Space Habitats Beyond LEO: A Short Step Towards the Stars

by Paul Gilster on January 8, 2016

Building a space infrastructure is doubtless a prerequisite for interstellar flight. But the questions we need to answer in the near-term are vital. Even to get to Mars, we subject our astronauts to radiation and prolonged weightlessness. For that matter, can humans live in Mars’ light gravity long enough to build sustainable colonies without suffering long-term physical problems? Gregory Matloff has some thoughts on how to get answers, involving the kind of space facility we can build with our current technologies. The author of The Starflight Handbook (Wiley, 1989) and numerous other books including Solar Sails (Copernicus 2008) and Deep Space Probes (Springer, 2005), Greg has played a major role in the development of interstellar propulsion concepts. His latest title is Starlight, Starbright (Curtis, 2015).

by Gregory Matloff

dr_matloff

The recent demonstrations of successful rocket recovery by Blue Origin and SpaceX herald a new era of space exploration and development. We can expect, as rocket stages routinely return for reuse from the fringes of space, that the cost of space travel will fall dramatically.

Some in the astronautics community would like to settle the Moon; others have their eyes set on Mars. Many would rather commit to the construction of solar power satellites, efforts to mine and/or divert Near Earth Asteroids (NEAs), or construct enormous cities in space such as the O’Neill Lagrange Point colonies.

But before we can begin any or all of these endeavors, we need to answer some fundamental questions regarding human life beyond the confines of our home planet. Will humans thrive under lunar or martian gravity? Can children be conceived in extraterrestrial environments? What is the safe threshold for human exposure to high-Z galactic cosmic rays (GCRs)?

To address these issues we might require a dedicated facility in Earth orbit. Such a facility should be in a higher orbit than the International Space Station (ISS) so that frequent reboosting to compensate for atmospheric drag is not required. It should be within the ionosphere so that electrodynamic tethers (ETs) can be used for occasional reboosting without the use of propellant. An orbit should be chosen to optimize partial GCR-shielding by Earth’s physical bulk. Ideally, the orbit selected should provide near-continuous sunlight so that the station’s solar panels are nearly always illuminated and experiments with closed-environment agriculture can be conducted without the inconvenience of the 90 minute day/night cycle of equatorial Low Earth Orbit (LEO). Initial crews of this venture should be trained astronauts. But before humans begin the colonization of the solar system, provision should be made for ordinary mortals to live aboard the station, at least for visits of a few months’ duration.

Another advantage of such a “proto-colony” is proximity to the Earth. Resupply is comparatively easy and not overly expensive in the developing era of booster reuse. In case of medical emergency, return to Earth is possible in a few hours. That’s a lot less than a 3-day return from the Moon or L5 or a ~1-year return from Mars.

A Possible Orbital Location

An interesting orbit for this application has been analyzed in a 2004 Carleton University study conducted in conjunction with planning for the Canadian Aegis satellite project [1]. This is a Sun-synchronous orbit mission with an inclination of 98.19 degrees and a (circular) optimum orbital height of 699 km. At this altitude, atmospheric drag would have a minimal effect during the planned 3-year satellite life. In fact, the orbital lifetime was calculated as 110 years. The mission could still be performed for an orbital height as low as 600 km. The satellite would follow the Earth’s terminator in a “dawn-to-dusk” orbit. In such an orbit, the solar panels of a spacecraft would almost always be illuminated.

For a long-term human-occupied research facility in or near such an orbit, a number of factors must be considered. These include cosmic radiation and space debris. It is also useful to consider upper-atmosphere density variation during the solar cycle.

The Cosmic Ray Environment

From a comprehensive study by Susan McKenna-Lawlor and colleagues of the deep space radiation environment [2], the one-year radiation dose limits for 30, 40, 50, and 60 year old female astronauts are respectively 0.6, 0.7, 0.82, and 0.98 Sv. Dose limits for men are about 0.18 Sv higher than for women. At a 95% confidence level, such exposures are predicted not to increase the risk of exposure-related fatal cancers by more than 3%.

