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Pages: 1
Author Topic: Europa Direct!

Europa Dreamer
Posts: 9
Europa Direct!
on: November 16, 2013, 01:47


Aggressive exploitation of indigenous water resources for realization of life-support and
cryogenic propulsive liquids may be the key to relatively near-term manned exploration of the
Solar system, particularly its ‘’middle’’ portions, e.g., out to the Jovian and Saturnian ice-bearing
moons. The basic point, of course, is that leaving-LEO mass-budgets for effectively one-way
missions – ones which fully exploit water at their destination-point for life support there, and for
return-to-Earth propellants – are exponentially smaller than for round-trip ones. Now it is
currently unfashionable to send even volunteers on one-way, i.e., settlement-committed,
Government-sponsored space missions, in the manner in which the East Coast of the United
States was initially settled. Thus, it is necessary at present to consider mission architectures that
return expedition crews to Earth after comparatively brief stays at their outbound destinations.
The corresponding Gordian knot may be slashed by equipping expeditions to places such as
Ganymede and Europa (and ice-bearing asteroids, and icy Saturian moons, and . . .) with
equipment quite similar to the Mars Water Plant which we discussed above, so that they can reequip
themselves for the return segment of the trip – as well as support their local living and
exploration activities – entirely with products derived from local water at their destinations.
It might appear difficult to photovoltaically energize the equivalent of a Mars Water Plant
for a location as distant from the Sun as Europa, let alone Titan, simply because the intensity of
sunlight is 1-4% of that on Earth at the Saturnian and Jovian orbits, respectively, and use ofphotovoltaic arrays for generation of the required electric power thus would appear to be
impractical. Actually, this isn’t the case, since direct band-gap semiconductors, e.g., GaAs, are
more than two orders-of-magnitude more mass-efficient than indirect band-gap ones, such as Si,
in photovoltaic conversion, i.e., ≤1 micron thicknesses of GaAs are optically thick to most of the
solar spectrum whereas >100 microns is required for equivalent solar-spectrum photo-opacity of
Si. Very thin sheets of direct band-gap semiconductor, strengthened appropriately with an
underside polyaramid layer, thus may be expected to provide practical, ≥1 W/gm specific
photoelectric electric power production as far out as Saturn’s orbit, i.e., in 14 W/m2 sunlight.
A manned mission to Europa is challenged by the nominal 6.3 km/sec trans-Europan
insertion _v from LEO, which has added to it the 6.8 km/s of _v required to brake to a softlanding
on the near-vacuum surface of Europa upon entering the Jovian system on a Hohmann
transfer trajectory. Even the use of RL-10-based propulsion systems, with their restartability and
their 4.9 km/s exhaust speeds, seemingly implies mass-ratios of 14.5 for such one-way missions.
Actually, a Minovitch (gravity-assisted) Earth-Venus-Jupiter trajectory can reduce the outbound
insertion _v to 4.4 km/s without a significant increase in outbound trip-time and a Jovian-system
capture-burn at Io’s depth in the Jovian gravity-well, followed by more Minovitch maneuvering
among the Galilean moons before a powered touchdown on Europa can trim the total circum-
Jove maneuvering _v to 5.1 km/s. The total outbound mission _v can be thereby reduced to no
more than 9.5 km/s. This, in turn, implies a Rocket Equation multiplier of 6.95 on the leaving-
LEO mission-payload mass of ~25 tonnes (corresponding to a total mission-time of about 7
years, including a year on the Europan surface), so that the reference Europan expedition’s total
leaving-LEO mass is only 173 tonnes. The numbers for a crew-of-four expedition to Callisto or
Ganymede are essentially the same. (Of course, the same expedition might care to average down
its outbound-and-return ‘’travel overheads,’’ and touchdown successively on more than one icy
Galilean moon, ‘’while in the neighborhood,’’ refueling at each stop.)
The corresponding leaving-LEO _v on a Minovitch trajectory to Titan is only 4.7 km/s
(!), reasonably assuming use of aerobraking for a Titan touchdown (although use of highly masseconomized
photovoltaic arrays on the Titanian surface, where wind momentum flux densities
might be quite large, cannot be assured until confirming meteorological data, e.g., from the
Huygens probe of Cassini, is in-hand). Soft-landing on a vacuum-shrouded, ice-bearing
Saturnian moon naturally would be significantly more expensive in _v, unless the first stop in the
Saturnian system were made at Titan, thereby sinking the interplanetary _v. In this case,
refueling could be done first at Titan, and then the tanks could be ‘’topped off’’ as indicated at
successive stops on other icy-albeit-vacuum-shrouded Saturnian moons prior to Earth-return
from the final one of them. The corresponding Rocket Equation multiplier for the Titan
expedition is (only!) 2.6 on a characteristic Saturnian mission-payload mass of ~40 tonnes, so
that the leaving-LEO mass for a manned expedition to the surface of Titan is (only) 104 tonnes!
The total mission time would be about 14 years; assuming 1.5 years were spent on the surface of
Titan (as well as skipping among the icy Saturnian moons). In both the Europa and Titan
expedition cases, the total impulse required for lift-off of the surface and insertion into a trans-
Earth trajectory isn’t larger than the total outbound impulse, so that propellant tankage reuse is
entirely feasible: the expedition’s transit-vehicle touches down at the icy destination with dry
cryopropellant (and water, and oxygen) tanks and lifts off with (in the case of cryopropellants,
partly-) full ones reloaded with local water products. These Jovian and Saturnian system
exploration data are summarized in Table II, along with those of the baseline case for Mars.
These relatively very modest leaving-LEO masses for round-trip manned expeditions to
Solar system destinations hitherto considered to be unattainably distant relative to contemporary
human technology should motivate serious thought about mounting such expeditions during the
next few minimum-energy ‘’launch windows’’. That most all of the leaving-LEO mass in all of
these cases is comprised of water products – LH2 and LO2 – and thus of material which may be
Earth-orbited in convenient-sized parcels with high-acceleration, potentially low-cost means,
should be especially thought provoking.

