Cheapest way ever to orbit

[rogers_buck]

This is too interesting a thread to let die. Are there any updates on JP aerospace?<br /><br />Anyone have any additional thoughts on the tethered fueled lift stage. I eyeballed my original calculations and I still think they are correct. It would be pretty easy to test the tractor tether + high explosive system on a small scale. A modest sized helium baloon, a small solid propellent rocket, the explosive clad kevlar tether, all readilly achievable at modest cost.

 

[white_noise]

errrr..........don't get it. how does the explosion gives forward momentun to the tether?

 

[rogers_buck]

The principal is essentially the same as a rocket. Think of it as an incredibly long solid propellent rocket engine with an axial tether and shiethed by a thin outter jacket. Unlike a typical solid propellent rocket the fuel has an impossibly high energy content. Light off a shuttle-booster sized canister of that stuff and you would level the pad and everything around it for miles. By stretching the fuel for kilometers you make the energy of the detonation front manageable (or at least that's the idea) by stretching the specific impulse from milliseconds to tens of seconds. Sane amount of explosive, same total energy released, it just blows up slower.<br /><br />As the high-explosive blasts from the surface of the tether it is deflected rearward as it evaporates the outer jacket, there is a coupling of the released energy to the tether. I estimated 20%, but that is little more than a WAG. <br /><br />The "take-up" rocket on the baloon end of the tether takes the slack out of the accelerating tether. With respect to the stretched rubber band model, this is to make up for the fact that the tether is largely inelastic. Other than that, the stretched rubber band is a good model. The propagating detonation point being analagous to energy being released from the stretched elastic bonds.

 

[chris_in_space]

I can see 2 major problems.<br /><br />First I am really not sure that the tether will be strong enough even if it's made of nanotubes. I mean you have calcultaed the energy realeased by the explosion but I really don't think that the tether will be able to "absorb" all that energy. It's like putting a bomb behind a car calculating the energy released and the speed the car would obtain with that energy and say that will be the final speed of the car. It's quite probable that the car will blow into pieces right from the beginning. So the important factor is will the car resist not is there enough energy in the bomb.<br /><br />Second, even if the tether is strong enough there would be a major problem. With so many energy into the tether it would pull too hard on the payload, exercing hundreds if not thousands of Gs on the payload and I can't think of too many payloads capable of resisting accelerations of 100 g. Plus the tether pulls so hard and with such a high acceleration that the paylad will reach incredible speeds while still very close above the ground so it's very likely that the drag of the atmosphere will make everything melt down.<br /><br />It's a little the same thing as putting a payload into a cannon and trying too shoot it into space like Jules Vernes wanted. I'm sure that today we could build cannons powerful enough to shoot a payload into orbit but before it will reach it, there would be nothing left of the payload.<br /><br />But still found this an original and interesting idea (I mean we really need people thinking of something else than the most complex launch systems using the most advanced quantum physics). <br />Who knows maybe once we get on the moon or in a high earth orbit and we want to send a payload to Mars we could use this tether system with much lower strain in the tether in order to have more reasonable accelerations. But I really doubt that from the orbit or from the moon there won't be a more efficient way to move things round.<br /><br />

 

[blairf]

Pah! you lightweights - this is the cheapest way to orbit<br /><br />0 - Have Breakfast<br /><br />1 - Winch 500 tonne cylinder of lead (it sounds a lot but it is only c50 cubic meters) to top of very very tall cliff There's a 6000m cliff somewhere in the Himalayas I think<br /><br />2 Build a small rocket that can get from 4000 meters per second up to the required 11000 meters per second - don't bother about air friction. This baby is going to be way out of the atmosphere when we fire it up.<br /><br />3 Build yourself a VERY strong 10 times multiplying block and tackle on the top of your mountain - make sure it is securely fixed into the mountain.<br /><br />4 Lower your rocket and lead weight about 10% of the way down your cliff - obviously still attached to the block and tackle. Make sure you do NOT snag your ropes on anything on the way down.<br /><br />5. Retire to safe distance<br /><br />6. Drop weight <br /><br />7. Watch in awe as planet earth accelerates your lead weight up to about 400 meters per second<br /> <br />8. Pray your block and tackle have been well made. <br /><br />9 Gaze in wonder as your 10 tonne rocket soars into the sky with only its sonic booms as acoustic accompaniement.<br /><br />10 About 10 seconds later as the rocket leaves the atmosphere strain your eyes to catch the rocket ignition<br /><br />11 Go back to retrieve your weight, pick up another rocket. Have some lunch and then prepare for your afternoon launch.<br /><br />You gotta love medieval rocket science!