Al Globus and Joe Strout have considered the radiation environment experienced within Earth-orbiting space settlements below the Van Allen radiation belt [3]. This source recommends annual radiation dose limits for the general population and pregnant women respectively at 20 mSv and 6.6 mGy (where “m” stands for milli, “Sv” stands for Sieverts and “Gy” stands for Gray). Conversion of Grays to Sieverts depends upon the type of radiation and the organs exposed. As demonstrated in Table 1 of Ref. 3, serious or fatal health effects begin to affect a developing fetus at about 100 mGy. If pregnant Earth-bound women are exposed to more than the US average 3.1 mSv of background radiation, the rates of spontaneous abortion, major fetal malformations, retardation and genetic disease are estimated respectively at 15%, 2-4%, 4%, and 8-10%. Unfortunately, these figures are not based upon exposure to energetic GCRs [3].

In their Table 5, Globus and Strout present projected habitat-crew radiation levels as functions of orbital inclination and shielding mass density [3]. Crews aboard habitats in high inclination orbits will experience higher dosages than those aboard similar habitats in near equatorial orbits. In a 90-degree inclination orbit, a crew member aboard a habitat shielded by 250 kg/m2 of water will be exposed to about 334 mSv/year. To bring radiation levels in this case below the 20 mSV/year threshold for adults in the general population requires a ~12-fold increase in shielding mass density [3].

But Table 4 of the Globus and Strout preprint demonstrates that, for a 600-km circular equatorial orbit, elimination of all shielding increases radiation dose projections to about 2X that of the habitat equipped with a 250 kg/m2 water shield. If shielding is not included and this scaling can be applied to the high-inclination orbit, expected crew dose rates will be less than 0.8 Sv/year [3]. This is within the annual dose limits for all male astronauts and female astronauts older than about 45 [2].

Early in the operational phase of this high-inclination habitat, astronauts can safely spend about a year aboard. Adults in the general public can safely endure week-long visits. Pregnant women who visit will require garments that provide additional shielding for the fetus. Some of the short-term residents aboard the habitat may be paying “hotel” guests. As discussed below, additional shielding may become available if development of this habitat is a joint private/NASA project.

Is Space Debris an Issue?

According to a 2011 NASA presentation to the United Nations Subcommittee on the Peaceful Uses of Outer Space, space debris is an issue of concern in all orbits below ~2,000 km. About 36% of catalogued debris objects are due to two incidents: the intentional destruction of Fengyun-1C in 2007 and the 2009 accidental collision between Cosmos 2251 and Iridium 33 [4].

The peak orbital height range for space debris density is 700-1,000 km. At the 600-km orbital height of this proposed habitat, the spatial density of known debris objects is about 4X greater than at the ~400 km orbital height of the International Space Station (ISS) [4]. As is the case with the ISS, active collision avoidance will sometimes be necessary.

Atmospheric Drag at 600 km

An on-line version of the Standard Atmosphere has been consulted to evaluate exospheric molecular density at orbital heights [5]. A summary of this tabulation follows:

Atmospheric Density, km/m2 at various solar activity levels

heightLowMeanExtremely High
400 km5.68E-133.89E-125.04E-11
5006.03E-147.30E-131.70E-11
6001.03E-141.56E-136.20E-12

Note that atmospheric density levels at 600 km are in all cases far below the corresponding levels at the ISS ~400 km orbital height. But orbit adjustment will almost certainly be required during periods of peak solar activity.

Since the proposed 600-km orbital height is within the Earth’s ionosphere, there are a number of orbit-adjustment systems that require little or no expenditure of propellant. One such technology is the Electrodynamic Tether [6].

Habitat Properties and Additional Shielding Possibilities

A number of inflatable space habitats have been studied extensively or are under consideration for future space missions. Two that could be applied to construction of a ~600-km proto-colony are NASA’s Transhab and Bigelow Aerospace’s BA330 (also called B330).

Transhab, which was considered by NASA for application with the ISS and might find use as a habitat module for Mars-bound astronauts, would have a launch mass of about 13,000 kg. Its in-space (post-inflation) diameter would be 8.2 m and its length would be 11 m [7]. Treating this module as a perfect cylinder, its surface area would be about 280 m2. Transhab could comfortably accommodate 6 astronauts.

Transhab-cutaway

Image: Cutaway of Transhab Module with Crew members. Credit: NASA.

