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NASA Space Science

Using Fuel Depots Instead of Giant Rockets 202

EccentricAnomaly writes "The New York Times has a story about a leaked NASA study that showed it would cost $80 BIllion less and get astronauts to an asteroid sooner if NASA used fuel depots instead of developing a new rocket. According to the article, NASA's response to the leaked study is to start developing fuel depots in addition to continuing its new rocket program. Because, after all, who doesn't need more cool stuff."
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Using Fuel Depots Instead of Giant Rockets

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  • I doubt that any current rocket could make use of an on orbit depot, because 90% of all current rockets are discarded before orbit is achieved, not to mention that none of them are designed to be refueled after use (most rockets are ignited externally for example). It might just be that a new rocket is needed as well...

    • Re:Uhm... (Score:5, Informative)

      by Temkin ( 112574 ) on Sunday October 23, 2011 @11:19AM (#37809712)

      The Apollo J-2 was designed to restarted way back in 1967, as was the Aerojet AJ-10 from the late 1950's.

      AJ-10 variants were used for both the Apollo SM engine, and the Shuttle OMS pods. They were designed to remain fueled for long periods of time and be re-ignightable. This is a solvable problem.

      • And that's my point - how many of those are integrated into the currently active generation of rockets? Just because it's a "solved" problem doesn't mean it's still available on current systems.

        • What? (Score:5, Informative)

          by FatLittleMonkey ( 1341387 ) on Sunday October 23, 2011 @01:48PM (#37810630)

          Did you read the report? It was comparing the cost of SLS launched missions to the moon or an asteroid, with depot enabled versions of missions to the moon or an asteroid. They weren't trying to argue that every rocket in the world is refuellable, nor even most, they were saying that launching a LTO transfer stage empty, then fuelling it in orbit, is cheaper to develop and fly than building a Really Big Rocket.

          You can launch a 100 ton lunar transfer stage on SLS, say, with a 25 ton dry weight and 75 tons of fuel.

          Or, you can launch the 25 ton stage empty on a Falcon Heavy or a Delta IV Heavy, plus three fuel missions on similar rockets, and it will cost billions of dollars less. (Their scenario is more detailed, obviously.)

          SLS is an expensive and harmful way to do these missions. It actually makes us less likely to go beyond Earth orbit, and wastes two to three decades and many tens of billions of dollars doing so.

    • Uhhhmmmm - didn't they shut down the engines, and restart them, on the shuttles? And, didn't they refuel those shuttles, and fling them back into space on new missions? DUHHH! I really can't remember hearing about the shuttle pilots going outside, so that they could strike a match, to start up the rocket engines, either.

      • No, the main Shuttle engines were never shut down and restarted, once they were shut down that was it. Take a look at a shuttle launch video sometime - the main engines were started by sparks generated by systems on the ground, not onboard systems. The OMS engines were restarted, but they use a different type of fuel to that used for the launch.

        Also, the Shuttles were never refueled in orbit. Infact, it's pretty hard to suggest they were ever refueled, because the fuel tanks were discarded and burned up.

        • I specifically said "shuttle engines", as opposed to booster engines, used to get the shuttle into high atmosphere/low orbit. Shuttle engines. Ignited, and extinquished, repeatedly, I believe. Let me find some kind of a reference, alright?

          The first thing I find, using the search terms that I chose to use, refers to the Apollo flights, where engines were repeatedly ignited and extinquished. Let's remember - the fuel has to be pumped to the engines. Extinguishing an engine is at least somewhat like shutt

          • "Re-entry and landing
            The vehicle began re-entry by firing the Orbital maneuvering system engines, while flying upside down, backside first, in the opposite direction to orbital motion for approximately three minutes, which reduced the shuttle's velocity by about 200 mph (322 km/h). The resultant slowing of the Shuttle lowered its orbital perigee down into the upper atmosphere. This OMS firing was done roughly halfway around the globe from the landing site."

            From "Space Shuttle" on [].

            Once shut down, about 12 mi

            • While that seems to be true, my original claim was that rockets can be extinguished then reignited. Those OMS rockets are an example, and the moon missions are another example. It might seem that SOME rockets aren't designed to be shut down, then used again, but it's very possible to shut off fuel flow to other rockets, then ignite them again, at a later time.

