Date: Mon, 12 Aug 1991 02:08-EDT From: space-tech@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #85 Contents: Karl Dishaw 2-stage tether system Henry Spencer Re: 2-stage tether system Dominic Herity Optimum Rocket Acceleration Henry Spencer Re: Optimum Rocket Acceleration ------------------------------------------------------------ Date: Fri, 9 Aug 91 01:15 GMT From: Karl Dishaw <0004244402@mcimail.com> To: space-tech To: Dani Eder Subject: 2-stage tether system Some comments on Dani Eder's proposed launch system (if they seem late, it's cause I just subscribe to the digests): GAS GUN CUSTOMERS: If collector robots are needed it'll severely limit the number of orbital planes your customers can be in. Shuttle and satellite launch missions usually pick their plane according to the satellite's target orbit or other considerations. Having to rendevous with a collector would put a big constraint on mission planning. Since these fuel pods are going to need a circularization thruster and a homing beacon anyway, it might be better to give them a cold-gas nozzle and a few more microchips so the customer can guide them in w/o needing a robot already in place. The space station would be a good spot for a collector robot--IF it goes up. 2-STAGE TETHER DESIGN The numbers I was using are: radius tip vel. tip accel. stage 1 120km 1000m/s 8.33m/s/s stage 2 24km 1000m/s 42m/s/s total delta-V delivered: 4 km/s Making the second stage have the same tip velocity gives a much higher tip acceleration. Compression will only be an issue for the middle 20% of the second stage, once the structure is spinning. The whole tether will probably have to be reinforced for the spin-up loads. I took the 5:1 length ratio from Dani's 6 Jun message, but I think we'd need an even ratio, since this is also the ratio of the tethers' periods and we want to have the vehicle on the outside point of the 2nd stage's cycle after a half rev (separation point). Constructing the tether is a pretty impressive project. From the numbers Dani tossed around it'll run around 400 tons. That's 3 Saturn V launches, or four Energiyas. Launching this in 10kg segments means not even a full truss cell in each package. Assembling it with humans in EVA is too expensive for this kind of plan, but using robots/teleoperators is a bigger technological risk than the tether itself at this point. Digging up the capital for some big launches might be easier and cheaper. SSTO VEHICLE The vehicle is going to need a lot of extra structure to handle the accelerations from the catapult and tether. The catapult boosts it at 10g. Most rockets are built for 3g, so you'll need to make the tanks and other structure a lot stronger. Hooking up to and separating from the tether will have some transient loads, I'm not sure of how great they'd be (depends on how precise the hookup point is matched). If the vehicle is hooked up to the tether so the centrifugal forces are in the same axis as the boost loads it won't have much of a penalty from the second stage. The first stage's centrifugal acceleration will put a load of nearly 1g on it from all directions in the plane of the tether. All this adds up to a structure much stronger than your typical booster. I haven't had a chance to run numbers on the structure, but the SSTO would be too heavy to compete without the tether's help. Are you thinking of a winged SSTO? The wings would have to be real strong to withstand this kind of shaking around. WRAPUP Kibitzing aside, I hope it works. But a simpler plan like Gary Hudson's Phoenix or the Saturn V revial (see AW&ST) would probably be a lot likelier to actually get up. Once we've got big traffic in space we can afford a tether, but not starting out. Karl Dishaw, 2LT, USAF, 5th Satellite Control Sqdn "Tower to pilot: In case your base is closed before you return from this mission, here are your alternate landing sites...." ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 9 Aug 91 13:39:36 EDT To: Karl Dishaw <0004244402@mcimail.com> Cc: space-tech Cc: Dani Eder Subject: Re: 2-stage tether system Small quibble: > The vehicle is going to need a lot of extra structure to handle the > accelerations from the catapult and tether. The catapult boosts it > at 10g. Most rockets are built for 3g... No, most rockets are built for loads approaching 10G, because that's typically what they impose just before burnout. Only the shuttle holds things down to 3G, and it takes some performance penalty to do so. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Fri, 09 Aug 91 20:08:46 BST From: Dominic Herity Subject: Optimum Rocket Acceleration (was Re: 2-stage tether system) > Only the shuttle holds > things down to 3G, and it takes some performance penalty to do so. I thought that until I did the maths. It turns out (neglecting air resistance) that a 1.5g acceleration ramping up to 3g, (which is about what the shuttle does) adds only a few per cent to delta-v compared to bone-crushing acceleration. Yes. I know mas is expenential wrt delta-v, but its still small. Furthermore, I calculated that any savings gained by increasing acceleration would be offset by extra engine mass (neglecting extra structural mass). Dominic. ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 9 Aug 91 15:46:42 EDT To: Dominic Herity Subject: Optimum Rocket Acceleration (was Re: 2-stage tether system) >I thought that until I did the maths. It turns out (neglecting air >resistance) that a 1.5g acceleration ramping up to 3g, (which is about >what the shuttle does) adds only a few per cent to delta-v compared to >bone-crushing acceleration... The performance penalty isn't large, as witness successful operation of the shuttle, but it's not trivial either. Even a few percent is a real and substantial loss, especially if you are trying something demanding like single-stage-to-orbit. You also need throttlable engines, which are significantly harder to develop and less reliable (due to greater complexity). Nobody wants to use high acceleration -- the gentler the better, even for unmanned payloads -- but most designers have found that it pays off. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ End of Space-tech Digest #85 *******************