Leaving-LEO _v, km/s 4.0 4.4 4.7
(Venus fly-by, for Europa and Titan)
Destination maneuvering _v, km/s 0 5.1 0
(Aerobraking at Mars, Titan)
Total outbound _v, km/s 4.0 9.5 4.7
Rocket Equation mass-multiplier 2.26 6.95 2.61
(RL-10 5:1 LO2:LH2; vexhaust = 4.9 km/s)
Leaving-LEO mission payload mass, T 18 25 40
Leaving-LEO total mission mass, T 41 174 104
Outbound trip-time, years 0.9 3.0 6.2
Stay-time at destination, years 1.1 1.0 1.6
Leaving-destination _v, km/s 6.0 5.1 4.8
Return trip-time, years 0.8 3.0 6.2
Total mission-time, years 2.8 7.0 14.0

Europa Dreamer
Posts: 4
Re: Europa Direct!
on: January 13, 2014, 05:02

That was hard to follow and I'm not sure what the topic is.

Europa Dreamer
Posts: 20
Re: Europa Direct!
on: January 14, 2014, 08:19

It has been suggested on other forum pages that we collect hydrogen from Europa’s surface and use it to fuel a rocket for a return journey. While I’m absolutely supportive of getting our guys back safely, I’m equally aware of how challenging it would be. Notice at the bottom of this page it states that Europa’s escape velocity would be 5.1km/s (thanks to Jupiter’s proximity) compared to Earth’s 4-4.7km/s. This tells me that we would need enough hydrogen to power a rocket similar to those used to achieve Earth’s escape velocity. While it may be possible to collect that amount of hydrogen, getting that kind of launch vehicle all the way to Jupiter and landing it on Europa’s surface would almost certainly raise the cost and complexity of the mission beyond feasibility.

A similar and more practical proposal would be to use hydrogen gathered in this fashion to power a vehicle off of Europa’s surface and into Jupiter’s orbit. This would be closer to achieving the moon’s escape velocity (which we’ve done before). From there, we would still need some kind of booster coming from Earth to leave Jupiter’s orbit. It would still be very costly and complex, but more practical.

Europa Dreamer
Posts: 4
Re: Europa Direct!
on: January 15, 2014, 03:35

It wouldn't be that hard. Escape velocity is not needed, as it only applies to ballistic projectiles.
Also, earths escape velocity is 11.2km/s.

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