  Within its Voyage 2050 planning cycle, the European Space Agency (ESA) is
considering long-term large class science mission themes. Gravitational-wave
astronomy is among the topics under study. Building on previous work by other
authors, this paper studies a gravitational-wave interferometer concept, dubbed
"LISAmax", consisting of three spacecraft, each located close to one of the
Sun-Earth libration points L3, L4 and L5, forming a triangular constellation
with an arm length of 259 million kilometers (to be compared to LISA's 2.5
million kilometer arms). We argue that this is the largest triangular formation
that can be reached from Earth without a major leap in mission complexity and
cost (hence the name). The sensitivity curve of such a detector is at least two
orders of magnitude lower in amplitude than that of LISA, at frequencies below
1 mHz. This makes the observatory sensitive to gravitational waves in the
{\mu}Hz range and opens a new window for gravitational-wave astronomy, not
covered by any other planned detector concept. We analyze in detail the
constellation stability for a 10-year mission in the full numerical model
including insertion, dispersion, and self-gravity-induced accelerations. We
compute the orbit transfers using a European launcher and chemical propulsion.
Different orbit options, such as precessing, inclined orbits, the use of flybys
for the transfer, and the launch strategy, are discussed. The payload design
parameters are assessed, and the expected sensitivity curve is compared with a
number of potential gravitational-wave sources. No show stoppers are identified
at this point of the analysis.
