ISS cutbacks

Euler · 2004-08-08 22:49:25

The Hydrogen rocket at Delta-V of 10kms is about 89% fuel, while the Peroxide/RP1 is about 96%. That is, the dry mass is about three times higher for the Hydrogen, which even if the stage mass is much higher then that of H2O2/RP1, Hydrogen still wins

GNCRevenger, you seem to be arguing Mad Grad's case for him. His point is that while you need 3 times as much H2O2/RP1 to lift the same amount of dry mass as you could with LOX/LH2, you can fit 4 times as much of it in the same fuel tanks. This lends credibility to the idea of an H2O2/RP1 rocket.

If you are worried about the size of your fuel tanks (and you should be), then H2O2/RP1 would seem to have a clear advantage over LOX/LH2. However, there is another factor to consider: the engines. The H2O2/RP1 engine with the highest T/W ratio in Astronautix’s database has a T/W ratio of 70. This is about the same as the T/W ratio of the current SSMEs and other modern first stage LOX/LH2 engines. This means that since the H2O2 rocket will be 4 times heavier, it can expect its engines to also be 4 times heavier. This makes it so that building a SSTO spaceplane with either fuel would be very difficult, and possibly beyond our current level of technology.

GCNRevenger · 2004-08-08 22:59:11

Perhaps I am wording this in a confusing manner...

For a high Delta-V burn, the fraction of the rockets' mass that is fuel is greater for a lower Isp fuel than a higher one. Hence, you get "more rocket" for your total mass with higher Isp. Let me put in some numbers and see if it makes more sense... same setup as above, with a 100 ton hypothetical rocket fueled, means that your rocket can weigh 10.9MT with hydrogen or 4.08MT with H2O2/RP1 empty. So, if the rocket stage (tanks & engines, no payload) weighs - say - 3MT, then the Hydrogen rocket can carry 6.82MT more payload than the Peroxide rocket can. If the Hydrogen stage weighed double that of the Peroxide one, then it can still carry more payload...

Yeah, making an SSTO with chemical fuels of any kind is kinda out-there, but it certainly can't be done with low Isp fuels.

The numbers look a little skewed, but I used a big 10km/s Delta-V, most are a bit smaller than this on a per-stage basis, so the hydrogen rocket wouldn't have such a big advantage, but it can still overcome the extra tank and engine mass.

Euler · 2004-08-08 23:14:21

Let me put in some numbers and see if it makes more sense... same setup as above, with a 100 ton hypothetical rocket fueled, means that your rocket can weigh 10.9MT with hydrogen or 4.08MT with H2O2/RP1 empty.

Yes, but Mad Grad's argument is that if the rockets were the same size, then the H2/O2 rocket would weigh 400 tons. That would give it 16.32 MT to work with, versus 10.9 for the hydrogen rocket.

Euler · 2004-08-08 23:49:46

Then again, there may be another flaw in Mad Grad's argument. His argument assumes that the mass of a fuel tank is proportional to its volume. However, after looking at some examples from current rockets, I think that that may not be true. Rockets using lighter fuels seem to have a much better mass/volume ratio than rockets using heavier fuels. In fact, the core stage of Ariane (using LOX/LH2) actually has a better mass ratio than the core stage of Atlas V (using LOX/Kerosene).

RobertDyck · 2004-08-09 00:48:44

Hold on here, did you notice I suggested a scaled down MAKS on a 747? That replaces the first stage with a commercial aircraft. A 747 can land at any major airport on Earth. So a reusable orbiter launched from an aircraft and an expendable fuel tank can be a space taxi for 4 astronauts. That sounds like good engineering to me. That is an extension of SpaceShipOne, and just uses existing 747 aircraft instead of WhiteKnight.

Euler · 2004-08-09 01:44:45

Hold on here, did you notice I suggested a scaled down MAKS on a 747? That replaces the first stage with a commercial aircraft. A 747 can land at any major airport on Earth. So a reusable orbiter launched from an aircraft and an expendable fuel tank can be a space taxi for 4 astronauts. That sounds like good engineering to me. That is an extension of SpaceShipOne, and just uses existing 747 aircraft instead of WhiteKnight.

I am not sure how well that would work. White Knight and the An-225 were specifically designed to carry spacecraft, while the 747 was not. A 747 with a MAKS type vehicle on top of it looks like it would be top heavy and hard to maneuver. Also, you would have to worry about exhaust from the engines destroying the rudder of the 747.

The MAKS vehicle might also be difficult to scale down. If it is less than half as large, then the payload fraction it could get to orbit would likely shrink significantly. The complex tripropellant engines could also be difficult to shrink.