 

[rogers_buck]

The tether is important, but I don't think it has to be all that exotic. The energy being developed by the explosion has a roughly 25 second impulse. That's just for off-the-shelf stuff, I'm sure engineered impulses of 30-40 seconds could be achieved. So, in other words, about the same amount of energy as a shuttle SRB is developed over about the same amount of time. Accelerations would be manageable just as they are in the shuttle payload bay. <br /><br />This is really just a first stage replacement technology. Asi I indicated, about 10-15% of the 50km tether is actually traversed under power. As you say, that is in the atmosphere. This tech would simply be to get things moving in the right direction and at a good clip.

 

[chris_in_space]

You really believe in your idea so hard ?!<br /><br />So if some common sense isn't enough here are the numbers (it's all solid mechanics). Lets take a sort of carbon nanotube tape with a cross sectional area of A = 1 square meter (which is enormous but a smaller area would make it even worth). So material science says this tape can store elastic energy of Ee = 0.5*E*A*epsilon^2 where E is Young modulus (for carbon nanotubes something around 1e10 Pa ) and epsilon is the relative distorsion (or lengthening) of the tape. So if want to store the amount of energy you have given (159631950000 Joules), soving the above equation gives you an epsilon of 1.23 . This means the tape, when it is streched at the maximum, will be 23% longer than in it's initial state. I don't know if you realise it but this is enourmous (a steel rope for example breaks allready from an epsilon of about 0.001%). So I'm absolutely sure that at this epsilon the carbon nanotube tape will break. I can figure out the exact numbers but still let's suppose it won't break. Let's calculate the stress in the cable by relation stress=E*epsilon with E still the Young Modulus. You obtain a stress of 1.23e10 Pa. Ok so know let's see what happens when this stress arrives to the payload. Since the cross sectional area is 1 square meter it results in a force of 1.23e10 N. which will have to lift a payload of let's say 10000kg. That makes an acceleration on the payload of 1.23e6 m/s^2 or around 1.23e5 gees. OK so now I let you find a payload that won't have anything after such an acceleration and won't burn in the atmosphere when traversing the atmosphere at 1.23 km/s after just 0.001 seconds after take off...<br /><br />Hmm... Just wondering if all this was ment to be serious and if this post was all worth in order to refute "medieval rocket science"<br /><br />So roger I'm sorry if you really believed in your idea but it really isn't working...

 

[rogers_buck]

Good analysis, but I think your looking at the wrong thing. The analogy of the stretched rubber band with elastic energy was just an analogy. My assumption is that the tether is inellastic.<br /><br />Energy is supplied by the total weight of the explosive spread along the tether. The explosive front propogates up the tether and takes 20-25 seconds to burn-off. So the impulse of the total energy is 20-25 seconds.<br /><br />Nothing gets stretched (of course there is some stretching), the rocket at the far end is only there to keep the tether straight as it is accelerated from the bottom up.<br /><br />I will not be surprised if there is a problem with what I'm proposing, but your good analysis was (I believe) based on a wrong assumption about what I'm proposing.<br />

 

[chris_in_space]

