ESA Document - Emits - ESA

s
Storable (Isp 345) Cryogenic (Isp 450) Cryo + Storable
Mass to LEO (tonnes) 1728 969 1336
Table 2-14: Mass to LEO for different propulsion technologies
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 56 of 422
As shown in Table 3-15, the use of cryogenic propellant for all the mission can reduce the initial
mass to less than 1000 tonnes, but the problem of boil-off remains. An intermediate solution was
adopted: cryo propulsion system for the first propulsive manoeuvre (TMI) and storable for the
other two (MOI and TEI). This approach will allow a mass reduction in LEO and enables a
possible analysis of the two types of propulsion technologies in the framework of a mission to
Mars.
Boil-off represents the main problem in cryogenic systems. To reduce the volume of the
propellant (LOX and LH2), you must store them in liquid phase. For that purpose there are only
two solutions: keep them at cryogenic temperatures, or compress them to high pressure. The
second option has a big disadvantage in terms of mass, as the tanks have to support the internal
pressure. Therefore the best solution is to keep the propellants at low temperatures.
To completely isolate the propellants at cryo temperatures from any source of heat is practically
impossible, therefore the propellants will undergo some phase change from liquid to gas. This
propellant in gaseous form has to be depleted to avoid an increase in the internal pressure of the
tank.
G
L
Propellant tank
Tank insulation
Q
Figure 2-23: Boil-off process
During assembly of the vehicle in LEO, propellant mass is lost, and the ∆V capabilities of the
vehicle are reduced. Loss of propellant can be compensated by launching more propellant before
the composite departs. There is a point at which launching more stacks does not compensate the
boil-off effect, as the new propellant launched is less than the mass lost by all the other stacks
that are already there.