Onorbit staging

A recent published paper119 describes a concept called "on-orbit staging'' (00S) that the authors believe is critical to achieving the objectives of the human exploration initiative. The authors say: "We demonstrate with multiple cases of a fast (<245-day) round-trip to Mars, that using 00S combined with pre-positioned propulsive elements and supplies sent via fuel-optimal trajectories can reduce the propulsive mass required for the journey by an order-of-magnitude.'' The authors thereby imply strongly that such short round-trip missions to Mars can be made viable.

The basis for the claims made in this paper is well known. The more demanding a propulsion operation is, the more it benefits from staging the propulsion system into segments such that the amount of propellant used to accelerate propulsion stages (as opposed to payload) is minimized.

For moderately demanding propulsion steps, staging provides moderate benefits. For example, consider the case representing trans-Mars injection from Earth orbit, where the change in velocity (Av) is ^4,000 m/s, the specific impulse of the LH2/L0X propulsion system is 460 s, and the ratio of stage mass to propellant mass is 0.1. These correspond to a typical long-stay Mars mission. If the total initial mass in LE0 (sum of payload, stage, and propellant) is 1 mT, a single-stage propulsion system can send a payload of 0.338 mT on its way toward Mars. If this propulsion system is staged, the payloads that it can send toward Mars for various levels of staging are given in Table 5.22. The gains from staging are modest.

If we consider a round trip of a payload from LE0 to the Mars surface and return to trans-Earth injection, using L0X/LH2 propulsion for Earth departure and CH4/L0X thereafter, with propulsion used for all steps (aero-assist is not used here), the values of Av for a long stay (500-600 days at Mars) mission are estimated in Table 5.23. With these values, the effect of using staging for all steps is summarized in Table 5.24. Staging provides moderate benefits. However, staging beyond two stages produces diminishing returns.

119 Enabling Exploration Missions Now: Applications of On-Orbit Staging, David C. Folta, Frank J. Vaughn, Jr., Paul A. Westmeyer, Gary S. Rawitscher, and Francesco Bordi, American Astronautical Society Paper AAS 05-273.

Table 5.22. Payload mass that can be sent toward Mars vs. level of staging.

Payload/initial mass

Table 5.22. Payload mass that can be sent toward Mars vs. level of staging.

Payload/initial mass

1 stage

0.338

2 stage

0.354

3 stage

0.358

4 stage

0.360

Table 5.23. Estimated values of Av for Mars

long-stay round trip.

Step

Av (m/s)

Earth departure

4000

Mars orbit insertion

2500

Mars descent

4000

Mars ascent

4300

Mars departure

2400

Table 5.24. Payload

mass that can be sent on

round trip to Mars surface vs. level of staging

(long stay).

Round trip payload/

No. of stages

initial mass in LEO

1 stage

0.0097

2 stage

0.0116

3 stage

0.0122

4 stage

0.0124

Next, consider a short-stay mission with its much higher values of Av. For a short-stay mission, the Av values given by Folta et al. are summarized in Table 5.25.

With these higher values of Av, staging has a greater effect. For the round trip mission, we find that the mission cannot even be implemented with a single stage. Multiple stages are necessary or the rocket equation "blows up". The results are given in Table 5.26.

Table 5.25. Estimated values of Av for Mars short-stay round trip.

Earth departure 8320

Mars orbit insertion 8900

Mars descent 4000

Mars ascent 4300

Mars departure 9900

Table 5.26. Payload mass that can be sent on round trip to Mars surface vs. level of staging (short stay).

1 stage

2 stage

3 stage

4 stage

Round-trip payload/ initial mass in LEO

Cannot be done 0.0000647 0.0000893 0.0001010

Several things should be noted:

• Staging has a much greater percentage effect in this case. Even the fourth stage produces a significant percentage improvement in payload mass. Folta et al. placed primary emphasis on this percentage gain.

• The absolute values of payload mass are far inferior to the payload masses for a long-stay mission. Even with four stages of propulsion, the payload mass fraction is about 1% of that for a long-stay mission.

• Even allowing for the fact that the short-stay mission requires far less in the way of life support and other resources, and therefore the vehicle mass for the short-stay mission is likely to be lower by perhaps a factor of 2, the short-stay mission nevertheless requires 50 times as many launches as the long-stay mission.

Thus, with or without staging, the short-stay mission is infeasible and unaffordable. Folta et al. reached the opposite conclusion. The best way to describe the paper by Folta et al. is by an analogy.

Suppose an incredibly grossly overweight person weighing 1,200 lb discovered a new innovative diet. By following this innovative diet, he can reduce his weight by 30% to 840 lb. He is very proud that he can produce such a large percentage weight reduction since a mildly overweight person weighing, say, 200 lb, could only expect to reduce his weight by say 10% to 180 lb using the same method. Now, the first person would be pleased with his performance on a percentage basis, and well he should. He could brag to the smaller person about his percentage gains. However, even after completion of his diet, he has a bloated weight. The smaller person, while undergoing a much smaller percentage reduction in weight, has an absolute weight that is much more appropriate.

5.7 TRANSPORTING HYDROGEN TO MARS 5.7.1 Terrestrial vs. space applications

Hydrogen is being widely touted by the U.S. Government as a terrestrial fuel of the future for use in fuel cell powered vehicles.120 Government programs in fuel cell development and hydrogen storage have been implemented, although much of the apparent progress made so far has been in viewgraphs in PowerPoint rather than in hardware. There are a few overlaps between NASA needs and terrestrial transportation needs for hydrogen storage, although the requirements for H2 storage are very different for terrestrial applications than those appropriate for space missions. The requirements for terrestrial applications include:

• Low energy fill. [Note: Liquefying H2 requires about 1/3 of the heating value of H2, making liquid storage less attractive for terrestrial use. For space applications involving a single fill at the launch pad, this is irrelevant.]