I'm sorry if I misunderstood you. <br /><br />I think it was because of this passage:<br />"The resilience or blast propagation speed of the explosive is tuned to the speed of sound in the tether. As the explosion propogates the energy is absorbed in the <font color="yellow">elasticity</font>of the tether and thrust is ultimately imparted to the payload. By the time the explosion front propagates to top of the tether the payload (and it's end of the tether) is doing Mach 25. <br />"<br /><br />By the way if the tether is inelastic there's no sound in it so no speed of sound.<br /><br /><br /><br />But than I don't undrstand something. I don't see the difference with a rocket. I mean you replace the TNT by the fuel. Both "explode". You replace the cable by an engine nozzle. Both are quite inelastic and both "propagate" the energy of the explosion into forward motion of a payload...

 

[rogers_buck]

Yes, this is not my area so I am clumsy talking about it. Sorry for the elastic emphasis. The stretched rubber band analogy is pretty useless with respect to modeling, just mental imagery.<br /><br />There is no difference between the concept and a rocket that I can tell. The advantages are that PETN packs a lot more energy than the solid fuels employed today. The only reason you cant use PETN in a rocket is that it would get blown up. <br /><br />Project Orion (I think) was to use nukes as explosives and demo'd the concept using high-explosives. Firing them out the back. Small amounts of explosives detonated at a time.<br /><br />By stretching the fuel down a strong tether you get advantages of safety, cost, and simplicity. But in the end it is just a rocket towing a payload skyward.<br /><br />It seems that there might be a possible pony in that the point where the tether is at maximum tensile loading is identically the point where the tether could be at maximum compression as the explosive front detonates and ablates a cladding of the tether. I don't know if this could be usefull in making the tether virtually stronger or if it would make it weaker. Maybe shaping the forces at that level would be impossible on average so this is probably an irrelevent point.<br /><br />The reason I think this is the cheapest way to orbit (actually the cheapest first stage would be more accurate) is that the tether could be reused many times after cleaning and recladding. The cost of each shot would then be equivalent to the cost of that much primachord. I haven't bought primacord in 25 years, but it was only a few dollars for several meters when I did buy it.<br /><br />Let's say with automated processing that the cost of the tractor tether is $100/meter. That makes the total cost of a 50km first stage $5 million. If its 10 times that much it is still competitive.<br /><br />I think it is the safest launch platform because the fuel is all up front. If a failure occurs the supplim

 

[chris_in_space]

"By stretching the fuel down a strong tether you get advantages of safety, cost, and simplicity."<br /><br />Safety: maybe<br />Cost: I think it's really a little early to say<br />Simplicity: Hmm... Well you still have to find a way of transfering the energy from the explosion into forward momentum. Remember an explosion blows in all the directions so I don't see how the explosion around the cable will give it forward momentum. <br /><br />Well I hope you'll find some solutions but concerning myself I start losing my interest in the "project" since I start to see more and more (for me almost insurmountable) problems.<br /><br />But still it was nice to talk to you about it.<br />

 

[rogers_buck]

I agree. Also, you have to find a tether that can stand the heat, pressures, and still have the needed tensile strength. I don't think there is much question about the energy transfer, just the coupling. I don't know if the coupling is .001% or 50%. That part should be predictable at least.<br /><br />Thanks for the discussion.<br /><br /><br />Let's just say it has potential.<br />

 

[glutomoto]

err, I keep thinking that maybe the hardest trick with rockets is making the back and front ends go in the same direction.<br /><br />I know you addressed this problem with your idea of the take-up rocket. Wouldn't the take-up rocket need to, be able to accelerate faster than the prima cord, during all phases of the prima cord detonation sequence ? <br /><br />WoW that would really have to be one hot rocket, skip the prima cord and ride the rocket.<br /><br /><img src="/images/icons/smile.gif" /><br /> <div class="Discussion_UserSignature"> </div>

 

[bobvanx]

>take-up rocket<br /><br />Yeah, there's the rub. The tether has nearly zero rigidity, so the slightest inequality in the thrust of the burning end is going to make it merely snap around like a snake. And the take up rocket is going to be doing most of the work.<br /><br />Now, if you could get the front end of the exploding tether to direct the energy downwards, you might have something. In this scenario, the tether would be a micro-machined object, in effect a vast number of very small rocket nozzles. If each discrete nozzle were shaped like an aerospike ramp, and the explosive were wrapped around those, you just might be able to impart a forward vector.<br /><br />You'd run a real risk of the connection between one nozzle and the next burning through too soon.

