# Gemini: We can rebuild it, we have the technology

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#### arobie

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I would think that LOX would be included in the propellant mass. I am not entirely sure though.<br /><br />As for how much less LOX there would be to ethanol, I found a mixture ratio of *1 : 81* for *LOX : Ethanol* on this PDF document about propellants. Scroll down untill you see a chart on the 4th page. On that chart, it also gives LOX/Ethanol a Isp of 342 sec, but that's probably just the best case scenario.

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#### mrmorris

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<font color="yellow">"I found a mixture ratio of *1 : 81* for *LOX : Ethanol* "</font><br /><br />At that ratio -- it matters little whether the LOX mass is included or not. Assuming 374 kg of ethanol, it would only be about 4.6 kg of LOX.<br /><br />However here I found a 1:73 ratio.<br /><br />"Propellant Formulation: LOX/Alcohol-92.5%. Isp: 338.00 vac. Isp: 284.00 sl. Optimum Oxidiser to Fuel Ratio: 1.73. Density: 0.99 g/cc. Temperature of Combustion: 3,390.00 deg K. "<br /><br />Using this ratio -- we get 5.12 kg. We'll be generous and assume 5.5 kg of LOX. The same site gives us a LOX density of 1.14 g/cc. <br /><br />5120 g * 1.14 g/cc = 5836.8 cm3<br /><br />The 18" (45.7 cm) spheres have a volume of 50,040 cm3. Each then can hold approximately 10 times the amount of oxidixer needed for the RCS/OMS. Since I was planning on combining the LOX storage of the thrusters and the ECS -- this excess is perfectly fine.<br /><br />The way I visualize the RCS/OMS system at the moment is for the thrusters to be mounted in 'blisters' that protrude slightly from the hull exterior at the four compass points around the circumferences of the X3 -- as close to the CG as practical. Each of these blisters will contain five thrusters: four at 90 degree angles forward, back, left and right, plus a fifth in the center that points radially outward. With this arrangement -- the thrusters can be separated into two distinct systems -- top & bottom, left & right. If either system fails -- the other can still provide thrust in any direction required.<br /><br />The propellant/LOX tanks will be split between the two -- each having two primary 22.5" ethanol tanks and one 18" LOX tank. In the event one of the thruster systems fails -- the primary ethanol/LOX tanks for each thruster set will have the capability to crossfeed to act as a backup to the other.<br /><br />The space under my

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#### mrmorris

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I love it when a plan comes together. Reading today's SDC article on Car Satellite TV -- I thought -- "Hey -- phased array antennas for cars -- at low prices". Checking out the RaySat and KVH web sites -- they don't have what Gemini-X3 needs for its broadband Internet connection yet. But they may well have it before 2010. Cool -- I was leery of having to depend on Boeing. G-X3 will be an unwelcome spacecraft to their mind. Cheap, flexible -- and built by someone else.

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#### najab

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I know it's not my rocketship, but I'd be a lot happier with a non-cryogenic, hypergolic RCS system. Call me paranoid, but I'd feel a lot happier knowing that whatever happens, they can't get stuck in space.

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#### mikejz

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I agree, maybe a cold gas system should also be carried as a backup. Maybe a two tank system where liquid nitrogen is boiled off and brought to pressure as needed. Or have a backup compressed air tank for the crew that can double as as a cold gas RCS like on SS1.<br /><br />I have also had a thought: It might be possible to build batteries into the actual crew seats with polymer Li-Ion batteries. Just an idea. Also, you might want to look more to the crew seating, I noticed in that drawing that the back to seats where right up against the back, and depending on the landing some shock-absorbers in the seats might be a good safety idea.

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#### mrmorris

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<font color="yellow">"I'd be a lot happier with a non-cryogenic, hypergolic RCS system. "</font><br /><br />Well the storage specs I used for the ethanol were assuming a temperature of 20 degrees C -- so it's not cryogenic. I suppose if you were really to be paranoid -- you could use supercompressed oxygen (like Mercury did for ECS) in lieu of LOX. I can't see this buying any additional safety, though.<br /><br />As far as worrying about the igniters. Yep that's paranoid. There's 1000 things that can go wrong with a hypergolic OMS/RCS system. There's 1001 things that can go wrong with a non-hypergolic one.<br /><br />A hydrazine-free craft has numerous advantages. Based on everything I've read on the subject -- enough to offset the slightly reduced complexity of one which uses it.