According to Wikipedia, the BA330 would have a mass of about 20,000 kg. Its length and diameter would be 13.7 m and 6.7 m, respectively. The Bigelow Aerospace website reports that the approximate length of this module would be 9.45 m. It could accommodate 6 astronauts comfortably during its projected 20-year operational life.

Both of these modules are designed for microgravity application. Since the study of the adjustment of humans and other terrestrial life forms to intermediate gravity levels might be one scientific goal of the proposed 600-km habitat, the habitat should consist of two modules arranged in dumbbell configuration connected by a variable-length spar with a hollow, pressurized interior. The rotation rate of the modules around the center could be adjusted to provide various levels of artificial gravity. Visiting spacecraft could dock at the center of the structure. It is possible that the entire disassembled and uninflated structure could be launched by a single Falcon Heavy.

b330split1

Image: The pressurized volume of a 20 ton B330 is 330m3, compared to the 106m3 of the 15 ton ISS Destiny module; offering 210% more habitable space with an increase of only 33% in mass. Credit: Bigelow Aerospace.

One module could support the crew, which would be rotated every 3-6 months. The other module could accommodate visitors and scientific experiments. It is anticipated that visitors would pay for their week-duration experience to help support the project. Experiments would include studies of the effects of GCR and variable gravity on humans, experimental animals and experiments with in-space agriculture. The fact that the selected orbit provides near-constant exposure to sunlight should add a realistic touch to the agriculture studies. These experiments will hopefully lead to the eventual construction of in-space habitats, hotels, deep-space habitats and other facilities.

The possibility exists for cooperation between the developers of this proposed 600-km habitat and the NASA asteroid retrieval mission. Under consideration for the mid-2020’s, this mission would use the Space Launch System to robotically retrieve a ~7-meter diameter boulder and return it to high lunar orbit for further study [8]. The mass of this object in lunar orbit could exceed half a million kilograms. It is conceivable that much of this material could be used to provide GCR-shielding for Earth-orbiting habitats such as one considered here. As well as reducing on-board radiation levels, such an application would provide valuable experience to designers of deep-space habitats such as the O’Neill space colonies.

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References

1. S. Beaudette, “Carleton University Spacecraft Design Project; 2004 Final Design Report, “Satellite Mission Analysis”, FDR-SAT-2004-3.2.A (April 8, 2004).

2. S. McKenna-Lawlor, A. Bhardwaj, F. Ferrari, N. Kuznetsov, A. K. Lal, Y. Li, A. Nagamatsu, R. Nymmik, M. Panasyuk, V. Petrov, G. Reitz, L. Pinsky, M. Shukor, A. K. Singhvi, U. Strube, L. Tomi, and L. Townsend, “Recommendations to Mitigate Against Human Health Risks Due to Energetic Particle Irradiation Beyond Low Earth Orbit/BLEO”, Acta Astronautica, 109, 182-193 (2015).

3. A. Globus and J. Strout, “Orbital Space Settlement Radiation Shielding”, preprint, issued July 2015 available on-line at space.alglobus.net).

4. NASA, “USA Space Debris Environment, Operations, and Policy Updates”, Presentation to the 48th Session of the Scientific and Technical Subcommittee, Committee on the Peaceful Uses of Outer Space (7-9 February 2011).

5. Physical Properties of U.S. Standard Atmosphere, MSISE-90 Model of Earth’s Upper Atmosphere, www.braeunig.us/space/atmos.htm

6. L. Johnson and M. Herrmann, “International Space Station: Electrodynamic Tether Reboost Study, NASA/TM-1998-208538 (July, 1998).

7. “Transhab Concept” spaceflight.nasa.gov/history/station/transhab

8. M. Wall, “The Evolution of NASA’s Ambitious Asteroid Capture Mission”, www.space.com/28963-nasa-asteroid-capture-mission-history

{ 62 comments… read them below or add one }

Alex Tolley January 14, 2016 at 16:43

We should beware using some numbers to “prove” that an approach cannot work. In this case, the speed of the space shuttle launch should not be used as proof that a ranp or gun launch cannot work. Rockets attain high speed very quickly to get above the atmosphere as fast as possible as well as maximize the fuel (stand a rocket on its tail with thrust that counteracts g and it goes nowhere fast).