 

[nexium]

Perhaps I'm missing the picture, but it appears that the explosive front is pushing the tether. The tether will fold into a tangled mess unless the tether is stiff and/or the balloon is acelerating the other end almost as fast as the explosive front is pushing at perhaps 100 feet per second per second = 3g average.<br /> In 25 seconds at that accelleration v = at = 100 times 25 = 2500 feet per second = 2000 miles per hour. S = 1/2 at squared = 50 times 625 = 31,200 feet altitude at the end of the burn. That is likely high enough to avoid over heating the payload at 2000 miles per hour. The balloon will be dragged by the tether and payload, as the balloon has too much air resistance to rise at 2000 miles per hour even at 50,000 feet which will be about the maximum altitude unless a 2nd stage ignites.<br /> If the gross weight of the first stage is increased several times, without changing the kinetic energy of the the explosive front, the system will only rise a few hundred feet to perhaps a speed of 55 miles per hour. That might work quite well, but is a puny first stage. Neil

 

[rogers_buck]

Good analysis, but I added a rocket on the baloon end. When the explosive is detonated the rocket at the far end takes up the slack in the tether (the baloon is released). The model I was using was PETN at 1gm/cm which has a verry high resilance (sp) ~20kft/sec, but I'm sure a more favorable formulation could be made. <br /><br />You might want to back up a few posts and look at my calculation of the total energy imparted to the tether. It is pretty respectable with a 20% transfer and total impetus of 25 seconds (time it takes for the PETN to burn off). Then, second stage time but your hauling the mail!

 

[rogers_buck]

>> I know you addressed this problem with your idea of the take-up rocket. Wouldn't the take-up rocket need to, be able to accelerate faster than the prima cord, during all phases of the prima cord detonation sequence ? <br /><br />Yes, as fast as the tether and the payload. Not as fast as the explosive front which is traveling at 20k ft/sec or so. Its similar to the nozzle velocity not being the same as the payload velocity. The tether and the payload accelerate through the 20+ seconds the tether is exploding.

 

[rogers_buck]

>> Yeah, there's the rub. The tether has nearly zero rigidity, so the slightest inequality in the thrust of the burning end is going to make it merely snap around like a snake. And the take up rocket is going to be doing most of the work. <br /><br />Not really. The explosive front is moving verry fast, so defects will be of extreme high frequency. A systematic defect that is periodic might lead to harmonic problems, but defects should be random. Defects would have to be controlled only to the level that shear becomes a problem. I don't think that's too critical though.<br /><br />The take-up rocket only has to be able to accelerate the tether at the rate the payload end is accelerating. The weight of the tether is constantly being reduced and the definition of payload is constantly increasing as the explosive front propagates up the tether. So the take-up rocket accelerate less and less of the tether as the explosive burns off. It never has to accelerate the payload itself or that portion on the tether behind the detonation point.

