Exactly what I said... :)

Someone recently took potshots at them because "everything they've done has been based on NASA technology". To which I replied, "Well, at least SOMEONE is using NASA technology to move the ball forward-- WHY DOESN'T NASA MOVE FORWARD USING NASA TECHNOLOGY???

The comment was directed at the fact that a lot of the initial technology for the Merlin rocket engine was purchased from NASA. SpaceX is working on their FOURTH version of that motor now, already doing the hard work of going to regenerative cooling...

Bigelow bought NASA's technology related to inflatable space structures and is using that technology for the basis of their space-hotels and will probably end up selling inflatable space station modules to NASA for future space stations or even Lunar Orbit Stations or Lagrange point stations... even deep space habs on long-term missions, which could really use their low mass (compared with rigid structures). Again, MORE NASA technology NASA was doing nothing with!

Later! OL JR :)

I found the following line on this SpaceX page (see: http://www.spacex.com/falcon_heavy.php ) interesting:

"Despite being designed to higher structural margins than other rockets, the Falcon Heavy side booster stages have a mass ratio (full vs. empty) above 30, better than any launcher in history. By comparison, the Delta IV side boosters have a mass ratio of about 10."

A mass ratio of 30:1 is in "SSTO territory" (the Titan II first stage is capable of SSTO [Single-Stage-To-Orbit], although not with very much payload). With the new Merlin-1D engine, an ablative EEC (Extendable Exit Cone) deployable nozzle skirt, and the lightweight tanks, an expendable SSTO launch vehicle appears feasible. If it used the old thin-walled, pressure-stabilized Atlas-type "balloon tanks," an expendable SSTO SLV should be a practical (and useful) proposition.

The big difference between NASA's $1000 per pound claims is that NASA's came before a shuttle was built and flown, and there had never been anything even resembling the technology before it. The Falcon 9 has already been built and flown, so they know exactly what goes into it down to the last nut, bolt and man hour. Considering the FH is roughly three F9's and some extra hardware to make them all play together, they should have a real good estimate on its cost.

Exactly right... and if you read the stuff from 'back in the day' on the shuttle and how it came to be what it is and why it 'went wrong' (there are a number of EXCELLENT books on the subject) one of the main reasons was that NASA used COMPLETELY UNREALISTIC numbers in their studies to justify building a shuttle... it's like a bad case of accountants "go fever"... the prevailing attitudes seemed to be "we want it, now go pull numbers out of yer butt to justify it". So they did.... "figures don't lie, but LIARS FIGURE!" The flight rates they used, sometimes up to FIFTY flights a year, OF COURSE made everything look overly rosy! At those flight rates, your infrastructure costs, workforce overheads, etc. are amortized over SO many flights they drift into the "noise" money-wise... They used inflated flight rates to justify SRB's, since the costs associated with them would be divided out over so many flights, it made them 'disappear'. Same thing justified the reuse of the engines and everything else-- because they DRASTICALLY underestimated the turnaround time and amount of touch-labor and inspection and refurbishment between missions... They came at it with a 'pilot's walk-around inspection' would be "all that was required" type mentality-- like an airliner, "refuel and go", and it just wasn't EVER gonna happen that way... so OF COURSE shuttle has been more expensive than Saturn V, with only a TINY FRACTION of the payload capabilities and NO beyond Earth Orbit capabilities (it can barely stagger up to Hubble's orbit height!)

