site hut” during the
assembling of AGOS.
The initial stage of AGOS contains four rotating living modules with approx. 0.9 g, four zero-g
central modules (two of them rotate), a docking module, connecting tubes and structural framework
to stiffen the entire structure (Fig.2).
Fig.2 AGOS initial stage; length 78 m, span 102 m, rotation radius 40 m, rotation rate 4.2 rpm, crew: 24
The non-rotating framework carries 1600 solar panels. Two joints connect the rotating
elements with the non rotating parts of the station (see section 5.4). The entire initial stage will have
a mass of approx. 270 tons. Including the transport of robots, tools, etc., 15 launches will be
necessary to establish AGOS.
The living quarter modules have two floors, the “upper” floor for living, cooking and working,
the “lower” one is the dormitory for six persons. Each crew member has a private room of 9
including a small bathroom (Fig.3 and 4). The living quarter modules should have no windows, not
to disturb the “gravity illusion” of the crew. Instead of windows high-definition screens could show
the space environment to the crew, or simulate views of terrestrial landscapes. Windows should be
provided just in the docking module because of micrometeorite and cosmic ray danger. A
maximum crew of 24 persons could live and work in the initial AGOS stage and have a living area
of approx. 600 m².
Fig.3 Section of a living quarter module
Fig.4 Floor plans of a living quarter module
5.2. Enlargements
Due to its modular design AGOS can be enlarged easily by “plug-in” of additional modules and
structural framework. Figure 5 shows a possible stage 2 of AGOS with four additional living
quarter modules. When the framework and the modules are mounted, the rotation has to be stopped
temporarily. Figure 6 shows a possible final stage with a closed ring of 32 living quarter modules,
which are connected by two lateral toroidal tubes. The maximum crew may be approx. 180
persons. Along the central axis additional non-rotating cylinders, solar panels, etc., can be provided.
Fig.5 AGOS stage 2, with 8 living quarter modules , crew: 48
Fig.6 AGOS final stage, 32 living quarter modules, crew: approx. 180
5.3.Structural elements and mass
To minimize mass and launch weight nearly all structural components, the cylinders as well as the
stiffening framework are made of thin aluminum sheets and small trapezoidal aluminum sheeting.
The sheets are approx. 0.5 – 0.8 mm thick [16]. The caves between the trapezoidal sheets are filled
with foamglass for thermal insulation and micrometeorite shielding. Thus a typical structural
element like a cylindrical shell or a bulkhead has an average mass of 10 kg/m². Figures 7 and 8
show the lightweight construction of a bulkhead, built of trapezoid sheeting. Figure 9 is a cutaway
drawing of a living quarter module.
Fig.7 Bulkhead inside a living quarter module Fig.8 Bulkhead detailed rendering
Fig.9 Cutaway drawing of a living quarter module
The furniture and technical equipment of the modules may be 40% of the structural mass. The
entire payload and the number of launches for AGOS stage 1 is estimated in Table 2.
Table 2 Payload and number of launches for AGOS stage 1
Payload Launches Mass (tons)
4 living quarter modules (20 tons) 4 80
2 central rotating modules (20 tons) 2 40
2 central non-rotating modules (22 tons, including roto -joints) 2 44
1 docking module 1 20
4 radial “spokes” (3 m diameter, 2 x 20 m length) 1 20
4 connecting tubes (to connect living quarter modules) 1 12
1810 m tubes for structural framework (7.5 kg/m) 1 14
1600 m² solar panels (25 kg/m²) 2 40
Total 14 approx. 270 tons
If we assume one additional launch for machinery and the assisting robots we need 15 launches to
assemble AGOS stage 1 in LEO.
5.4. The roto -joints
To connect the non-rotating cylinders with the rotating part of AGOS we propose two magnetic
liquid rotary seals. Magnetic liquid rotary seals operate nearly without maintenance and extremely
low leakage even in vacuum. They provide a hermetic seal by using a so called “ferrofluid”, an oil-
based liquid which is suspended in place by a permanent magnet [17] . Figure 10 shows the design
of a roto – joint : the ferrofluid is held magnetically between the rotor and stator in a labyrinth seal.
Additional ball bearings provide the centering of the rotor within the seal gap and support external
loads.
Fig.10 The roto -joint: (1) rotor, (2) stator, (3) auxiliary ferrofluid tank, (4) ball bearings,
(5) seal gap filled with ferrofluid, (6) air lock, (7) fire door
The two roto -joints are also used as electric motors, which adjust the rotation of the non rotating
part of AGOS. Additionally the rotating living quarter modules are equipped with small reaction
control thrusters to accelerate or decelerate rotation. By modifying the rotation rate, different g-
levels can be simulated to study their effects on human health. To know these effects may help to
design future interplanetary spaceships with similar or smaller rotating facilities.
6. Conclusions
To establish AGOS stage 1 in space a strong effort by international cooperation will be necessary.
As shown in Table 2 we need at least 15 launches with payloads of 12-22 tons. According to Table 1
we assume five different types of launchers to be available now: one Russian rocket, one European
launcher and three different US launchers. The Space X Falcon 9 Full Thrust is the only one which
has a fully reusable first stage and may reduce costs significantly. Maybe in the 2020ies launchers
with higher payload capacities will have been developed. To minimize the time frame for the entire
project the US, Europe, Russia and other emerging space nations like China may coordinate their
work and should time launches carefully. For research, planning and production of the modules and
the entire construction one may assume ten years, the 15 launches and the assembling may take 18
months. In a best case scenario AGOS stage 1 may be completed in 2029. The present ISS should
work until 2029 to be used as an auxiliary device during the assembling of AGOS. In any case a
space station in LEO will be necessary, not only for scientific research but to keep the gate to the
universe open for next generations.
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Post-ISS plans: What should be done ? REACH-Reviews in Human Space Exploration
  • A Garcìa
  • A Lamb
  • A Sleptsov
  • C Moreno
  • M Victorova
  • N Glazkova
  • V Shteyngardt
Garcìa A, Lamb A, Sleptsov A, Moreno C, Victorova M, Glazkova N, Shteyngardt V. Post-ISS plans: What should be done ? REACH-Reviews in Human Space Exploration 1 (2016) 63-73.
China Dream, Space Dream: China´s Progress in Space Technologies and Implications for the United States. A report prepared for the U.S.-China Economic and Security Review Commission
  • K A Pollpeter
Pollpeter KA (2015). China Dream, Space Dream: China´s Progress in Space Technologies and Implications for the United States. A report prepared for the U.S.-China Economic and Security Review Commission, 1-116.