RobertDyck · 2004-08-09 06:51:29

Another option, instead of a winged orbiter use a lifting body. That means a bulked-up HL-20 with a scaled-down RD-701 engine. Rocketdyne made the reusable SSME, since then they were bought by Boeing, could Boeing Rocketdyne develop a reusable tripropellant engine?

SpaceNut · 2004-08-09 12:42:27

So with a delta wing or up swept-ed outer wing edges what kind of thermal tiling or RCC panel problems could we have?
All for use as a taxi to the ISS, sounds ok on paper but what would the cost be for development and per flight use?

RobertDyck · 2004-08-09 14:51:05

Well, no, not just a taxi to ISS. What I keep suggesting is a reusable space taxi from Earth surface to LEO and back. This can be used for a lunar mission using Earth orbit rendezvous instead of Lunar orbit rendezvous; the lunar transfer vehicle goes from Earth orbit to lunar surface and back. The space taxi can also be used for a manned mission to Mars: launch the habitat unmanned and slowly raise it to high orbit or a libration point such as L2. Use the space taxi to carry astronauts to orbit, then a geospace transfer vehicle to carry them to the Mars vehicle.

Euler · 2004-08-09 15:47:37

Another option, instead of a winged orbiter use a lifting body. That means a bulked-up HL-20 with a scaled-down RD-701 engine. Rocketdyne made the reusable SSME, since then they were bought by Boeing, could Boeing Rocketdyne develop a reusable tripropellant engine?

Lifting bodies generally mean using linear aerospike engines, as they can be fit onto the back of the vehicle more easily than ordinary bell nozzle engines. A reusable linear aerospike tripropellant engine? I'm sure that it could be built, but it sounds complicated and expensive.

Are you still planning to use an external tank with the lifting body? I thought that the main advantage of a lifting body design is that it could carry more fuel internally than a winged orbiter.

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With all this discussion of reusable SSTO vehicles, has anyone ever considered building an expendable SSTO? As it is still expendable, it would not be able to achieve the order of magnitude reduction in costs that spaceplane proponents hope to get with RLVs. However, an expendable SSTO might be able to compete with traditional multistage vehicles for launching payloads to LEO.

Consider the Ariane V core stage. It is a LH2/LOX rocket with a very good mass ratio (around 14:1). If it was launched with the fuel tanks about 2/3 full, it could probably get to LEO. If you added a second Vulcan 2 engine, it should be able to get around 8 MT to orbit, which is not bad for a 186 MT vehicle. It should also cost less than other vehicles that are about the same size, due to the simplicity of only having one stage.

It does have one big disadvantage though: it can only send payload to LEO, and most commercial satellites are currently launched to GEO.

Mad Grad Student · 2004-08-09 17:31:52

Then again, there may be another flaw in Mad Grad's argument. His argument assumes that the mass of a fuel tank is proportional to its volume. However, after looking at some examples from current rockets, I think that that may not be true. Rockets using lighter fuels seem to have a much better mass/volume ratio than rockets using heavier fuels. In fact, the core stage of Ariane (using LOX/LH2) actually has a better mass ratio than the core stage of Atlas V (using LOX/Kerosene).

Well, for one thing the Ariane V is a higher-tech rocket than the Atlas V. The original Atlases had extremely good mass ratios with their "ballon tanks" supported solely by pressure; in fact they could actually orbit themselves after dropping two of the original three tanks (Stage and a half to orbit, some called it). However, when Lockheed switched to conventional rigid tanks in the late '90s, they became no more exceptional than your average expendables.

H2O2/kerosene rockets can have very light tanks for the amount of fuel they carry because A) they don't need two seperate tanks for two propellants, B) the fuels aren't cryogenic, and C) they actually need to be made out of composite materials like fiberglass or carbon fiber. GCN, I don't know how to make it plainer than this. Let's go back to the Centaur for a second. Filled with H2O2 it weighs four times as much as it would with LOX/LH2, and this allows it to achieve higher delta-Vs because it can pack more fuel into the stage without changing the dry mass of the stage at all.

Let me ask you something, is the shuttle a shining example of how to build a spacecraft? Every component in it is overdesigned to the extreme. Where NASA could have used a spark plug they built an elaborate, expensive component that took many man-hours to design that did the job of a, well, spark plug. Every component on it is a re-invention of the wheel, and the most complex way to solve a problem was taken. If doing things simply isn't the way to go, is this the right way?

The KISS principle, Occum's razor, not adding epicycles all make great sound-bites because they are ideaologies that work.