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#### mrmorris

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<font color="yellow">"...maybe a cold gas system should also be carried as a backup."</font><br /><br />CG systems cannot reasonably produce a deltaV in the ranges required. I've considered the possibility of putting one in for maneuvers very close to the space station, but it wouldn't be a true 'backup' to the capabilities of the main OMS/RCS.<br /><br /><font color="yellow">"It might be possible to build batteries into the actual crew seats with polymer Li-Ion batteries. "</font><br /><br />I plan to use the space *under* the seats for stowing carry-on luggage -- as with an airplane. The people going to the station have to have a few pair of clean undies after all. The batteries for the craft are planned to be Li-Ion -- but I was expecting them to be up in the front along with the electronics. I'm trying to keep the vast majority of the electrical equipment in close proximity to mimimize on the wiring runs required.<br /><br /><br /><font color="yellow">"...I noticed in that drawing that the back to seats where right up against the back, and depending on the landing some shock-absorbers in the seats might be a good safety idea." </font><br /><br />The seats in the capsule are simply placeholders. I'm using them to determine height/width/depth requirements for crew positions. I don't expect to get into a huge level of detail on the mechanics inside the PV. There are simply too many variables. <br /><br />I have considered mounting the seats to the frame via shock-absorbers, though and wondered if COTS car shocks would actually be useable in this capacity. I see no reason why they could not be, and the expense of them is probably 1/1000th what NASA would pay for a functional equivalent. Of course that would be assuming the manufacturer didn't <b>pay</b> a few hundred thousand to have their shocks used for the marketing value thereof.

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#### najab

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><i>There's 1000 things that can go wrong with a hypergolic OMS/RCS system. There's 1001 things that can go wrong with a non-hypergolic one. </i><p>Truth be told, I'd like solids for reentry. I'd be happy with either hydrogen peroxide/silver catalytic thrusters or good old hydrazine/nitrogen tetroxide. I know it is paranoid, but retro fire is one of those things that absolutely, positively <b>has</b> to happen. I have visions of frozen LOX valves ("Hey, I <i>thought</i> the grease would work!"), igniters that don't ignite, depleted batteries ("I thought <b>you</b> turned the lights out!") - yeah, I <b>am</b> paranoid!<p>I like the idea that if everything else fails, the crew can open a manual bypass valve and come home.</p></p>

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#### najab

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><i>...and wondered if COTS car shocks would actually be useable in this capacity. I see no reason why they could not be...</i><p>Be very careful. Greases have been know to have bad reactions in 100% oxygen atmospheres - we would have to test very carefully to be sure.</p>

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#### mikejz

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Just to clarifiy by backup i do not mean a system with the ability to complete the mission, only to regain attiude control and possition for a retroburn (aka if the RCS system goes you have something to get you back home).<br /><br />I understand your concern for hydrazine, however if you keep the plumbing out of the PV--It should reduce the risk. If you use Lox based RCS I would look into how much fuel would be needed to keep a torch lit on each engine for the duration of the mission, this would be a big step towards reducing the risk of non ignition.

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#### scottb50

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Rather than keeping it as simple as possible I think redundancy is more important. The flexibility of a liquid system would allow emergency operation immediately, while solids would pretty much limit your options. Dual, autonomous propullsion systems with individual redundancy in systems makes more sense. <br /><br />As for hypergolics they have proven very effective, but they are also highly corrosive and have limitations as far as storage time, Soyus being an example. Shuttle seems to handle them much better but it only has to do so for a relatively short time. I would bet pergolics would prove limiting also if we are talking about a vehicle that could be refurbished and relaunch on a regular basis.<br /><br />I also think, if the plan is to use a parafoil landing system the shock absorbing provisions of the seats would not have to be too exotic, more in line with that required for business or airline seat construction.<br /><br />While it is a plus to keep cable runs to a minimum, putting the avionics in the front defeats the dynamics of a capsule design. The major mass should be as close to the back as possible to increase the inherent stability. What I would think would work far better would be two pilots forward and four passengers side by side behind them, then major components, electronics, propellant and such behind the passengers and in front of the heat shield. I also think putting a docking adapter on the front is the best idea that would require putting as much mass in the rear as possible to counter act it's mass.<br /><br />I've been thinking it might be possible to have a maneuvering stage that attaches to the docking adapter for launch and acts as an escape system in an emergency. Once reaching a destination it could be dettached to allow docking and either reattached to allow return, burning up in the atmosphere or de-orbit the vehicle and return to the station for re-use, sort of the Tug idea in it's early stages.<br /><br />I still think we should conce <div class="Discussion_UserSignature"> </div>