As the gun launch makes clear, the frictional heating on exit from the barrel can be solved using ablative material. In the case of ramps and winged craft, the launch velocity does not need to be as fast as the shuttle, but can subsequently be achieved using a mix of air breathing engines and rockets, much like the proposed Skylon, which will take off on a long runway.

I an dubious of ramp takeoffs, but they were once popular. In the movie, “When World’s Collide” a mountain ramp was used for takeoff. For British readers. Jerry Anderson’s early puppet series, “Fireball XL5″ used a short ramp with rocket assist.

Air launch, at least for smaller vehicles, seems to be the preferred approach today, with the aim of reducing atmospheric drag rather than providing high launch speed. As always, reducing costs should be the aim so that activities in space can expand.

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Brett Bellmore January 15, 2016 at 8:07

I’m not entirely clear why the XL5 used a ramp on takeoff, since it didn’t seem to need one to return. Nor did the ships seem to have any fuel limitations. In the end it was just a puppet show. Though a very good one!

When Worlds Collide was a much more serious effort, with more thought going into it. By ramp assist standards, I think even the ramp in that movie was rather short.

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J. Jason Wentworth January 15, 2016 at 18:11

Michael: My apologies on the launch mode/location mix-up (I’ve been juggling online time at home with multiple medical appointments, which has required quick action with the former). My 12 year-old computer can’t open your posted video’s link, unfortunately. But scramjets in various forms (as projectiles fired from long tubes, or powering the lower [and even upper] stages of winged space transporters) have great promise, and have been seen as desirable for decades. As with nuclear thermal rockets, scramjets have proved more difficult to perfect than their early advocates expected (they are conceptually very simple, yet dynamically complex, particularly as regards their “internal ballistics”), but they have finally begun to live up to their potential. Being able to “leave the oxidizer outside” greatly reduces the size and mass of space vehicles.

Indeed, the near-Earth “junk yard” of dead satellites and spent upper stages is a potential resource for future orbital construction projects. With appropriate low-thrust, high-efficiency tugs, lunar colonization projects might also benefit from using the metals now whizzing uselessly (and dangerously, for active spacecraft) around the Earth. Ironically, legal and political considerations concerning melting down old satellites might be more troublesome than the technical ones.

Antonio: I’m not an engineer, but I know a few who work in the space program, including an uncle who worked on the Saturn V, Skylab, the Space Shuttle, and the ISS at Boeing. They have given me insight into the difficulties of developing reliable space hardware, and of qualifying it for flight with humans aboard. Apollo looked so easy (relatively speaking; it cost three astronauts’ lives) only because it had virtually unlimited resources. They had plenty of money and talented people to correct the many design errors and technical snags that occurred. The Shuttle and the ISS did not enjoy such largess, and their speed of progress show(ed) it. Absent some overwhelming incentive to send people to Mars, such expeditions–whether public or private–won’t have Apollo-like resources either; this is partly why NASA Mars expeditions are always 30 years in the future (Congress’ refusal to fund Mars expeditions as new project starts makes such “another Apollo situation” very unlikely).

I am abundantly familiar with that report, and with Robert Zubrin’s Mars mission studies. I think ISRU should be pursued, and that it will work. But it is a very long–and expensive–way from building an Earth-bound Sabatier chemical reactor demonstrator to having a flight-qualified unit that can be relied upon to provide a Mars crew with their means to come home. It must land gently and undamaged, and it has to work the first time, because fixing a damaged, leaky, or otherwise balky ISRU unit under Martian surface conditions, in pressure suits, would not be easy, to put it mildly.

You are, in essence, in the position of a Reaction Motors, Inc. engineer in 1946, who–just after having delivered the X-1’s rocket engine–says: “Next, we’re going to build the engines for the Atlas ICBM.” That did occur in due time, but it took over a decade, and it was only after the Atlas was given a top priority (meaning that whatever the program asked for in money and manpower, it got) that it became possible to rapidly perfect its engines. In the IRBM and ICBM program (as with NASA’s Apollo later), the USAF and the contractors had the luxury of trying multiple solutions at once, with very generous budgets, because time was of the essence for military reasons, and Apollo was similarly “well-fed” because of its political and prestige value. With an Apollo-like effort, a Mars-ready ISRU unit could be developed fairly quickly, but unless something extremely interesting is found on Mars (which makes sending people there a compelling goal), a flight-qualified ISRU unit will take many years to develop, because it won’t have the benefit of priority funding and manpower. It will be developed in stages–and likely at some point in subscale form, as a test during a robotic Mars sample return mission. But even that unit won’t be human-rated, which will take still more years and money. I think it will come, but it isn’t just around the corner.