              Those OMS thrusters seem to have been used during the shuttle's ascent, then later to maneuver for docking and/or to change orbits, then later again

              • OMS engines were just variants of the standard hydrazine rockets, similar to the ones used on any satellite. (Although much more complex for human-rating.) Hobby rockets. You can turn them on and off all day.

                The SSME's are big LH/LOx rockets. Massively complex systems, and shut down is brutal.

      • by IrquiM ( 471313 )
        of course not - they were hand cranked
    • You're not trying to refuel the launch vehicle, you're trying to refuel the payload. It could be an injection stage for an interplanetary probe, the stationkeeping propellant for a satellite, or the fuel required to take an Apollo-style capsule to the moon.

      So you'll need re-designed payloads, but not redesigned launch vehicles. The idea is that this reduces the need for a $50B monster like SLS by allowing larger missions with our current stable of launchers.

  • Let's have both. (Score:5, Insightful)

    by Sasayaki ( 1096761 ) on Sunday October 23, 2011 @10:42AM (#37809522)

    We could easily have both. Easily. Let me show you how:

    NASA's budget: $18.724 billion (Fiscal Year 2011) (source: [])

    Department of Defense's budget: $663.8

    Which does NOT including Iraq and Afghanistan, which together are approximately $900 billion, and does NOT including the care for the approximately 33,000 wounded veterans those wars have produced... which is probably a few billion, but I couldn't find an easy source so let's just go with nothing. But remember it's there.

    Adding those into DoD's budget gives: $1,563.8 billion. (source: [])

    That means that DOD gets 83 times as much as NASA gets. They could reduce their budget by 1/83rd and double Nasa's budget.

    A country needs defense. I get it. But seriously -- NASA is one of those organizations that, if your pour money into it, does AMAZING things. Things that give so much back to the scientific community -- things like computers, insulation, search and rescue, navigation, everything. So, so, so, so, so, so, so, so many technologies can be traced back to the space program... and while DoD are great inventors too, especially in medical treatment, materials, transportation... NASA gives so much back too and no brown people have to die.

    Can't we just have a couple less B2 Stealth Bombers (B-52's bomb brown people just fine) and a couple less F-22's (F-15 Eagles still have never been defeated in combat) and GET THE FUCK TO MARS?

    • by hedwards ( 940851 ) on Sunday October 23, 2011 @10:46AM (#37809546)

      Yes, but they also study climate change which automatically negates all the good stuff they do to the smaller government lobby.

    • Can't we just have a couple less B2 Stealth Bombers (B-52's bomb brown people just fine) and a couple less F-22's (F-15 Eagles still have never been defeated in combat) and GET THE FUCK TO MARS?

      The B-2 is no longer produced, so nothing but sunk cost and amortised operating costs there, and the F-22s production is finishing very soon.

      • The B-2 is no longer produced, so nothing but sunk cost and amortised operating costs there, and the F-22s production is finishing very soon.

        I think you're - deliberately? - missing the point. The grandparent poster wasn't suggesting the cancellation of those specific programs (I'm giving him the benefit of the doubt here); but rather that the scale and cost of military procurement and R&D tends to be orders of magnitude beyond what is permitted to the space program, and that proportionately small cuts to DOD programs would permit proportionately enormous relative increases in NASA funding.

        If you prefer, you can substitute "F-35" for "F-

    • Oh, so you actually think THEY want to go to mars? Or any such thing.

      The problem is that the people that tend to rise to power are megalomaniacs and control freaks. All they care about is maintaining power and exerting their ego on others. The have no vision beyond that. And worse still, most of them are just as happy to murder people in unjust wars as to do something good for humanity. And the murder thing pays better $, so there you are.

    • Re: (Score:3, Interesting)

      by bmajik ( 96670 )

      Here here!

      The ENTIRE APOLLO PROGRAM cost 160 billion in ADJUSTED 2005 Dollars!!

      That's all missions over the entire program, all support technologies that had to be invented for it, -- everything, the whole enchilada.

      So, _every year_ we spend about 6 _APOLLO PROGRAMS_ blowing up people that don't even matter to us. We borrow 9 APOLLO PROGRAMS every _year_.

      I recommend 1 "Apollo Program" as the new unit of measure of government stupidity. All things the federal government does should be measured in terms of

      • Re: (Score:2, Insightful)

        by DanDD ( 1857066 )

        > So, _every year_ we spend about 6 _APOLLO PROGRAMS_ blowing up people that don't even matter to us. We borrow 9 APOLLO PROGRAMS every _year_.