What's TRULY sad is that NASA has made the "same old mistakes all over again" (Deja vu all over again!) The ESAS study was on track to recommend a vehicle very much like WHAT IS BEING CONTEMPLATED RIGHT NOW for SLS... two launches of a 'mid-sized' shuttle derived HLV, but the PTB at NASA, Mike Griffin and Co, wanted the 'biggest rocket ever built' in Ares V and justified building it by using Ares I "the stick" crew launcher to develop the bigger SRB booster and J-2X engine for it, touting it as the 'safest crew launcher ever conceived' and all this BS... so through some inside pressure and putting a thumb on the scales, tipped the figures in the study to favor the "little and big 2 rocket launch" program over the "2 launches of the same mid-size rocket" program, (which incidentally launching the same rocket TWICE raises the flight rates and amortizes the infrastructure/workforce costs, whereas launching 2 seperate rocket designs ONCE for each mission has the opposite effect-- additional facilities, lines, tooling, workers, equipment, etc. for the second rocket, increasing costs... even 'shared components' like the J-2X and first stage SRM booster, (when it was still common between Ares I and V) can't save enough to cover the added costs of a SECOND ROCKET DESIGN). SO here we are, 7 years and 9 billion dollars later, with a 2/3 finished capsule and NO ROCKET TO PUT IT ON, because of the stupidity of "fudging the numbers" to justify the predetermined choices the PTB wanted...

Then, when the engineers and the folks who actually design the stuff came back and told them "won't work (as advertised)" or "its gonna cost twice as much as we thought" NASA HQ turned a blind eye, killed the messenger, and told the troops to go put ten pounds of crap in a five pound bag... like you can dictate physics... :rolleyes: THAT was the REAL travesty of the whole thing... and then when the money didn't appear, instead of simply saying, "OK, this is too expensive and we're gonna have to find a way to do it cheaper-- even if it means smaller rockets, less capabilities, whatever", they simply "pushed the schedule to the right" (pay as you go) which is a bankrupt way of trying to do it anyway-- It's about like hiring a bulldozer operator, but not having the money for a bulldozer yet... you still have to pay the operator to keep him 'ready to go' when you get your machine, but you CAN'T DO ANYTHING until you get the dozer... and the salary of the operator eats up your budget for buying the bulldozer... Better to wait to hire the guy, and put the money you save from not hiring him towards the down payment on the dozer... you get both sooner and can actually do something with it.

That's why this NEW SLS/flex path nonsense is doomed to failure, on the path it's currently on. It's why everything NASA does now is terminally screwed up before it starts-- when it takes a DECADE to get a new rocket to the test phase, or you base an 'exploration program' around a mission to an asteroid every 2-3 years or so, the costs are going to be ABSOLUTELY ASTOUNDING! You may only be flying your rocket once every two years, or every three, and you may wait ten years for the first flight, then wait another ten before you go to Mars, but YOU'RE STILL PAYING OVERHEAD/SALARIES WHILE YOU WAIT... and all those costs add up... the longer you wait, the more you pay, and the more it drives up the cost of the mission when you eventually fly it. That was what was SO patently rediculous in the speech Obama made before the election-- "delay the moon return 5 years and put NASA in a standdown, and shuffle the money saved to education". Ya right. It's not like a Chevy plant in Detroit-- just send everybody home for 5 years on furlough and lock the doors, turn out the lights, and pay a night watchman to make sure someone doesn't burn the plant down... No, with NASA, you have to 'maintain the capability'. You don't get world class rocket engineers from the want ads-- and they're not gonna sit by the phone for 5 years eating pork-n-beans waiting for a phone call from NASA to come back to work-- You have to keep those guys on the payroll, maybe doing busy-work or "polishing wrenches" but you have to keep them if you want them, because if you don't they'll be somewhere else and most won't come back after getting canned (see the post-Apollo "brain drain"). So if you have to keep everybody on the payroll, and keep the factories from rusting down and the lights on and the security guards at the front gate, even if NOTHING IS BEING BUILT, then were exactly are the SAVINGS going to come from to 'reinvest in education'??? The post-Challenger and post-Columbia standdowns PROVED that it costs ALMOST as much to 'keep the capability' (keep folks on the payroll and the lights on in the shop) and NOT FLY AT ALL as it does to fly 2, 3, or 4 times a year...