The astronaut food, water, oxygen, and shielding problems are inter-related, because they all involve mass which must be accelerated and decelerated. The INSPIRE Mars project, which would have sent two people on a Mars flyby mission later in this decade, has stalled partly due to concerns about radiation exposure during the mission, particularly from solar flares. The mass required for shielding takes away from the mass alotted to food, water, and air. A larger launch vehicle could be used, but that requires still more money. I mentioned the Bellcomm Venus (and Mars) flyby mission studies to illustrate the limitations of the Saturn V, including the long trip times (you had suggested that the Saturn V could send Mars orbit, and perhaps landing, missions).

Aerocapture (into Mars *orbit*, as opposed to a Mars landing) has not yet been tried. It will require more precise guidance because it will be a briefer and likely shallower brush with the Martian atmosphere, with unattractive failure modes (a skip-out into solar orbit, achieving a much higher [or lower] than intended orbit around Mars, and a burn-up in the Martian atmosphere). Reaching a too-high (or too-low) orbit could require more propellant to correct it than the mission could spare. I wouldn’t want to see a human crew be the first ones to try an aerocapture arrival at Mars (the variable density of the uppermost Martian atmosphere with varying solar activity will also be a complicating factor for aerocapture). Like ISRU, I think aerocapture will work, but it will require a lot of testing before it is well-understood and can be relied upon to bring a human crew safely into their intended initial Mars orbit.

I wish you had specified what “The rest of your comment doesn’t make much sense” was, but I’ll take a shot at it (based on an unavoidable guess at what you were referring to). You are very confident in human crews’ abilities to adapt to both free fall and fractional-Earth gravity conditions. So am I. But in the real world of human spaceflight, no flight surgeon will certify that every astronaut is capable of spending the durations of time in free fall and 3/8 Earth gravity that a Mars mission will require, including for those mission scenarios that involve using centrifugal artificial gravity, even at 1 Earth gravity strength. (Even in a 1 Earth gravity rotating spaceship, it is not known if the Coriolis Force might cause problems for people’s balance mechanisms, or–if it does–what radii of rotation and rotation speeds are long and slow enough, respectively, to avoid nausea. This is why near-Earth rotating stations are necessary)

Not until many more people have lived in free fall for long periods (and in rotating stations, which has yet to be done at all) will space medicine specialists be able to select Mars crews with any degree of certainty that they will all remain healthy throughout the trip. I want humans to go to Mars–and to the stars–but only in science fiction do they just “wing it,” technically and medically–in real-world space travel, all risks are carefully evaluated and are then retired through rigorous testing before each new step is taken. The farther we send people from Earth, the more careful we must be, because the consequences of errors and incorrect assumptions are worse.

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J. Jason Wentworth January 15, 2016 at 18:30

Alex Tolley and Brett Bellmore: My interest in launch ramps is more in the way of one option that shouldn’t be discounted, because for true spacefaring there will likely be many viable ways to launch payloads. Depending on the payload size and purpose (and the desired orbit), different launch and landing methods will have their favorable niches.

Mountain ramps have the disadvantage of fixed launch azimuths and high initial costs, but they are reusable like rocket sled tracks, and they offer reuse of the lowest captive stage (or “drop tank sled”). But for some payload mass range and type(s), they may be competitive. Dan DeLong’s “almost-SSTO” air-launched spaceplanes, whose wings assist their ascent trajectories, are attractive for smaller (a few tons) payloads, and they can return payloads as well. For larger payloads (including ones requiring safe return), ground-takeoff TSTO or SSTO reusable ballistic ships may be best. No option should be left untried, if analysis suggests it could work; that is my only admonition.

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Alex Tolley January 15, 2016 at 21:33

I’m currently reading G. Harry Stine’s “Halfway to Anywhere: Achieving America’s Destiny in Space”. It is about the development of the SSTO DC-X.
Now if SSTO is doable, and the claim is that it is, then assisted launches are really irrelevant, as a unitary vehicle can place a payload in orbit and return for relaunch, preferably with a minimum of turnaround time. This seems more cost effective and flexible than building space launch infrastructure.