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#### najab

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><i>The flexibility of a liquid system would allow emergency operation immediately, while solids would pretty much limit your options.</i><p>I'd appreciate if you clarified that statement. <b>THE</b> reason solid rockets are used in escape systems is that their operation is instantaneous.</p>

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#### scottb50

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You were talking about a means of de-orbiting a vehicle, not a means of escape in an emergency. My point was if a problem required a vehicle to return quickly the flexibility of a liquid system would provide more options than a solid system. Say in chosing a location to land in. With a solid you use it all or nothing, with a liquid system you could control the burn, or burns, to assure an optimal landing area.<br /><br />In an escape system solids would have a distinct advantage, at least a few seconds, but when you factor in reaction time to a serious situation it becomes less relevent. The main advantage offered by a solid system is storability, it is simpler and much cheaper to put a solid rocket under an ejection seat than a liquid motor, but if you were using it for other reasons, enroute and returning it would make more sense to use a liquid system. <div class="Discussion_UserSignature"> </div>

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#### mrmorris

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<font color="yellow">"Truth be told, I'd like solids for reentry. "</font><br /><br />???? The SRBs <b>are</b> still the DO thrusters. The OMS/RCS is for docking and fine-tuning re-entry to reduce the landing footprint.

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#### mrmorris

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<font color="yellow">"...100% oxygen atmospheres ..."</font><br /><br />Not planning on 100% oxygen. That's been pretty much labeled a bad idea ever since Apollo. I may be thinking too simplisitically, but I was planning on the original cabin mix being Earth-Normal (i.e. whatever is sitting at the launch site when the astronauts hop in). This will be supplemented in flight with O2 from the LOX tanks and scrubbed of CO2 by Lithium-Hydroxide filters. Gemini-X3 might also need a tank of Nitrogen to keep the mix normal -- but I am (possibly incorrectly) assuming the base Nitrogen from launch won't ever be going away -- at least in the limited timeframe that G-X3 will be in flight.

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#### mikejz

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You could always go for a Oxygen-Helium mix to save weight. (Plus its just fun!)

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#### mrmorris

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<font color="yellow">"I also think, if the plan is to use a parafoil landing system..."</font><br /><br />While that's my definite preference -- it's also one of the biggest question marks. I don't want it to be the make-or-break of Gemini-X3. The second-best option is a parachute landing to a desert locale -- possibly making use of an airbag system to cushion the final impact. With a GPS-assisted re-entry and the ability to use the RCS system to make corrections on the way down -- the landing footprint should be very small. Another reason not to use hypergolics -- they are generally considered too dangerous to have onboard during a landing.<br /><br /><font color="yellow">"...and they can be transported as water, which is demonstratably the safest thing we could be taking to orbit. "</font><br /><br />No. They can't.<br /><br />The reason for putting chemical propellents/oxidizers onboard a spacecraft is because they generate large amounts of energy when combined. Water is the <b>end state</b> of the LOX/LH reaction (i.e. the lowest energy level). To convert water to atomic hydrogen and oxygen requires electrolysis -- a <b>very</b> energy intensive operation. The batteries can't supply this power. Solar cells could over time, but not in the timeframe required.<br /><br />Beyond that -- after electrolysis -- the gasses must then be separated using fractional distillation, then pumped into storage vessels at high pressures to make them available to the OMS/RCS. I have some spare room so far on X3 -- but not enough to take an entire LOX/LH manufacturing facility to orbit.