        Ahem. Please keep this kind of generalization to yourself. All humans matter to me, especially those that need blowing up. However, I do appreciate your sentiment that our priorities are severely skewed.

    • Re:Let's have both. (Score:5, Informative)

      by CrimsonAvenger ( 580665 ) on Sunday October 23, 2011 @11:16AM (#37809692)

      Adding those into DoD's budget gives: $1,563.8 billion.

      Umm, no.

      You added eight years worth of supplemental and emergency appropriations ($900 B) onto last year's military budget.

      The same article showed the correct amount to add for this year: $37 billion.

      Which would make the correct value $700.3 billion, less than half what you asserted.

      • by jfengel ( 409917 )

        Still, a goodly sum of money. And a lot of people are prepared to see it reduced.

        The question is where you direct the reduction. There's a lot of demand to see it put into debt reduction; that is, reducing DoD spending AND not increasing NASA's spending, so that the deficit isn't quite so big.

        There are good arguments to be made for either. I'll let you know if I ever hear anybody engaging in one, rather than spouting ideological and partisan talking points.

      • You are correct, and I stand corrected.

        It's still what I call a metric fucktonne of money, when the entire budget for NASA is a rounding error for the DoD.

      • Mod Parent up. Thanks.

        I'm still in favor of using some of that $700.3 billion to double NASA's Budget.

    • by medcalf ( 68293 ) on Sunday October 23, 2011 @11:17AM (#37809698) Homepage
      I'm all about getting to Mars. Heck, offer me a one way ticket and I'm off. But here's the deal: NASA is not going to get us there. Today's NASA is not the entrepreneurial NASA of the 1960s or even the 1970s. This is an Iron Law bureaucracy whose job is to keep working, which they do by spreading money across a lot of important Senators' districts. Note the important fact left out of the summary: this finding of getting there cheaper with fuel depots was buried by NASA for months because they didn't want to interfere with the SLS funding, which like Constellation before it is almost certain to never, ever fly. Consider that the last successful NASA development program for rockets was run in the 1970s, with the Shuttle. (And that was only successful if "success" means "getting people into space" as opposed to meeting cost or capability targets.) The only new rockets since then have been commercial, and NASA is in a love/hate relationship with those.
    • Space is no longer about access. It is about economics. The SLS is based on a design that was the best that we could nearly 50 years ago. Now, we can do much better.

      Donate here []. Convince your friends to donate also.

      If they can't do it on donations, not enough people actually care about doing it.

    • by tiqui ( 1024021 )

      Many misconceptions here

      First, we stopped buying B-2 bombers and F-22 fighters long ago

      Second, Bush43 and Obama have both dramatically under-funded a number of vital weapons systems, like ships and submarines, that wear-out over time and must be periodically replaced. The money that should have been used to keep stuff current over the past decade was largely diverted into the wars and to various various emergency/unforeseen systems like the move from Hummers to MRAPS, the move from recon drones to armed dro

  • by Graff ( 532189 ) on Sunday October 23, 2011 @10:46AM (#37809536)

    This is an old idea that should have been implemented long ago. Fuel tanks can survive much higher g-forces and can be built and launched relatively cheaply compared to satellites and personnel.

    In fact, fuel is just about the perfect candidate for a mass driver [] where energy can be stored up and then released in a burst into a linear induction motor or similar technology. This means that much less expensive and less polluting energy sources can be used in the launch as opposed to most rocket fuels. It's also inherently safer since you don't have a 5000 degree F flame that you need to feed and control.

    Once the fuel tanks are exhausted they can be converted into modules for space stations or spacecraft, probably much more efficient than building them to survive a re-entry to get re-used. Why waste all the energy it took to get them up there and the energy it would take to send up a pre-built module when you can design the tanks for re-use?

    Yeah, there's a lot of complexity that I'm doing some hand-waving around but it's still a great concept that should be developed further.

    • by Graff ( 532189 )

      In fact, fuel is just about the perfect candidate for a mass driver where energy can be stored up and then released in a burst into a linear induction motor or similar technology.

      To add to this, the energy doesn't have to be a massive amount used up in a quick burst. You can perform some of the acceleration over a period of time on a circular racetrack and then launch it once it has a good deal of its final energy. Obviously this will require a good deal of engineering to get right but there's already been a lot of work done on this topic.