To get back on topic, I REALLY hope that Elon succeeds... because if he doesn't, we're not going ANYWHERE ANYTIME soon... IF EVER. The NASA bureacracy seems more interested in spending money than actually doing anything, and the gov't seems more than content to let them do it, so long as the money gets funneled to the right Congressional districts to keep them getting reelected. You get a President with strong backing from 'the space states' you see more money and support for NASA-- you get a president that didn't play well in the South and west, NASA gets cut or neglected to punish the local electorates... (exactly what we're seeing now).

It's time for a different modus operandii for space... Good luck Elon!

Later! OL JR :)

Well, in all fairness, there's more to it than just raw thrust at liftoff... in fact the upperstage mass fraction and ISP plays a HUGE role in how much payload a given rocket can lift. While the Merlin upper stage engine has good ISP for it's size and design, it's still limited in potential ISP by the fact that it's a kerosene based engine. Raw thrust is the most important metric at liftoff and during flight through the lower atmosphere (up to Max Q or so) and thus for first stages, kerosene is probably THE ideal fuel-- it's dense, meaning smaller tanks for a given amount of fuel, the ISP with advanced engine design can be pretty darn high (MUCH higher than solid propellants!) which means you get more 'bang for your buck' (payload lifted per pound of fuel). BUT, once at altitude, and when you have enough velocity that gravity losses aren't bleeding you to death, the main metric for payload capability becomes ISP, not raw thrust. Thrust levels are only important to the effect that you have to keep accelerating at a high enough rate to prevent gravity losses from 'stealing you blind' as you get to orbit altitude/velocity. (you have to have a sufficient T/W ratio) BUT ISP becomes more and more important on ascent, and ESPECIALLY once you're in space, and if you have to relight your engines for a transfer trajectory burn (GTO, TLI, etc.) In space, ISP is the most important metric.

Hopefully Elon and the gang will get their Raptor LH2/LO2 engine for their upper stage (s). At altitude where ISP begins to supplant raw thrust levels in importance for performance, LH2 REALLY shines because of the tremendous theoretical ISP it can deliver (with advanced engine design). That's part of the reason that Delta IV isn't optimized-- it uses LH2 for first stage propellant, making the first stage HONKIN' ENORMOUS due to the low density of LH2, and requires a MASSIVE fuel tank to hold enough LH2 to feed a high-thrust engine (especially one with relatively low ISP for a hydrogen engine like RS-68 is). If Delta IV were powered by a pair of highly efficient SSME's it's performance would improve quite a bit. BUT the much smaller kerosene powered first stage of Atlas V, coupled with high-ISP advanced kerosene engines of high thrust, is still probably a better choice... smaller tankage means a better first stage mass fraction, though mass fraction isn't as important in booster stages as it is in upper stages, to be sure). The slight increase in upper stage size due to larger LH2 tankage due to LH2's low density is more than made up for by the higher ISP a good LH2 engine can provide at altitude...

I wonder why SpaceX doesn't just buy RL-10's off the shelf?? PWR has said that with some slight work and high order rates, they can get the price per unit of RL-10's down to the price of a helicopter gas turbine engine-- quite a bargain in the rocket launch world! RL-10 is one of the best hydrogen engines in the world, and has been flown since the early 60's. Its main handicap is the relatively low thrust, requiring clusters for higher-mass stage applications (like S-IV stage on Saturn I Block II). Of course had NASA completed RL-60 that would largely have addressed that issue... :rolleyes:

I know SpaceX likes to "do it themselves" but I think that they should really fast-track Raptor-- I sorta see the point of what they're doing-- highly efficient, more powerful thrust level kerosene booster engines will improve performance, but when they get a good LH2 engine they'll have a world beater-- they're playing to their strengths, because LH2 stuff IS hard, and they have a good track record with kerosene technology, which is much easier than LH2. They're probably also looking at the fact that the Russians have some world class kerosene engines that can ALMOST compete with run-of-the-mill LH2 engines, and so they're playing to their strengths by using what they know to develop a world-class kerosene engine. Once they have the know how to do that, a GOOD LH2 engine will be that much easier... So it makes sense when looked at from that perspective...

