The majority of a rocket's fuel is used shortly after ignition, when the rocket is slowest and heaviest. I propose a launch tower that is about 1km high, which continuously fuels the rocket in the first stages of ascent. This can be done either with uncoiling fuel lines or series of injection systems along the length of the tower. when it reaches the top of the tower it starts it's untethered flight under it's own power, having received an initial boost.-- simonj, May 01 2009 Variation on the theme http://www.hammache...9-sMCFRBafgodOLUAUgImagine rocket sitting on a kind-of "half stage" that applied initial lift and velocity. The first stage would ignite as the half-stage reached its limit of hose-length. [Vernon, Feb 24 2015] What if the fuel hose was attached to a pulley making it "neutral bouyancy" so that the rocket could carry the unfurling hose without additional weight imposed?
Still, that period where most of the fuel is burned is more like 10km than 1.-- DIYMatt, May 01 2009 Yep the hose can be attached to a motorized gantry that matches the speed of the rocket. And yes 10km would be better but unworkable. 1km should give a significant boost to make it worthwhile. Bigger payloads to space.-- simonj, May 01 2009 Your pumps at the bottom are going to have to supply enough power to lift the fuel that distance, at sufficient flow rate to make it worthwhile. Your hose is going to have to tolerate the pressure involved. Your disconnect is going to have to come off without leaving an explosive fuel cloud for the engines to ignite. And the hose is going to want to keep going - ballistically to several times the altitude at which it released. (Point of release is the highest attained speed, note...)-- lurch, May 01 2009 1. The fuel could be siphoned in rather than pumped 2. high pressure hoses are common 3. An iris-seal on both the hose and the fuel tank should work 4. The hose will not keep going once it reaches the end, it will be secured at one end and damped at the other.-- simonj, May 01 2009 hmmm - but if you're building a 10km tall launch tower it might be easier to have the rocket accelarated by something on the tower - like an electric linear motor-- hippo, May 01 2009 rube goldberg, rocket scientist.-- WcW, May 01 2009 How much fuel would you save by simply starting the rocket from the top of the 1km high tower?
Or perhaps ask for 1,000 volunteer bungee jumpers to jump from the tower and grab handles provided on the rocket when they get to the lowest point of their bungees?
Note that the tallest building to date is the Burj Dubai at 818m. At it's base it covers an area 'the size of Watford' (UK metaphor for anything larger than we wanted it to be).-- Twizz, May 01 2009 // 'the size of Watford' (//One Watford = 23 milliWales, or 98 London Bus squared.Rather than build a ridiculously tall tower, why not launch from the bottom of a really deep hole?-- coprocephalous, May 01 2009 I thought this was a way to feed actors before they do balcony scenes ...-- Aristotle, May 01 2009 //How much fuel would you save by simply starting the rocket from the top of the 1km high tower?
then you're starting with zero inertia, so you're back to square one. The beauty of my proposal is the rocket is being fueled while it's accelarating.
And you could still have the solid boosters that detach a couple of minutes after clearing the tower.-- simonj, May 02 2009 I agree with hippo's comment - if you're going to use an external power supply to accelerate the rocket, putting fuel through its motors is a bad, bad idea. Much more sensible to accelerate it more directly. Personally, I'd make the launch tower into a tube and accelerate the cylindrical rocket using a compressed gas, airgun-wise.-- MaxwellBuchanan, May 02 2009 Actually, I wonder if this would make sense for the N-Prize? A purely gun-type launch is unrealistic, but I wonder if a 100m tall potato-gun made from drainage pipe could give a significant velocity boost to an otherwise conventional rocket? The gun itself would be ground-equipment and so, apart from fuel and refurbishment costs, wouldn't contribute toward budget. Also, an N-Prize rocket will stand much higher G's than would be possible with a human-carrying projectile or a larger rocket.-- MaxwellBuchanan, May 02 2009 Just thinking about people's comments...it might be an idea to accelerate the "launchpad" up the tower using booster rockets rather than refueling the engine. there are many possibilites within the concept.-- simonj, May 03 2009 [Bigs] yes, that's a 'rockoon', which is well baked.-- MaxwellBuchanan, May 03 2009 I would imagine that the ongoing amount of energy to maintain (never mind manufacture) this //a launch tower that is about 1km high,// would negate any energy saving. I think that that is the main problem with permanent high altitude platforms, removed from the equator. Launching from the equator gives us the most bang for our buck, so to speak. So you need to be looking around there. To move machinery and parts, to either use an existing geostructure, or create your own, will be massively energy intensive. Not likely to be recouped by several hundred launches.