 

[rogers_buck]

The stretched rubber band analogy has its limits, so I'll make another one that I think is more appropriate. <br /><br />Imagine 2 meters of fish line with a fishing pole balanced as a lever tugging on the line from the top and a 1kg weight sitting on the ground tied to the fish line. Your goal is to attempt to pick up a 1kg weight with greasy fingers slid along the fish line. The fingers slip along the fish line but there is some friction so the weight starts to rise. The pole at the far end tips and keeps the line taught. Eventually your fingers will arrive at the top and you can provide no more lift to the weight - that's burn out of the first stage.<br /><br />The friction between your fingers and the fish line is the energy transfer between the explosive and the tether. If it is high you can really get the weight moving and you'll need a ladder to keep pulling on the fish line. If it is low, you reach the top without hardly having budged the weight.<br />

 

[glutomoto]

<font color="orange">Re: Its similar to the nozzle velocity</font><br /><br />Since you brought up nozzle, I must have missed how did you say your system directs the enery release to actually be "thrust" on the desired vector ?<br /><br /><font color="orange">Re: Yes, as fast as the tether and the payload </font><br /><br />So, in 20+ seconds, the take up rocket has to accelerate from balloon speeds to ... what was that payload melting speed you calculated ? is that possible with a rocket ??<br /><br /><br />Also does the addition of the take up rocket still keep the cost low enough to be the cheapest way ever to orbit ?<br /><br /><img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[rogers_buck]

The energy gets directed along the coaxial tether for the following reasons:<br />1) The explosive is essentially a stack of small pancake charges that are stacked the length of the tether. Such shaped charges create blast waves normal to the detonation surface.<br />2) The outter cladding of the explosive which is vaporized instantly tends to deflect the blast wave in the unconstrained dimension. This should partially compensate for the small size of the pancakes.<br /><br />Let's say you tune the tether explosive so that you get comparable performance to an SRB when scaled to your payload mass. To rephrase your question in that light, can a rocket at the far end of the tether accelerate the integral of the dead weight of the tether down to -0- mass at the same rate that the payload + integral of -0- mass to dead weight of the tether sans explosive is being accelerated? The answer is of course.<br /><br />As to whether the requirement for a take-up rocket spoils the economics, I don't really know if any of this is economical. I think the take-up rocket would be of the same scale as an escape rocket with a variable fuel grain. Full throttle at the start to -0- in a steady curve. The tether+PETN doesn't weigh all that much and you have to accelerate less and less of it as the seconds tick by. It is also possible to reuse the take-up rocket or at least elements of it. So, anybody's guess.<br /><br />I did some economic postulating in an earlier post, and there was a sizeable advantage in cost even if my guesstimages were off by a factor of 10.<br /><br />I believe most of the risk in this concept rests on the tether. Were asking it to be surrounded by high temperature compression wave down its entire length while experiencing tensile loading. I don't know that CNT has the right properties or if such a material will ever exist for the 50km, 2-3G, 10-50 tonnes parameters of this system. It would be interesting to get some comments from someone who has studied the tethe

 

[crossovermaniac]

What about Robert Truax's big dumb booster concept like the Sea Dragon?

 

[rogers_buck]

From what I read about it, the Sea Dragon looked pretty cool to me. Definately out of the box.

 

[rogers_buck]

Thought I'd bump this old thread to take things up with a variant I mentioned in the Missions forum.<br /><br />The idea would be to use this technology as a 3rd stage to leave LEO. A small rocket would play out a few km of the special primachord with a kevlar strenght center chord. It seems this would be pretty feasible since the thrust requirement would be lower and thereby remove the requirement for CNT type materials.<br /><br />You would want to use an explosive that would give you a resilance of 1km/sec so you didn't over stress the chord or the payload, yet you would still want a high energy output from the explosive to keep the weight down. So my guess is the best tradeoff would be for a light grain and a long chord. Maybe a conductive wire could be used to deploy the chord in the earth' magnetic field so the rocket at the far end wouldn't fire until the chord was fully extended. Something along those lines...<br /><br /><br />

 

[virusxp]

I am by no means a specialist, but why exactly you do think, that TNT (or any other high explosive) has more energy to offer that the traditional booster propellants?<br />According to wikipedia, TNT has energy density of 4.184MJ/kg. Even diesel fuel beats it at 47, not to mention liquid hydrogen, which has 120 MJ/kg.<br />The hydrogen has pretty poor energy density per volume, though.