Go SpaceX! OL JR :)

There is more to an LH2 stage than just the engines. I have been following with much interest the articles you have been posting about proposed uses of Saturn technology. The S-IVB stage, with a single J-2 engine, was a very reliable piece of hardware. The five J-2 S-II stage, not so much; maybe we just did not fly enough of those to perfect it?

Or is the problem building larger LH2 tanks? The Space Shuttle external tank has a problem with shedding foam; maybe NASA should have looked into burning the insulation at ignition like the Delta IV?


Bill

S-II had teething problems, supposedly because of North American's "mismanagement" of the program. From what I've read, NAA basically says it wasn't their fault, that the S-II was "at the bleeding edge" for the time and was REALLY difficult to build because of it's size... the S-IVB was smaller and therefore easier to build and perfect. Plus, S-II was one of the last elements of Saturn V to be completed-- S-IVB had been around for a while already and therefore had more time for improvements to be incorporated into it and for data coming back from actual flights to prove where things could be changed, lightened, needed strengthening, etc. S-II turned out to be a good stage, but it took longer to get there, frankly, because it was harder and it was started later. NASA's 'reset' of NAA management DID clean house and get S-II back on track-- there were some stupid mistakes made (like trying to do tanking tests on S-II with LIQUID WATER instead of LH2-- water is WAY WAY heavier than LH2 and ended up splitting the stage open like an overfilled water balloon, because it was designed to handle the weight of liquid hydrgoen not liquid WATER.)

Probably the neatest configuration to come out of these studies that I've been summarizing so far is the ground-lit S-II stage-and-a-half proposals... Strap 4 Titan III SRB's to the outside of a beefed up S-II stage, slap yer payload on top, and instant 1.5 stage to orbit. When you look at that, and the fact that in most of the Saturn V uprating studies, the S-II was pretty much 'spot on' as far as propellant capacity was concerned. The only time the S-II stage was stretched (much) was when the engine count was increased from 5 to 6 or 7, or when MUCH higher thrust engines were installed (like HG-3 or toroidal aerospikes in the 300,000+ lb thrust range) or both. S-II's 930,000 lb fuel load was in the 'sweet spot' for the 5 J-2's it carried, and was pretty close for 7 J-2's on some of the upgraded Saturn V's, especially when being used with a third stage, where staging altitude and velocity were less important than on a two-stage vehicle where the stage would deliver the payload to orbit (or just shy of it for stage disposal).

The shuttle External Tank, especially in its present SLWT form, is one of the most amazing stage structures ever designed. It has the best mass fraction ever, (part of which is contributable to it's engines being mounted in another vehicle, granted) but it would make an EXCELLENT starting point for a new core vehicle stage. There's even room for improvement if you could eliminate SRB's and their necessary cross-beam which passes through the intertank, which would allow for the design of a common bulkhead, saving a HUGE amount of weight! But even with SRB's and the cross-beam, it's still excellent tankage and should be the basis for future stage design...