Euler · 2004-08-09 18:12:05

Well, for one thing the Ariane V is a higher-tech rocket than the Atlas V. The original Atlases had extremely good mass ratios with their "ballon tanks" supported solely by pressure; in fact they could actually orbit themselves after dropping two of the original three tanks (Stage and a half to orbit, some called it). However, when Lockheed switched to conventional rigid tanks in the late '90s, they became no more exceptional than your average expendables.
H2O2/kerosene rockets can have very light tanks for the amount of fuel they carry because A) they don't need two seperate tanks for two propellants, B) the fuels aren't cryogenic, and C) they actually need to be made out of composite materials like fiberglass or carbon fiber.

If a dense fuel rocket had the structural mass/volume ratio that the Ariane V has, it would have a mass ratio of nearly 60:1. Can you find any stage of any rocket ever built that has a mass ratio even close to that? If tank weight is only dependent on volume of fuel, then you should be able to find some rockets that approach this ratio.

RobertDyck · 2004-08-09 18:42:30

Lifting bodies generally mean using linear aerospike engines, as they can be fit onto the back of the vehicle more easily than ordinary bell nozzle engines. A reusable linear aerospike tripropellant engine? I'm sure that it could be built, but it sounds complicated and expensive.
Are you still planning to use an external tank with the lifting body? I thought that the main advantage of a lifting body design is that it could carry more fuel internally than a winged orbiter.

Um, no. Lifting bodies have nothing to do with aerospike engines. They don't necessarily go together, and aren't necessarily exclusive. You can use a bell cone on a lifting body just as easily as a winged vehicle. Lockheed Martin planed to use a linear aerospike engine with a lifting body on VentureStart but that doesn't mean they go together. The sole advantage of an aerospike engine, whether linear or axial, is its relatively even specific impulse a pressures from 1 atmosphere to vacuum.

A totally reusable vehicle will have to wait for supersonic combustion. It is possible to do single stage to orbit (SSTO) and it is possible to build two stage to orbit (TSTO) like Space Ship One / White Knight. However, as GNCRevenger pointed out it is difficult and leaves very little mass margin for cargo. Following the model of MAKS permits a 4-crew space taxi launched from a 747 only if it includes NO cargo and an expendable external tank.

The advantage of a lifting body is lower surface area per unit volume of interior space. Lower surface area means less hull to carry and smaller heat shield. That translates to smaller and lighter vehicle. If you want to build a SSTO like VentureStar, yes one of the advantages is more fuel internally; however that isn't the only reason to use it. It's just more efficient than a winged vehicle.

Euler · 2004-08-09 19:41:08

Um, no. Lifting bodies have nothing to do with aerospike engines. They don't necessarily go together, and aren't necessarily exclusive. You can use a bell cone on a lifting body just as easily as a winged vehicle. Lockheed Martin planed to use a linear aerospike engine with a lifting body on VentureStart but that doesn't mean they go together. The sole advantage of an aerospike engine, whether linear or axial, is its relatively even specific impulse a pressures from 1 atmosphere to vacuum.

There is another difference between linear aerospikes engines and bell nozzle engines: linear aerospike engines are rectangular, bell nozzles are circular. In a typical lifting body design (like venturestar), a rectangular engine fits in more efficiently than a circular engine.

The advantage of a lifting body is lower surface area per unit volume of interior space. Lower surface area means less hull to carry and smaller heat shield. That translates to smaller and lighter vehicle. If you want to build a SSTO like VentureStar, yes one of the advantages is more fuel internally; however that isn't the only reason to use it. It's just more efficient than a winged vehicle.

It has more volume, but most of it is awkwardly shaped. Much of the volume could not be efficiently used as a cargo bay, but it could be used as a fuel tank.

Mad Grad Student · 2004-08-09 22:37:28

If a dense fuel rocket had the structural mass/volume ratio that the Ariane V has, it would have a mass ratio of nearly 60:1. Can you find any stage of any rocket ever built that has a mass ratio even close to that? If tank weight is only dependent on volume of fuel, then you should be able to find some rockets that approach this ratio.

Not necessarily. I immediately thought of http://astronautix.com/lvs/bealba2.htm]Beal Aerospace when you said that, but they didn't even try to get a high mass ratio because they instead went for the big dumb booster approach. Let's see, one pound of rocket for every 80 pounds of fuel (In order to get a decent amount of payload up). Yep, that ought to be possible, I don't see why today's materials couldn't support something like that. Carbon fiber is ridiculously light and immensly strong. For example, each half of the shell of the Beech Starship mentioned earlier only weighed 20 pounds, for a pretty good amount of surface area. Nobody wants to mention carbon fiber beacause of the death of the X-33, but you could make it work on an SSTO. A problem many point out is where to get an autoclave big enough to make the shell, but it turns out you don't even need one. Instead of using an autoclave, carbon fibers can be wound individually around a male plug, left to cure, then pulled off in the right shape. Why the X-33 guys didn't even consider this method is beyond me.