• Low-cost mass-produced units.

• Safety issues are different for vehicles than for space.

• Volume may be more important than mass for terrestrial use.

• The DOE seems to be interested in storage systems capable of containing >6% H2 by weight.

In regard to terrestrial hydrogen storage for vehicles, if one starts with room temperature hydrogen at 1 bar, and stores this hydrogen cryogenically in a tank, roughly 30% of the ultimate heating value of the hydrogen stored is required to cool and liquefy the hydrogen. If the hydrogen thus stored is utilized in a fuel cell that is, say, 40% efficient, only 0.7 x 0.4 = 28% of the original heating value of the hydrogen is actually used in the final application. For vehicle use, this would be a very serious negative factor.121 For one-time applications to space, it would not matter.

120 The hydrogen economy concept has been shown to be a hoax by Robert Zubrin. ''The Hydrogen Hoax,'' The New Atlantis, Winter, 2007.

121 A recent report Why a Hydrogen Economy Doesn't Make Sense by Ulf Bossel elaborates on this point. See http://technology.physorg.com/top_news/

Whereas 6% H2 by weight might be acceptable for terrestrial vehicles, it would be prohibitive for any imaginable space missions. In fact, one might as well transport water that contains 11.1% hydrogen. Space applications are driven by mass, volume, and power requirements. Furthermore, the energy required to cool and liquefy hydrogen is very high, and this makes storage as liquid hydrogen unsuitable for terrestrial applications where hydrogen tanks have to be repeatedly refilled. However, for space applications, typically involving a single fill of a tank for transport to a distant point, the energy required to initially liquefy the hydrogen is unimportant. Therefore, storage as a liquid has great advantages for space.

The DOE programs seem to be heavily immersed in go-go language: "Breakthrough", "Grand Challenge", and "Revolutionary". They have ambitious schedules and goals for future accomplishment. However, funding does not seem to be commensurate with the goals, and the use of many small parallel research activities, coupled with frequent turnover in management, casts doubt on whether these goals will be accomplished. The DOE-funded work appears to be heavily centered on solidstate sorbents for H2 storage with a considerable amount of nano-technology (all of which appear to be under-funded and over-viewgraphed). It is a matter of concern that mass is not often mentioned in most publications and news releases, and year-end reports for FY02, 03, and 04 are similar, suggesting that progress has been slow.122 Aside from the matter of technical progress, the whole concept of a hydrogen economy doesn't make much sense when net energy is compared with other alternatives.123

For our purposes, we are primarily interested in the requirements for (a) transfer of hydrogen from LEO to the Mars surface or lunar surface, and/or (b) long-term storage on the Moon or Mars. This hydrogen could be used as a feedstock for ISRU, or directly as a propellant, or for use in fuel cells for power generation.

Hydrogen is a potential propellant for space vehicles that has the advantages of high specific impulse (450 s in chemical propulsion with O2; 900 s in a nuclear thermal rocket). It also has the logistic advantage that it can easily be produced from water, and therefore it is a natural propellant to use based on in situ resource utilization (ISRU) on the Moon or Mars, if accessible indigenous water supplies are available. If indigenous water is not available, transporting hydrogen to the Moon or Mars to be used as a feedstock in ISRU may also provide benefits. In addition, hydrogen is an appropriate fuel to use in fuel cells that would likely be elements of power systems involved in human exploration of the Moon and Mars.

Space applications for hydrogen include:

In present use:

• Use in Earth Launch Vehicles.

• Use as a propellant for chemical (H2/O2) propulsion for Earth departure from

LEO toward the Moon or Mars or wherever.

122 http://wwwl.eere.energy.gov/hydrogenandfuelcells/

123 "Does a Hydrogen Economy Make Sense?'', Ulf Bossel, Proceedings of the IEEE, Vol. 94, No. 10, October 2006.

Planned for mid-term use:

• Use as a propellant for lunar orbit insertion and descent to the surface of the Moon.

• Use as a feedstock for regenerative fuel cells. Probably less likely to be used:

• Use as a propellant for ascent from the Moon (no foreseeable plans to do this— may ultimately depend on outcome of search for lunar polar ice).

• Transport from Earth to Moon for use as a feedstock for regolith-based ISRU.

• Transport from Earth to Mars for use as a feedstock for atmospheric-based ISRU.

• Use as a propellant for a nuclear thermal rocket for Earth departure toward the Moon or Mars (although it appears to be unlikely that the NTR will actually be developed and implemented).

However, the only systems that presently utilize hydrogen as a propellant are Earth Launch Vehicles and Earth departure propulsion systems, although it seems likely that the human exploration initiative will use hydrogen for propulsion as far out as descent to the lunar surface, within a week after launch. The reason that hydrogen is not used more widely in space missions is due to the perceived difficulty in storing and transporting hydrogen, particularly for extended periods of time, as well as problems meeting volumetric constraints. For Launch Vehicle applications, the hydrogen tanks can be topped off in the Launch Vehicle shortly before takeoff and it is burned up in rockets in a comparatively short time, before boil-off becomes a serious problem. For space applications, the key figures of merit for hydrogen storage are:

Fj = Initial mass fraction

= (initial mass of H2)/(initial mass of H2 + mass of storage system)

Fv = Usable mass fraction

= (usable mass of H2)/(initial mass of H2 +mass of storage system)

The usable hydrogen mass is the initial mass of hydrogen, less the boil-off prior to use and any residuals left in the tank at the end of use. In the case of active systems, the mass of the storage system includes the active cryocooler system and its power system.

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