The problem with shedding foam is the shuttle design... Remember the liftoff footage of the old Saturn V's, Ib's, Atlases, etc... hundreds of pounds of frost and ice shaken loose when the engines fired up, raining down in a hailstorm of ice and snow onto the MLP/pad as the rocket lifted off... That was the first thing that had to be done away with when the shuttle was designed-- it's soft chalk-like tiles would be RIPPED TO RIBBONS by that avalanche of ice and frost at liftoff... the solution was spray on foam insulation. Saturn V and the other rocket's LOX tanks were poorly insulated, which let ambient heat from the outside air and in the metal itself cause vigorous boiloff of the LOX in the tank, which was constantly replenished and the GOX (gaseous oxygen) vented to prevent the tanks from overpressurizing and rupturing. That supercooled the structure, making it SO cold that the moisture in the air touching the skin of the stage froze and stuck to it as ice and frost, building up a layer inches thick. This brittle layer flaked off when the structure started vibrating at engine start, causing a minor avalanche on the pad. LH2, which is MUCH colder, can actually LIQUIFY AIR and droplets of liquid air at several hundred degrees below zero dripping everywhere can cause havoc with equipment, so the hydrogen tanks were better insulated, in the case of the Saturns from hand-fitted insulation tiles fitted to the walls INSIDE THE TANKS, which was an EXPENSIVE way of doing it! Since the shuttle tank was to be tossed after each flight, they had to find a cheaper way-- thus, externally applied spray-on foam insulation. Now the problem is, spray foam isn't a particularly strong structural component... it sticks pretty well, but mechanically it's not very strong. As long as there's a good bond with NO VOIDS and only the tiniest of bubbles in the foam (and not larger bubbles imbedded in the foam like holes in a loaf of bread!) and as long as it's not applied too thick, it can stick amazingly well. The problem with the foam was, they applied it VERY thick in certain areas (like the ice-frost ramps which doomed Columbia) and it tended to rip off when the rocket was flying through turbulence in the slipstream at hundreds of miles an hour in the dense atmosphere... and another problem was, if there were any sizeable voids or bubbles in the foam, the air trapped inside them would ACTUALLY LIQUIFY inside the bubble in the foam on the side of the LH2 tank... of course this LIQUID AIR is MUCH smaller in volume than the gaseous air in the atmosphere, which means that the air pressure in the atmosphere surrounding the tank foam would squeeze the foam down around the bubble until the pressure was equalized in the bubble (reduced volume of the bubble by squashing it flat against the liquid air). When the rocket lifts off, and the aero-heating starts to warm the foam up, the liquid air boils off again into gas, and expands back to it's previous volume. Additionally, as the rocket is ascending and exiting the atmosphere, the outside air pressure is constantly falling toward vacuum, and the air trapped in the bubble starts to expand and push outward on the bubble, trying to equalize the pressure until the gas escapes or the bubble pops. Squash a piece of foam and then try bending it-- both greatly weaken the foam and make it want to rip apart or come off whatever it's stuck to MUCH easier, hence the "foam shedding"... Careful application to eliminate all large bubbles and voids reduces the liquid air problem, along with careful foam formulation to keep the bubble size in the foam itself small and uniform... keeping it thin enough so that it's physical bond strength is sufficient to keep it 'glued' to the tank walls is also important-- the thicker the foam, eventually it's weight starts to pull it free from the tank due to air drag, gee forces, and inertia... so the frost ramps and other 'thick spots' in the foam were removed and replaced with HEATERS to warm the metal or thinner foam up enough to prevent the formation of ice and frost in those spots...

Delta IV's "burning the foam" has nothing to do with the insulation, foam, shedding, ice formation, or anything like that... it's an artifact of the startup procedure for RS-68, which vents a huge cloud of gaseous hydrogen from the engine just before it fires up. This GH2 ignites into a 'fireball' that engulfs the rocket as it comes up to pressure and lifts off, scorching the surface of the foam... shuttle would produce a smaller but similar fireball, but the much smaller amount of vented GH2 from the SSME's is continuously ignited and burned off by the "shower of sparks" at the base of the shuttle engines that is set off in the MLP flame hole just prior to SSME startup. Delta IV is actually looking at a way to reduce this "fireball" effect on liftoff, especially if the vehicle is ever to be used for manned spacecraft....