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#### najab

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><i>The SRBs are still the DO thrusters.</i><p>Oh, okay. I thought you were using the 'Shuttle-esque' definition of OMS (as you can tell, I skipped some posts <img src="/images/icons/wink.gif" />). Liquid motors are okay for RCS, though I still like the idea of hypergolics. Have you considered hydrogen peroxide/silver thrusters?</p>

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#### mrmorris

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<font color="yellow">"I still like the idea of hypergolics. Have you considered hydrogen peroxide/silver thrusters? "</font><br /><br />Nope. The only two I've really considered are hydrazone and LOX/Ethanol. They each have advantages.<br /><br />Hydrazine<br /> - Is pretty much the standard for orbital maneuvering. <br /> - Lots of choices in subcontracting the RCS/OMS system out. Lots of companies have systems using it.<br /> - Has a long history of reliability.<br /><br />LOX/Ethanol<br /> - Will reduce maintenance costs of the ship considerably.<br /> - Will reduce recurring beauracratic overhead/expense, since there are no environmental concerns.<br /> - LOX can be shared between the RCS/OMS and the ECS.<br /> - Is considerably less dangerous to the workforce that has to maintain the craft.<br /><br />Finally NASA is pursuing this as an upgrade to the shuttle thrusters. I assume... OK I <b>know</b> that they are more knowledgeable in making such a decision than I am. I don't know anything about the H2O2/Ag thrusters. It sounds pretty expensive to be throwing silver out the back of a spacecraft -- but I could easily be wrong. Obviously I haven't worked out the quantities involved. I also don't know what its advantages are.

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#### najab

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><i>It sounds pretty expensive to be throwing silver out the back of a spacecraft...</i><p>Yes, that would add up quite quickly. Fortunately, the only thing getting thrown out of the spacecraft is steam. <img src="/images/icons/smile.gif" /> Hydrogen peroxide decomposes on contact with silver (and a few other catalysts) producing steam at about 600C. It makes for a really simple low-thrust engine since it is basically a monopropellant. Peroxides require careful handling on the ground since they oxidize like crazy, but they aren't nearly as difficult or hazardous to handle as LOX or hydrazine. The only drawback is that it isn't the most stable fuel for long time storage.</p>

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#### mrmorris

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<font color="yellow">"This is an article for Nanosat, but you might get a few ideas"</font><br /><br />An interesting read, but yes -- too small for Gemini-X3. Initially when I was researching propulsion systems, I was automatically ignoring articles aimed at unmanned satellites. After all -- propulsion systems for satellites <b>had</b> to be too small for use on a manned spacecraft. Then I smacked myself in the forehead and realized that G-X3 masses less than a heckuva lot of satellites launched today.<br /><br />Nanosats are another matter, of course... <img src="/images/icons/smile.gif" />

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#### mikejz

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Yeap, but the idea of solid RCS is a very odd and intresting idea. <br /><br />Anyways, given that you are think of using LOX/Ethanol <br />do you envision using it also to drive a fuel cell--as ethanol fuel cells are being worked on with a might higher power output per pound vs. traditional batteries. <br /><br />Also, I'm no expert here, but because ethanol is 35% Oxygen to begin with, is it possible to use it would an adding Lox (I know that it will not be the proper blend---but will it burn?)

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#### mrmorris

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<font color="yellow">"...do you envision using it also to drive a fuel cell..."</font><br /><br />No. I dropped the FC from the capsule early on. They don't come into their own for the duration trips Gemini-X3 will be used for. Batteries are the more mass and space--efficient energy source for brief voyages. If the X3 were going to be used for trips in ranges of 5 days or more, it would be a different story.<br /><br /><font color="yellow">"... ethanol is 35% Oxygen to begin with...but will it burn?"</font><br /><br /><b>NO.</b><br /><br />Water is 33% oxygen (counting each atom once) or about 80ish% oxygen by weight. It won't burn, period. Chemical reactions don't work that way. Ethanol needs an oxidizer to burn.

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#### scottb50

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<<No. They can't.>><br /><br />I'm not refering to doing it on board the X-3, there it would be simpler to use cryogenic tanks, like those used on Shuttle for the fuel cells. I'm refering to general operation in LEO and beyond with the electrolyisis equipment being part of the station. If the X-3 could fuel at the station it would also increase the payload to orbit capability. <br /><br />I had the numbers handy, at one time, comparing Hydrazine/Nitrogen Tetroxide and LH/LOX pound for pound. Off the top of my head it took nearly twice the mass of a gallon of water for hydrozine to provide the equivelent amount of power available from that gallon of water. <div class="Discussion_UserSignature"> </div>

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