    • We need chemicals for a short time. BUT, for moving around in space, Chemicals do not provide much promise long term. Instead, moving to using solar to move cargo around LEO makes good sense. For humans, we will use chemicals to move humans, but far safer, faster and cheaper to use nukes in the moderate haul on. And as to launch, today, we use rockets, but that is going to change. Scaled Composites launch of cargo will happen in about 3 years or less. It will be a fraction of the costs of SpaceX. Likewise,
      • Likewise, it is certain that Skylon will work.

        Uh, no. It's not certain at all. They've been working on it for a long time. No plane just yet. They've made some progress but it is a long, long way from 'working'.

        You have a funny idea about engineering. You seem to think if they build it, it will work. The current NASA missions are based on tech from the 1940's and 50's. It's taken them that long to be sure they work and to get the kinks out. Yes, the newer commercial companies can work off the shoulders of giants, but that does not imply or guar

        • The engineering on Skylon has been since the 50's as well. In fact, most of the tech for it was developed not by UK, but by USAF, and later NASA, though HOTAIL. Even reagan's NASP was an attempt at this, HOWEVER, it was quickly shown that the failure was material. We have exceeded past that back in the 90s. Sabre does not have a materials issue. What they have is mild engineering, mild testing, and mostly economics. Well, the engineering check was already done. They are building the test engine now.
    • This is an old idea that should have been implemented long ago. Fuel tanks can survive much higher g-forces and can be built and launched relatively cheaply compared to satellites

      Well, no. A booster doesn't care what's on top of it - it costs just the same regardless. Nor can the depot itself be built in any manner that can be construed as 'cheap'. It needs a GNC system, and an RCS system, and a power system, and insulation (and active cooling for cryogenics), and a docking system, and a propellant trans

    • by DanielRavenNest ( 107550 ) on Sunday October 23, 2011 @01:05PM (#37810324)

      Fuel is a good candidate to launch with a high-g device, but a mass driver is not the most economical way to get it off the Earth. It is fairly easy to show that a pipe will cost less per foot than induction coils and a frigging huge power supply to feed it, for the same job of accelerating a projectile. Generally, these type of devices are called "hypervelocity guns", defined as when the muzzle velocity is hypersonic (ie more than Mach 5 or 1500 m/s). This is roughly twice the muzzle velocity of large military guns.

      In 1993 I was the study manager at Boeing for using a large gun to deliver fuel to a depot, which then was used to send communications satellites to GEO. The savings was you needed 75% less conventional rocket to launch the satellite dry. Hypervelocity guns are not new, they have been used for ballistic and re-entry testing for about 40 years now. NASA owns several of them. Mainly they need scaling up and "industrializing" - setting them up for regular operations, rather than research use.

      To reach the highest muzzle velocity, you want to use the lightest gas (Hydrogen), and heat it, so the speed of sound is as high as possible. Speed of sound is the same as speed of pressure waves in the gas, and when your projectile exceeds that speed, there is no way for the gas at the back end to affect the projectile any more, because it outruns the pressure waves. So the gun gets very inefficient at that point. To make hot hydrogen, it is easiest to store it at room temperature in pressure tanks, then run it through a heat exchanger before it gets to the barrel. There is nothing that goes "boom" like a small gun, it's closer to natural gas pipeline operations (in fact, we sourced the gun barrel from a pipeline maker in the study). Find a suitable mountain, such as Cayembe in Ecuador (the highest point on the equator, and the right slope), and put a 2 km long x 60 cm I.D. pipe pointing up. Load a 600 kg projectile about 4 meters long into it, and it will accelerate at 900 g's, and come out with a muzzle velocity of around 5600 m/s. You lose around 1 km/s of that to air drag, and then use an onboard rocket to finish getting to orbit. Net payload to orbit is around 100 kg, which does not sound like much, but if your launcher is at the equator, you can potentially launch 15 times a day to a single depot destination. Over the course of a year that comes to 550 tons (minus downtime for maintenance).

      For launching people and delicate cargo, Hawaii is the best location. Assume a 20 km pipe x 10 m diameter, pushing a 500 ton vehicle. It works out the pressure in the barrel needs to be 2 atmospheres (200kPa, 30 psi). That gives you 3 g's acceleration, safe for humans and satellite parts. Muzzle velocity is 1100 m/s (Mach 3.6), which is not a huge fraction of orbit velocity, but a nice running start before you light up your on-board rocket. Given those starting conditions, a reuseable non-cryogenic rocket should have a payload of around 35 tons, which along with a 10 meter diameter should be plenty for any cargo or people you want to launch. This is the upper end of what you might want to build, for your first low-g cargo launcher you can go a lot smaller.