The propellant costs for getting into orbit cost maybe a dollar per pound at tops. There is absolutely no reason why we can't send stuff to orbit for $10 a pound. It's simple math.

Morris · 2004-08-10 02:06:08

The KISS principle, Occum's razor, not adding epicycles all make great sound-bites because they are ideaologies that work.

As a natural iconoclast, I can't resist this one. It sounds too crazy to be true but it is. Some months ago I read a science article which said that some professional astronomers, even today, use the Ptolemaic epicycles to make their predictions of planetary movements because they are computationally much simpler than using the actual ellipses, and for the relative velocities under consideration are just as accurate!!

SpaceNut · 2004-08-10 07:53:59

No matter what type of propellant or the design of the shape takes it still boils down to funding and of the complete rocket plus launch ect.. cost to operate it.

We must keep costs down in the design phase, construction and in its normal use or we will fail.

Cutting back on the ISS is being influenced by these as well as other problems in addition. For one getting the shuttle or some other vehicle designed and rated for manned use.

GCNRevenger · 2004-08-11 09:48:09

GCN, I don't know how to make it plainer than this. Let's go back to the Centaur for a second. Filled with H2O2 it weighs four times as much as it would with LOX/LH2, and this allows it to achieve higher delta-Vs because it can pack more fuel into the stage without changing the dry mass of the stage at all.
Let me ask you something, is the shuttle a shining example of how to build a spacecraft? Every component in it is overdesigned to the extreme. Where NASA could have used a spark plug they built an elaborate, expensive component that took many man-hours to design that did the job of a, well, spark plug. Every component on it is a re-invention of the wheel, and the most complex way to solve a problem was taken. If doing things simply isn't the way to go, is this the right way?
The KISS principle, Occum's razor, not adding epicycles all make great sound-bites because they are ideaologies that work.

$10 a pound? What are you talking about? Thats just silly, if Beal claimed this then he is definatly a crackpot... just getting technitions to launch the thing would make it cost more then that. Again, comparisons with airplanes are not valid, because they aren't anything even remotely similar to rockets... the Beech Starship doesn't have to haul hundreds of tons of fuel at 3-5G at high Mach numbers with a running rocket engine and survive the wild temperature and pressure swings... And the "KISS good, no KISS bad" attitude again, "Why the X-33 guys didn't even consider this method is beyond me.". Well, yeah, I think it is beyond you, because you aren't a rocket designer. Same thing with basicly all the rockets in the world using Hydrogen fuels in at least one stage (Even the Soyuz Onega and SpaceX's Falcon-VB), they obviously aren't stupid people, so they must have a good reason. Occams Razor.

Speaking of hydrogen stages, now the Peroxide Centaur weighs far more than the Hydrogen one does, the total mass is the primary concern... Try replacing a 100MT upper stage with a 400MT one the rocket will hardly even get off the ground... Needing to launch several times the mass of fuel for a TLI/TMI stage? What about the need for far higher thrusts (more, bigger engines) to push the heavier stage? The extra mass for heavier structure? ...And if you want to consider much heavier stages, say that 400MT "Peroxide Centaur" one with a 10km/s Delta-V then the Hydrogen rocket can haul roughly tripple the mass with that much fuel... For an equivilent vehicle dry mass at high Delta-V, you will still need a rocket thats far heavier, which is a bad thing in rocket science.

And the Space Shuttle? Oh come on, surely you are kidding. Of COURSE the Shuttle was a mistake, a big mistake, obviously! Thats somthing of a straw-man and not even a valid point of contention... It is worth noting however, that the sucessors to the Saturn series, the NOVA rockets, were most likly going to have all-cryogenic engines with large SRMs.

My argument remains that the more complex solution that offers the higher performance is often justified. Not that the complex solution is good just because its complex Take the Saturn-V rocket, without the Hydrogen powerd S-IV and 3rd stage, then that arcitecture would have failed. It would have failed because the simpler solution could not possibly provide the performance needed to perform the mission. You would need a far more powerful rocket, and the Saturn-V was already borderline insane with its 7 million plus pounds of thrust, to perform the same mission.