SSTO reusable ballistic ships may be best

I would agree with that. What I would like to know is what does SpaceX know that directed them to continue with multi-stage rockets (“ammunition” as Stine disparagingly calls them), although it happily appears they will be able to refill the cartridges.

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Antonio January 16, 2016 at 4:33

J. Jason Wentworth: Since you didn’t try to rebut any particular point I made but only made comments of the type of “it’s too difficult” or “you don’t know enough”, or worse, you insist in denying the calculations in saying that there is a need of huge amounts of food or radiation shielding, I will not reply to your post.

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Michael Million January 16, 2016 at 5:17

@J. Jason Wentworth January 15, 2016 at 18:11

‘Michael: My apologies on the launch mode/location mix-up (I’ve been juggling online time at home with multiple medical appointments, which has required quick action with the former). My 12 year-old computer can’t open your posted video’s link, unfortunately.’

Trying opening it in google chrome.

‘But scramjets in various forms (as projectiles fired from long tubes, or powering the lower [and even upper] stages of winged space transporters) have great promise, and have been seen as desirable for decades.’

They have got the device working but the wings keep falling off, if we had no wings the engine would run just fine.

Just thinking if we got a complete rocket up into space we could refuel it from the water on the moon as it cost less to get water from the moon than from the surface of the earth and roam the solar system. Although infrastructure is needed to collect the ice from the moon in the long term it will be worth it.

Also if we used the inflatable design concept and then connected it up to a tank section from a rocket it would allow a huge space to be used. The tank section could be rotated slowly about its axis giving a small G just enough for the growing of plants for food, building materials, oxygen and play areas. The tanks are already robust enough to contain pressure and the tank could be designed with a rotation enabled hub connection already in place, just vent or collect the unused fuel and then dock and spin it up.

These inflatable habitats may also find use in the atmosphere of Venus in the goldilocks layer, they don’t need to be as robust as the space ones due to the pressure and temperature advantage.

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Brett Bellmore January 16, 2016 at 11:05

Ramps, like many other reusable launch systems, (Mass drivers, rotating skyhooks, LEO ‘landing strips’…) make more sense with higher traffic. Right now we’re entering a virtuous cycle where cheaper launches mean more traffic, which means that cheaper launch systems become economically feasible, which lowers prices, which increases traffic…

Even SpaceX’s reusable booster wouldn’t make sense at lower traffic levels.

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Michael Million January 16, 2016 at 16:01

Just thinking a bit more about the inflatable habitat concept, perhaps we could have one in between the oxygen and hydrogen tanks (the interspace) and have each tank with a hatch connected to it. When the whole rocket is in orbit the tanks are vented and the habitat inflated the hatches are then opened. We would then have the volume of the oxygen and hydrogen tanks available as well, each could have a different gravity setting on rotation. The nose cone fairing could have a re-entry insulation configuration which would then allow the rocket motors and turbines to return to earth if needed for reuse.

Any good?

Has anyone else looked at this website, lots of info on mans expansion into Space?

http://www.permanent.com/index.html

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Brett Bellmore January 16, 2016 at 16:49

Over at NBF, the question has arisen of what is meant by saying the orbit is “Sun-synchronous”. The altitude doesn’t appear to allow one orbit per year.

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Brett Bellmore January 17, 2016 at 7:59

Google is your friend, I looked it up. But these Sun synchronous orbits are highly inclined; Doesn’t that greatly increase delta V to reach them?

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Michael January 17, 2016 at 11:29

Interesting bit this,

‘Saving the tank would also allow more of the tank’s leftover fuel to be used (by a slow burn at lower tank pressure). An engineering study by the tank’s manufacturer, Martin Marietta, shows that the Shuttle can take an extra ton of cargo to orbit if the tank is saved.’

http://www.permanent.com/recycling-fuel-tanks.html

So if we were smarter we could have had around 130 odd shuttle (ET) tanks in orbit for delivering 130 odd extra tonnes in orbit or thereabouts. However getting rid of the shuttle earlier on would have saved a lot more wastage. I hope in future designs they use the tanks more effectively, it is one hell of waste in my book.

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