Later! OL JR :)

PS... I read or was told somewhere along the line (can't recall which ATM) that the shuttle tiles are SO brittle it can't even fly through a rainstorm-- the raindrops would crater the tiles like BB's shot at a foam block... If you've never handled a shuttle tile, the closest consistency I can think of is that freeze-dried astronaut ice-cream you can get at Hobby Lobby-- that block of freeze dried foam is about the same weight and physical strength as a shuttle tile, which makes sense, because they're both "frozen foam"-- Shuttle tiles are basically just glass foam when it comes down to it. Just like with astronaut ice cream that can easily be crushed to dust, dented with a fingertip squeezing it, or gouged by a fingernail, shuttle tiles are almost equally easy to damage... you can easily gouge it with your fingernail... so imagine what an avalanche of ice chunks and clumps of frost would do!!!

Sticking with a single (and easier-to-handle) oxidizer/fuel combination such as LOX/kerosene for all stages of a launch vehicle yields lower non-recurring costs and lower recurring costs for building (or modifying), operating, and maintaining a launch complex and its propellant storage and transfer facilities. The Soviets developed LOX/LH2 engines (the Energia used them), but they prefer LOX/kerosene engines (with nitrogen tetroxide/hydrazine engines a rather close [but waning] second, except for upper stages like the Briz) because they and their propellant-handling infrastructures are simpler and cheaper.

True, but propellant facility costs are a small part of the infrastructure and operating costs of a rocket vehicle system, and certainly not expensive enough to be the deciding factor. The gains from LH2 propulsion are significant enough to outweigh the added costs of supporting the fuel infrastructure at the pads. The REAL expense comes in designing, building, testing, and integrating hydrogen propulsion systems on the vehicle in question-- hydrogen is notoriously difficult to handle, it's small molecule size makes it extremely easy to leak, difficult to seal, and it's deep cryogenic temperatures (just a few degrees above absolute zero) make it VERY hard on structures and difficult to handle and use. BUT the benefits of it, being that it produces low molecular weight byproducts in combustion with high energy, meaning higher exhaust velocities, therefore higher ISP, means it's the best chemical propulsion that you can get, theoretically, and for all intents practically. That's also why it's ideal for use in nuclear engines as well, but that's another issue...

From what I've read and understand, the Russians have stuck with kerosene and to a lesser extent hypergolics for two reasons. First, their rocket development and engine programs were MUCH more closely tied to ballistic nuclear missile production than ours was; kerosene with LOX oxidizer is pretty suboptimal for missiles due to the fact that LOX is cryogenic and has to be constantly topped up due to boiloff; it can't be stored in the missile, so no "instant readiness" (using RFNA or other oxidizers with kerosene would obviate that problem, but the ISP is pretty low). Hypergolics, on the other hand, can be stored for years in the missile tanks, ready to go at the turn of a key. This "spilled over" into the Russian space program which has always been rather inextricably linked to their military space/missile programs. The second reason was that the Russians were VERY late to the party when it came to hydrogen propulsion research-- where we sent up the first Atlas Centaurs in the early-mid 60's, it was at least another decade before the Russians followed suit. There was no need for hydrogen propulsion for military missiles, so it wasn't a priority for the Russians. The Russians were also slower to develop solid rocket motor technology as well; their early SS-13 Savage, the first Russian solid propellant ICBM, was trouble prone and not very reliable or powerful, and was subsequently replaced by hypergolically fuelled ICBMs. It wasn't until much later that the Soviets perfected solid propulsion for their SLBMs and then employed it widely in their missile fleets. Solid propulsion STILL has virtually NO place in the Russian Space Program.

With the breakup of the old Soviet Union and the fact they have to lease their largest rocket base from a foriegn power (Khazhakstan), and the fact that their spent rocket stages crash land on the Khazhak steppes, unlike ours which disintegrate on impact with the ocean and sink, hypergolics have been increasingly falling out of favor with the Russians, hence the push to phase out Proton and replace it with a kerosene fuelled rocket like Angara or Rus-M. The Russians have always embraced the simplicity of kerosene fuelled vehicles since those and hypergolics were the two pillars upon which they had to build, and they have some world class kerosene engines; NK-33 which is being adapted to the new OSC Taurus II rocket as the AJ-26 engine, and it's arguably the best kerosene engine ever built anywhere in the world. The RD-180 powers the US Atlas V rocket, and while part of the military contract was to be able to build them in the US should it become necessary, and PWR claims they can, they have also said they STILL don't quite have the knack of how to make the special coatings metallurgically that allow the RD-180 to run oxygen-rich without melting down, which makes it so efficient and powerful. The Russians perfected that technology and it's WE who are playing catch-up.