  • I don't get it. If you don't have a rocket to get astronauts to the fuel depot, what good is the depot? Can someone explain this to me?
    • by Hentes ( 2461350 )

      In increments. The idea is, it's cheaper to get the fuel up using lots of small rockets currently available than trying to build a giant one on Earth.

    • You can launch fuel through a mass driver. Much safer and cheaper than a rocket. You could probably do same with food and other supplies, although depending on the cargo, medical might need a regular launch. Stick a couple guys on the fuel depot, fire up some food and air packs with the fuel, and replace them with a new crew every six months or so.

      • 'Cept that we don't HAVE a mass driver yet. Let's build one (even a teeny little one) before we start using it as a mission critical bit of infrastructure.

    • Private space has MANY rockets. They can take up fuel. Shortly, they will be taking up humans as well. Of course, a number of ppl in CONgress (hatch, hutchinson, shelby, coffman, wolf, etc) have worked hard to kill private space. In spite of those idiots, private space should win out.
    • Its not a replacement for all rockets. Its a replacement for giant rockets that are required to get huge payloads with lots of fuel to space.

      The Apollo capsule could have launched easily on many of our current launch vehicles (it was tested on the Saturn I). It was the fuel required to inject towards the moon that required the huge Saturn V.

    • If you want a manned mission the whole spacecraft has to be built to a higher standard of safety. That makes it heavier and more expensive. If you can get the fuel that would be needed for most of the mission into orbit (that's the hardest part) in a cheaper, lighter, unmanned rocket that's a more efficient use of resources. Once the fuel is up, providing it stays up and doesn't leak away - a hard task with liquid H2 - then you only need the expensive craft to be capable of lifting itself to meet the fuel d
    • by FatLittleMonkey ( 1341387 ) on Sunday October 23, 2011 @03:12PM (#37811310)

      As others have said, you use smaller rockets to launch the mission payload "dry", and to launch the fuel separately. But to explain the cost savings, let me use an example:

      The SLS is projected to cost over $60 billion to develop and around $1.5 billion per launch. The biggest version is supposed to launch 130 tons, but the first version will only launch 70 tons. (And remember, $60 billion is only for the rocket, it doesn't include the cost of the actual mission hardware.)

      SpaceX's Falcon Heavy will probably cost less that $0.5 billion to develop, and is already taking commercial orders at something like $0.125 billion per launch. It is intended to launch over 50 tons.

      For the price one 130 ton SLS launch, you could pay the entire development costs for Falcon Heavy and still have enough to buy 8 launches of 50 tons, or 400 tons total. And once Falcon Heavy is developed, each subsequent $1.5 billion could buy 12 FH launches, or 600 tons.

      600 tons for the same price as 130 tons.

      So instead of spending $60 billion to develop SLS, it could be spent on actual missions. Isn't that a more intelligent way to run a space program?

  • We are back to the discussion about if we should invest in a moon base or a satellite that sits between earth and the moon. IMHO a satellite would be a better investment.

    • by NEDHead ( 1651195 ) on Sunday October 23, 2011 @11:06AM (#37809636)

      What no one is saying is the real reason for the space efforts. As all slashdotters know, the tides are an important part of our ecosystem, and yet every day the moon moves further away, reducing the tides before our very eyes. The only practical solution is to put a stop to this disaster, by tying the moon to the earth with a stout cable. This will also provide an anchor at each end for the space elevator ( how all those nutters think a space elevator anchored at just one end is going to work is beyond me).

  • Picture one of these coming down to Earth, crashing onto a playground at the largest kindergarten in the world, deep frying ten thousand little tykes in a hellacious ball of fire. Sure, it will never happen -- cause it will never be built. Actually, it WILL be built, right after the government fixes the economy.
    • by medcalf ( 68293 )
      Oh, please! First, it would be in active use, so it would be boosted as the orbit decays (by the rockets it's refueling, most likely). Second, fuel depots wouldn't be reentry shielded and fuel is highly flammable, by definition. So if one were allowed to deorbit, all that would hit the ground is a bit of metal, and not much of that. Satellites fall all the time with little risk on the ground.
  • Say did Arthur C Clarke patent the idea of using Europa as a fuel depot?

