Quote from: MZ on 08/03/2009 08:59 pmWhy is the choice of fuels at a depot between hypergolics and LH2? I would think that there are fuels that are more easily stored and transferred than LH2, and that also has a better Isp than hypergolics.Just curious why the other options have been discarded.Methane, for example, which I believe doesn't need cooling. Both Armadillo Aerospace and XCOR (Jeff Greason's company) have recently constructed LOX/Methane engines:http://www.xcor.com/products/engines/5M15_LOX-Methane_rocket_engine.htmlhttp://www.hobbyspace.com/nucleus/index.php?itemid=14018The CEV was "originally" planned to use methane, but was switched to hypergolics in 2006:http://www.space.com/spacenews/archive06/Methane_013006.htmlQuoteHypergols such as the nitrogen tetroxide and monomethyl hydrazine combination the space shuttle burns to maneuver on orbit are considered highly reliable and easier to store than other propellants. But they offer lower performance than methane and other so-called green propellants and are highly caustic, requiring painstakingly careful -- and therefore expensive -- handling by workers on the ground who have to be extremely careful to avoid potentially lethal exposure.Scott Horowitz, NASA associate administrator for space exploration, said the decision to drop the methane-engine requirement from the CEV program came down to changing assumptions about the performance advantages and technical risk. There are no methane-fueled space propulsion systems in service today. Hypergolic systems, on the other hand, were used on board the Apollo command and service modules and the lunar landers.This paper on scalable depot design (the first one which came up from some googling) lists the following propellant options: LOX, LH2, LCH4, kerosene, H2O2, liquid xenon. http://www.ssdl.gatech.edu/Papers/Masters/Street_8900.pdfOn a related note, I'm not sure if anybody linked to this yet, but I came across a presentation by Boeing in 2007 on the "Potential Impact of a LEO Propellant on the NASA ESAS Architecture":http://www.boeing.com/defense-space/space/constellation/references/presentations/Potential_Impact_of_LEO_Propellant_on_NASA_ESAS_Architecture.pdf
Why is the choice of fuels at a depot between hypergolics and LH2? I would think that there are fuels that are more easily stored and transferred than LH2, and that also has a better Isp than hypergolics.Just curious why the other options have been discarded.
Hypergols such as the nitrogen tetroxide and monomethyl hydrazine combination the space shuttle burns to maneuver on orbit are considered highly reliable and easier to store than other propellants. But they offer lower performance than methane and other so-called green propellants and are highly caustic, requiring painstakingly careful -- and therefore expensive -- handling by workers on the ground who have to be extremely careful to avoid potentially lethal exposure.Scott Horowitz, NASA associate administrator for space exploration, said the decision to drop the methane-engine requirement from the CEV program came down to changing assumptions about the performance advantages and technical risk. There are no methane-fueled space propulsion systems in service today. Hypergolic systems, on the other hand, were used on board the Apollo command and service modules and the lunar landers.
Quote from: simon-th on 08/03/2009 08:26 pmQuote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. Just a couple of benefits:1. Costs. You get your fuel up to the depot with a number of smaller launch vehicles. High flight rates (>20 flights per year) reduces cost per launch significantly. On the other hand large rockets do have high fixed costs and only make sense at large flight rates (that you don't get if you have a 120mt vehicle).2. Flexibility. Your architecture basically involves your flight stack with an empty EDS going up to the depot and be fueled and then you go whereever you want to go. You can have an empty spacecraft (EDS and payload) in the 75mt range and after it's fueled you have a spacecraft with say 200mt ready to propel 60mt to a Mars trajectory.3. Creating a market for commercial rockets. You get fuel up to your depot constantly. A commercial provider with e.g. a 15mt to LEO rocket may be able to sell 20 launches per year to NASA for the fuel depot. That lowers this providers cost per launch quite significantly. That also means that the DoD and NASA can use these launchers for other payloads for quite a lower price.Thanks for all of the answers.