Like I said, I think it's terrific that SpaceX is playing to their strengths, taking what they learned from their previous engine iterations and applying it to the next generation and improving their engines. I'd love to see them build the BFE, the "Merlin 2" which would be on-par with the RD-180 or the F-1 thrust wise, maybe even better... (certainly better than the antiquated F-1 ISP wise). I'd like to see them build a kerosene engine that could rival the weight and power and efficiency of the NK-33, and put it to use on upper stages. BUT, good as they are, a GOOD hydrogen engine will STILL beat a good kerosene engine-- a GREAT kerosene engine might just be able to bump heads with a GOOD hydrogen engine, but a GREAT hydrogen engine can just blow a kerosene engine out of the water no matter how good it is, for in-space use anyway...

BTW... Von Braun originally planned for Saturn V to be all-kerosene. Quick figures on the fuel needed for TLI and then working backwards through the stages to ground launch quickly showed the benefits (necessity) of LH2, so the upper stages became LH2 powered. The benefits of kerosene's greater density and raw thrust, where ISP is far less important on first stages than raw thrust, yet still allows smaller stage tanks than low-density LH2, made it a natural choice to use a kerosene first stage (and also F-1 engine development had been going on for quite a while and it's use was a foregone conclusion).

Later! OL JR :)

The foregoing is all true. The way mission planners are looking at future manned lunar, asteroidal, and planetary missions, though, may make high-ISP upper stages less critical for logistical reasons.

The Apollo mission planners decided on LOR (Lunar Orbit Rendezvous) even though they preferred EOR (Earth Orbit Rendezvous) for safety reasons, because LOR allowed the mission to be accomplished using a single launch (EOR required two launches) carrying a much smaller lunar lander. LOR was also considered quicker to achieve (within JFK's "...before this decade is out..." timeline), cheaper (because only one Saturn V was needed per lunar mission), and simpler (because the then-untested in-space refueling wouldn't be necessary with LOR).

For future missions to the Moon (especially commercially-motivated ones to extract lunar polar ice), larger landers with greater payload "down-masses" to the lunar surface are envisioned. Since in-space refueling is now routine (although to date it has been limited to the relatively small propellant systems of the orbit-raising thruster systems of space stations), mission planners are looking at EOR-type missions (although NASA's Orion capsule/Aquila lunar lander mission architecture would not have involved in-space refueling).

With an EOR mission profile that utilizes refueling the upper stage (which will also serve as the lunar lander's TLI stage), maximum performance may not be necessary. An old, 1950s-vintage book that I have stashed away in a box ("Space Flight: The Coming Exploration of the Universe") describes a lunar mission that would have departed from a classic "wheel-shaped" space station in Earth orbit (a two-hour, 1,075 mile high orbit, if memory serves). Instead of reaching a TLI velocity of 25,000 mph, the three large Moon ships (with hypergolic propellant rocket engines) were intended to reach only 22,000 mph in order to economize on propellant. This would have enabled them to just *barely* coast over the "boundary line" where the Moon's gravitational field would have pulled them toward it. This "economy trip" would have taken five days instead of the three-day coast times of the Apollo missions.

I have nothing against LOX/LH2 upper stages, but looking at future missions through the "green eye shades" of the budgeter, less energetic but cheaper upper stages might--especially with in-orbit refueling and higher launch rates of cheaper rockets like SpaceX's--permit more lunar missions to be undertaken for less money, especially if "slower" lunar trajectories are used.