The KISS principle is a good way to keep the feet of engineers on the ground when they start putting elegance over functionality... but it is often taken to the extreme of putting the elimiation of elegance, like discounting the superiority of Hydrogen fuel in many rocket applications, above functionality. And that I will hold against stubborn engineers.

Morris · 2004-08-11 13:11:50

The KISS principle, Occum's razor, not adding epicycles all make great sound-bites because they are ideaologies that work.

In thinking about my previous post on this quote, I see that I may have come across as being more facetious than I actually intended to be. The point is that principles like Occam's Razor and KISS are heuristic principles and not universal laws. They work most of the time, but not always. For example, it is probably an efficient use of resources to give every project a "KISS review" but not to get hung up on a particular case where the principle may be false. In the example I gave about epicycles, using the "true", and conceptually simpler, Keplerian model turned out not to be the computationally simplest and thus not the most efficient use of computer resources.

When an idea goes from being a useful "rule of thumb" to an "ideology" there is an implicit raising of its status to that of a universal law. Since it is not, we can get into trouble with the exceptions.

Mad Grad Student · 2004-08-11 15:10:29

"Why the X-33 guys didn't even consider this method is beyond me.". Well, yeah, I think it is beyond you, because you aren't a rocket designer.

Lolololololol, mwah ha ha ha ha ha!!!!!

Oh, that was a good one, GCN. Aside from being a blatant insult, that comment actually helps put things in perspective. Are you a rocket engineer? No, I doubt it, there probably isn't a single rocket scientist surfing these message boards for inspiration either. Everything that we discuss, any points we make are about vaporware because in the grand scheme of things, anything we say here won't really affect anything. There was a great picture I saw earlier that sums up the situation pretty well. "Arguing on the internet is like the Special Olympics. Even if you win you're still retarded." Points like this are, well, pointless.

In the end, peroxide stages will need a good amount of mass in the propellant tanks because of their mass, but hydrogen stages will probably need more because of their bulk. You must admit that oftentimes an industry standard is set, and everyone wants to follow in their footsteps, this is a contributing factor in LOX/LH2's prevalence.

Hydrogen is great for it's upper stage because it gives you more bang for your buck where it counts, lots of "umph" from little propellant. This means that compartively small amounts of fuel can get the payload moving just as fast. Hydrogen peroxide is great for first stages because it delivers more bang for your buck where it counts down there, keeping tankage mass to a minimum. Every pound on the last stage used as staging instead of payload detracts a pound from the payload. First stages are already carrying so much mass in thier propellant you can save by using hydrogen peroxide instead.

Once the performance is good enough on a rocket to reach your payload goal, each boost you can get out of the design is just downhill. A few percent here, a little bit there, nothing that will change the world, though. That comes at a great expense, though, often requiring much more expensive-but lighter- components than ones that only bring marginal performance bonuses. Everything is a tradeoff, and all systems are the best in certain ways at the expense of others. Whatever gets the job done quickly and cheaply, and well enough to work, is the way to go.

Oh, yes, and the airplane comparisons actually do work. It will never be as cheap to go to orbit as it will be to cross the Atlantic, but aside from the vast amounts of extra fuel needed, there really aren't many fundamental differences here. I think it's reasonable to say that the cost of operating an SSTO could be brought down to ten times the cost of fuel, any objections?

John Creighton · 2004-08-11 20:54:38

I think the biggest reason for KISS in engineering is economics. If a simple solution is adequate why spend the money in components and design to create a more optimal solution. However if a simple solution is not adequate then KISS goes out the window. As an example I doubt anyone would try to apply the principle of KISS to a weather forecasting system that must predict the weather 2 weeks in advance for a town.

As far as fuels go, there are many tradeoffs and I am all for the designers exploring the alternatives.

SpaceNut · 2004-08-12 12:54:24

I hope we never get a hole in the station. If we can not fix one in the shuttle. How can we hope or expect Nasa to do anything but not be able to do it elsewhere.

John Creighton · 2004-08-12 13:31:37

I hope we never get a hole in the station. If we can not fix one in the shuttle. How can we hope or expect Nasa to do anything but not be able to do it elsewhere.

Can't they close off one section from another.

SpaceNut · 2004-08-12 13:42:38

They have air locks that can be sealed.
What if the section that is stuck is the one that has the space suits?

In addition I would say that they have neither minimal or no means of repair capability for such an occurrence should it ever happen.

SpaceNut · 2004-08-12 13:47:43

All the more reason to make a repair shop module, to store all the shuttle RCC panels and equipment for doing a real repair to either the shuttle or Iss. Throw in a few of the ceramic TPS tiles as well. Not to mention some sheet metal, drills and other needed tools to patch a hole.

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