Not infinite mass ratio, you would always be limited by your payload, but a mass ratio of ~9 million does seem pretty steep. Even for something modest like 100 kg, that’s a lot of air! One heck of a lot of initial thrust would be required to get you off of the ground… From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Nathan Mogk Sent: Wednesday, February 18, 2015 4:10 PM To: arocket@xxxxxxxxxxxxx Subject: [AR] Re: Way OT question: degerate matter thrusters? You are suggesting an infinite mass ratio. If you plug that in to the rocket equation, it becomes clear that with such a tank, you can gain up to an infinite amount of delta-v, regardless of your specific impulse. A better question to ask is what mass ratio would be required for propulsion system x to achieve orbit. If we assume 9.4km/s dv to get to orbit and an Isp of 60 (generous for cold gas thrusters), then you would need a mass ratio of 8,766,000. I believe the current state of the art for tank design is slightly lower than a mass ratio of 6. Engineering feasibility really is the limiting factor. I didn't actually address the compressed air propulsion. The results of that analysis would be a more accurate estimation of Isp, which will depend on the temperature of the gas as it leaves the nozzle. On Wed Feb 18 2015 at 1:55:16 PM Galejs, Robert - 1007 - MITLL <galejs@xxxxxxxxxx> wrote: Maybe I can ask my question once again, very precisely… Is it theoretically possible, assuming I give you a magical tank with zero weight and arbitrarily high tensile strength, to put an object into orbit using highly compressed air (think giga-atmospheres of pressure or more)? Or is there some physics limitation (like the tank freezing from the air expanding or some other thermodynamic gotcha) that would make this theoretical rocket not work? - Robert From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of rsteinke@xxxxxxxxxxx Sent: Wednesday, February 18, 2015 3:40 PM To: arocket@xxxxxxxxxxxxx Subject: [AR] Re: Way OT question: degerate matter thrusters? The engineering issue of tank weight is what would keep it from getting to orbit. Compressed air rockets work fine except that you just can't get very good mass ratio from a tank of air. Going to higher pressures doesn't help because the tank has to get heavier to hold the higher pressure. The reason that liquid fuels are better is that they are higher density than air at the pressures that generally make sense inside rocket propellant tanks. ----- Original Message ----- From: "Galejs, Robert - 1007 - MITLL" <galejs@xxxxxxxxxx> To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx> Cc: Sent: Wed, 18 Feb 2015 20:30:15 +0000 Subject: [AR] Re: Way OT question: degerate matter thrusters? I believe that that's a bad example. If I understand things properly, I believe that the ISP of the air alone is higher, but the mass flow with water is much better. But you missed my point entirely... I wasn't trying to optimize anything, I was just trying to see if using hyper-compressed air at some arbitrarily high pressure could be used to get something into orbit theoretically, ignoring obvious engineering issues like tank weight. - Robert -----Original Message----- From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Monroe L. King Jr. Sent: Wednesday, February 18, 2015 3:20 PM To: arocket@xxxxxxxxxxxxx Subject: [AR] Re: Way OT question: degerate matter thrusters? Here's a simple experiment to show you. Go down to the toy store and buy yourself one of the old pump up water rockets. Pump it up with air and let her rip. Then fill it with water and try again at the same pressure. Monroe > -------- Original Message -------- > Subject: [AR] Re: Way OT question: degerate matter thrusters? > From: "Galejs, Robert - 1007 - MITLL" <galejs@xxxxxxxxxx> > Date: Wed, February 18, 2015 1:10 pm > To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx> > > > So why is supercritical steam "better" than air? > > -----Original Message----- > From: arocket-bounce@xxxxxxxxxxxxx > [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Monroe L. King Jr. > Sent: Wednesday, February 18, 2015 3:01 PM > To: arocket@xxxxxxxxxxxxx > Subject: [AR] Re: Way OT question: degerate matter thrusters? > > Well so does HP air. Unless your using just a plain ol jet engine like > a ramjet. That would be HP air also. > > > -------- Original Message -------- > > Subject: [AR] Re: Way OT question: degerate matter thrusters? > > From: "Galejs, Robert - 1007 - MITLL" <galejs@xxxxxxxxxx> > > Date: Wed, February 18, 2015 12:55 pm > > To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx> > > > > > > Supercritical steam does not require heavy tankage? > > > > -----Original Message----- > > From: arocket-bounce@xxxxxxxxxxxxx > > [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Monroe L. King Jr. > > Sent: Wednesday, February 18, 2015 2:47 PM > > To: arocket@xxxxxxxxxxxxx > > Subject: [AR] Re: Way OT question: degerate matter thrusters? > > > > I think Supercritical steam would be better than air. High > > pressures require heavy tankage. > > > > Monroe > > > > > -------- Original Message -------- > > > Subject: [AR] Way OT question: degerate matter thrusters? > > > From: "Galejs, Robert - 1007 - MITLL" <galejs@xxxxxxxxxx> > > > Date: Wed, February 18, 2015 12:35 pm > > > To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx> > > > > > > > > > I know this is way off-topic, but it has always had me wondering > > > and it seems like Arocket has the appropriate knowledge base to > > > address this (or, at least wildly speculate). > > > > > > > > > > > > In some of Larry Niven's sci-fi stories, he imagines rocket > > > thrusters (between the ground and orbit) based on super-compressed > > > air (supposedly "nearly degenerate matter"). Would such thrusters > > > theoretically work, or are there some thermodynamic (or other > > > physics) limitations that come into play? > > > > > > > > > > > > Thanks, > > > > > > > > > > > > Robert
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