Quote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. Just a couple of benefits:1. Costs. You get your fuel up to the depot with a number of smaller launch vehicles. High flight rates (>20 flights per year) reduces cost per launch significantly. On the other hand large rockets do have high fixed costs and only make sense at large flight rates (that you don't get if you have a 120mt vehicle).2. Flexibility. Your architecture basically involves your flight stack with an empty EDS going up to the depot and be fueled and then you go whereever you want to go. You can have an empty spacecraft (EDS and payload) in the 75mt range and after it's fueled you have a spacecraft with say 200mt ready to propel 60mt to a Mars trajectory.3. Creating a market for commercial rockets. You get fuel up to your depot constantly. A commercial provider with e.g. a 15mt to LEO rocket may be able to sell 20 launches per year to NASA for the fuel depot. That lowers this providers cost per launch quite significantly. That also means that the DoD and NASA can use these launchers for other payloads for quite a lower price.
Can someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks.
Team-Any thoughts on this alternative to LH2 - LOX storage: What if the fuel was stored as Stable Water? Then when the time comes to transfer the fuel the water could be broken into H2 and O2 via energy gathered from solar panels. This could reduce the amount of loss due to the "boil off" effect, and would be more stable and less prone to "unfortunate incidents" on orbit.Thoughts?Drew Montgomery AKA TOG.
Is there really any advantage to LCH4 over RP-1 for refillable upper stages? The specific impulse is 3% higher, but the density is 20% lower.
Easier to keep the prop lines clean, I would think.
Team-This has probably come up elsewhere, but any thought of using one of the expandable modules that Bigelow is developing?Or how about (if we HAVE to use cryo genic fuels) puling an ET up from a SSTS or SDHLV?In other words, any thoughts on WHAT would hold our fuels?TOG
Quote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. LOL! The large rocket doesn't exist yet. Is that a good place to start!
Quote from: loomy on 08/03/2009 10:15 pmQuote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. LOL! The large rocket doesn't exist yet. Is that a good place to start!That's a bad argument. Propellant depots don't exist either. But from the hearing on July 30th, probably the strongest proponent on the panel for propellant depots, Jeff Greason said that he was no longer convinced that there wasn't a need for a 75m ton rocket. He said that the next generation HLV should be capable of lifting between 25mt and 75mt. But even with a 75mt rocket, he felt that propellant depots were still a game changer and needed to be a priority but he also felt that a large enough rocket would not need to rely on propellant depots until the propellant depots are ready.
Chemical in-space propulsion: Lunar return* Revolves around some kind of Earth Departure Stage** Considered three classes, which map to the three classes of launch vehicles (25mT, 75mT, 125mT)* 25mT vehicles require propellant transfer and a depot for human Lunar exploration** Still an open question whether mass/volume is sufficient for payloads** Number of launches requires time to place propellant, vehicles, and mass on orbit; managing boil-off, orbit maintenance, keep-alive power drives need for a depot for missions beyond the first few* 75mT vehicles can support some exploration missions with “top off” of one EDS by another, but can do significant exploration before depot is ready** Cannot match Ares V capability without top-off transfer* 125+ mT vehicles do not require propellant transfer for Lunar missions but would be greatly enhanced by them for Mars* Point of departure EDSs for these classes provided to NASA for additional architecture analysis now underway
Water, Water, Water. Locate at Emily-1. Start small, build-out to 2400m diameter, rotating ring.
That's a bad argument. Propellant depots don't exist either. But from the hearing on July 30th, probably the strongest proponent on the panel for propellant depots, Jeff Greason said that he was no longer convinced that there wasn't a need for a 75m ton rocket. He said that the next generation HLV should be capable of lifting between 25mt and 75mt. But even with a 75mt rocket, he felt that propellant depots were still a game changer and needed to be a priority but he also felt that a large enough rocket would not need to rely on propellant depots until the propellant depots are ready.
Quote from: loomy on 08/03/2009 10:15 pmQuote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. LOL! The large rocket doesn't exist yet. Is that a good place to start!That's a bad argument. Propellant depots don't exist either.