ESA Document - Emits - ESA
ESA Document - Emits - ESA 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.
s stack 9 Propellant mass of the new stack < propellant boiling-off stack 8 Boil-off = nr of stacks x boil off rate x time stack 7 Achieved ∆V (m/s) 2800 2700 2600 2500 2400 stack 6 stack 5 stack 4 stack 3 stack 2 Achieved ∆V ∆V required 8 Stacks 9 Stacks 10 Stacks 11 Stacks 12 Stacks Figure 2-24: Boil-off effects and ∆V capability loss HMM Assessment Study Report: CDF-20(A) February 2004 page 57 of 422 stack 1 The main parameters playing a role in the boil-off process are: • Boil-off rate, namely the mass of propellant lost per unit of time, which depends on the design of the system • Time prior to the usage of the propulsion stage, which mainly depends on the assembly time and therefore, on the launcher rate and the commissioning time An analysis was carried out to assess the influence of all the parameters involved in the boil-off process. The main contributor to the DV capabilities losses was the boil-off rate, but the launch rate and the commissioning time before departure also had an impact. Achieved ∆V (m/s) Commissioning time effect 4100 4000 3900 3800 3700 3600 3500 0 1 2 3 4 5 6 Commissioning time (months) Achieved ∆V (m/s) 4100 4000 3900 3800 3700 3600 Launcher rate effect 3500 0 1 2 3 4 5 6 Time in between launches (months) Achieved ∆V (m/s) 4000 3800 3600 3400 3200 3000 2800 Boil-off effect 0 150 300 450 600 750 900 1050 1200 1350 1500 Boil-off rate (kg/month) Figure 2-25: Effect of different parameters on the boil-off (11 stacks with a payload of 470 tonnes) Two scenarios have been analysed in more detail, fixing the commissioning time to three months and varying the time between launches between 1 and 3 months. The results have been compared with the storable propellant scenario, which provides the limit in terms of mass efficiency. Figure 2-26 shows the influence of the boil-off rate. The figure of merit represented is the number of propulsion modules (80-tonne cryogenic propulsion stacks) required to insert the payload into its trajectory to Mars. In the case of 3 months in between launches, a boil-off rate higher than 800 kg per month makes the mission not feasible with cryogenic propulsion. On the other hand, if only storable propellant is used, the number of stacks required is 16.
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s<br />
stack 9<br />
Propellant mass of the<br />
new stack < propellant<br />
boiling-off<br />
stack 8<br />
Boil-off = nr of stacks x boil off rate x time<br />
stack 7<br />
Achieved ∆V (m/s)<br />
2800<br />
2700<br />
2600<br />
2500<br />
2400<br />
stack 6<br />
stack 5<br />
stack 4<br />
stack 3<br />
stack 2<br />
Achieved ∆V ∆V required<br />
8 Stacks 9 Stacks 10 Stacks 11 Stacks 12 Stacks<br />
Figure 2-24: Boil-off effects and ∆V capability loss<br />
HMM<br />
Assessment Study<br />
Report: CDF-20(A)<br />
February 2004<br />
page 57 of 422<br />
stack 1<br />
The main parameters playing a role in the boil-off process are:<br />
• Boil-off rate, namely the mass of propellant lost per unit of time, which depends on the<br />
design of the system<br />
• Time prior to the usage of the propulsion stage, which mainly depends on the assembly<br />
time and therefore, on the launcher rate and the commissioning time<br />
An analysis was carried out to assess the influence of all the parameters involved in the boil-off<br />
process. The main contributor to the DV capabilities losses was the boil-off rate, but the launch<br />
rate and the commissioning time before departure also had an impact.<br />
Achieved ∆V (m/s)<br />
Commissioning time effect<br />
4100<br />
4000<br />
3900<br />
3800<br />
3700<br />
3600<br />
3500<br />
0 1 2 3 4 5 6<br />
Commissioning time (months)<br />
Achieved ∆V (m/s)<br />
4100<br />
4000<br />
3900<br />
3800<br />
3700<br />
3600<br />
Launcher rate effect<br />
3500<br />
0 1 2 3 4 5 6<br />
Time in between launches (months)<br />
Achieved ∆V (m/s)<br />
4000<br />
3800<br />
3600<br />
3400<br />
3200<br />
3000<br />
2800<br />
Boil-off effect<br />
0 150 300 450 600 750 900 1050 1200 1350 1500<br />
Boil-off rate (kg/month)<br />
Figure 2-25: Effect of different parameters on the boil-off (11 stacks with a payload of 470 tonnes)<br />
Two scenarios have been analysed in more detail, fixing the commissioning time to three months<br />
and varying the time between launches between 1 and 3 months. The results have been compared<br />
with the storable propellant scenario, which provides the limit in terms of mass efficiency.<br />
Figure 2-26 shows the influence of the boil-off rate. The figure of merit represented is the<br />
number of propulsion modules (80-tonne cryogenic propulsion stacks) required to insert the<br />
payload into its trajectory to Mars. In the case of 3 months in between launches, a boil-off rate<br />
higher than 800 kg per month makes the mission not feasible with cryogenic propulsion. On the<br />
other hand, if only storable propellant is used, the number of stacks required is 16.