    (I don't really think we have to worry about large black monoliths telling us not to go there

  • by WindBourne ( 631190 ) on Sunday October 23, 2011 @11:07AM (#37809642) Journal
    CONgress, namely Shelby(R), Wolfe(R), Hatch(R), Hutchinson(R), Coffman(R), Nelson(D), and many others, are pushing this nightmare. The only reason is not because they believe that it is needed for the space program, but these slimes have turned NASA into a jobs bill.

    The good news is that by 2014, SLS will be dead. The fact is, that once SpaceX launches FH, no president will support. It does not matter whom is in office. The project will be dead. Hopefully, we will then hold a COTS for 2 SHLV. At the same time, HOPEFULLY, these above slimes or whomever replaces them, will continue funding for NASA to support nuclear engines (not for launch on earth, but interplanetary travel and perhaps launch on the moon and mars), private space, ISS partners exploration of the moon, and NASA continuing their push for BEO.
  • by the gnat ( 153162 ) on Sunday October 23, 2011 @11:12AM (#37809672)

    According to the article, NASA's response to the leaked study is to start developing fuel depots in addition to continuing its new rocket program. Because, after all, who doesn't need more cool stuff.

    Undoubtedly NASA reacted this way because the US Congress has legislatively mandated development of a new heavy-lift rocket to preserve jobs in states with influential (Republican) senators, as a substitute for the cancelled Constellation program. It no longer matters what NASA itself thinks is the most efficient and technologically feasible solution. Even if the fuel depot plan would save twice as much, in practice it is ultimately subject to Congressional veto.

  • Is leaked the new catchy term?

    I do

  • I love this plan. (Score:4, Insightful)

    by trout007 ( 975317 ) on Sunday October 23, 2011 @01:25PM (#37810452)

    Here is the beauty. You never get really good at something unless you do it often. You need lots of practice. But currently we are stuck in a catch 22 with rockets. The payloads are very expensive so you want to make sure your rocket doesn't fail. This requires lots of money typically to verify everything.

    Fuel delpts turn everything upside down. Now you are launching something cheap, fuel. You can make the rockets as cheap as possible and even with a few failures it's no big deal since you are only out cheap fuel. But even with cheap rockets if you launch them often enough you will get very good at it and reliability will increase as you identify problems.

    This is great for NASA too since the majority of beyond earth orbit mass is propellant. It can launch they payloads dry and do that on much smaller and cheaper rockets. Also it can just pay for fuel delivered to the depot. Any failures are on the customers dime.

  • Prior art (Score:5, Informative)

    by savuporo ( 658486 ) on Sunday October 23, 2011 @02:11PM (#37810834)
    This does not come up in a lot of these conversations, but we have a "fuel depot" in orbit right now. It uses storable propellants, not cryogenic ones, but nobody says you cannot leave LEO with nitrogen tetroxide and unsymmetrical dimethylhydrazine (UDMH). In fact, that propellant combination was exactly the one being used to land on the moon in Apollo program.

    The fuel depot on orbit of course is ISS Zarya or better known as FGB. It gets fuelled up by Proton's on a regular basis, and ISS uses the propellant for station keeping. Considering the mass of ISS, boosting its orbit is no small feat.

    Russians also have a spare module, used to be called FGB-2 sitting somewhere in the hangar. It was proposed as various additions to ISS at some point.

    In summary, storable hypergolic orbital propellant transfer is a well known, well developed and currently used technology. Yes you need quite a bit more of it to do burns with delta-v in order of km/s, but the maturity of the solution and abundance of off the shelf engines and propulsion module designs using hypergols may well outweigh cryogenics in overall system designs.
    Propellant is also relatively cheap, even nasty stuff like hydrazine, and just lifting more of it would provide the much needed demand side for the globally stagnant launch industry, which has been in the oversupply mode for years, i.e. there are far more operational rockets than there are paid payloads.

    The point that "propellant depots" is nothing new, and in fact NASA's current flagship HSF program uses it needs to be made more often. Switching to cryogenics would be a new development even if not that complicated, and may or may not be worth it, depending on overall mission requirements and other elements of system architecture.
  • by Stele ( 9443 )

    Why build one when you can have two at twice the price?

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