For the n-prize it might work. You may be able to invest several hundred quad-baziltrillion into the platform, as it won't effect your claim to the prize. Heck, you could build a tower of babel 103 kilometers high and throw the fucking projectile into space...-- 4whom, May 03 2009 [4womb] If you look up a little, you'll see that I suggested a 100m tower. The idea is to find a reasonable compromise which gives you some useful velocity at a reasonable cost. I would imagine that a 100m tube could be put together fairly easily, even if sunk in a borehole. It would not be operated at gun-type pressures, and so with luck something like irrigation pipe could be used. OK, I'll do some numbers and get back in a bit.-- MaxwellBuchanan, May 03 2009 [4whom] The goal is not energy savings, it's lift weight savings. Assuming a 500m tower and a linear accelerator capable of 2g, you get a tower departure velocity of 140 m/s. This is tiny compared to the "escape velocity" a true orbital gun would require, but it is not insignificant. Assuming a shuttle orbiter as your launch vehicle, that extra 140 m/s represents a savings of about 350000 kg in fuel weight required on the launch pad (to low earth orbit). This can be treated as extra payload capacity, higher orbit capability, or what have you.
The point is if you don't have to carry all your fuel with you, you save not only the weight of the fuel, but the weight of the fuel required to lift that fuel as well.-- MechE, May 03 2009 OK, numbers. Start with 100 metres of aluminium irrigation pipe, 6" diameter. Currently on eBay in 10m lengths for $70 per length, so $700 total. Let's assume you can weld or otherwise joint the pipes to a reasonable standard.
Next, sink a 100m borehole, maybe 10 inches in diameter. Borehole machines do this routinely through porous rock such as chalk, with an extreme degree of verticality and straightness. We had one dug in our garden a while ago for a few hundred pounds, so say $1000 if you pay someone to do it for you.
Next, seal one end of your 100m pipe, but run a fuel line into the hole, and some form of igniter (a spark plug). Also, run three bolts through the pipe in a triangle, 40m from the sealed end.
Now sink the whole thing into the borehole, and pour concrete or the material of your choice around it to stabilise it and add a little reinforcement to the walls.
Now lower your rocket (a shade under 6" diameter, and possibly with a sabot or wad) until it sits on the three bolts.
Fuel the lower chamber with the flammable gas of your choice. Calculate the fuelling so as to give a pressure of 200 pounds per square inch (which I'm guessing is OK for concrete-wrapped irrigation pipe) when the fuel detonates and pressurizes the 40m of space below the rocket.
Press the button.
Assume the rocket weighs 200 pounds, for argument's sake. Pressure on the base of the rocket will be 5,652 pounds, giving an acceleration of about 30G to begin with. As the rocket climbs, the space behind it gets bigger and the pressure drops, dropping to 40% of its initial value (or about 80psi) by the time the rocket clears the tube, and hence a final acceleration of about 10G.
So, how much velocity do we get? About 135m/s, or 300mph. Which is a decent head-start, and will reduce the fuel weight needed on the rocket significantly.
If we go for a 100 pound rocket, and can pressurize up to 300psi, then we get a final velocity of about 240m/s or 536mph. (Note: the bursting pressure of a plastic drink bottle is something like 200psi.)-- MaxwellBuchanan, May 03 2009 [MechE] lift weight saving and energy saving are not horses of different feathers. I agree there are certain multipliers at certain escape altitudes , Burt Rattan agrees too (you could even evacuate the 500m verticle tunnel and get more out), none that cancel completely the energy required to construct and maintain such a structure, specifically at the equator, in fact Burt and SpaceShipOne have completely debunked the idea of a constructed launch tower that breaks even (ever). An idea that they were following, I might add.
No matter how far you go up, at the equator you still get more bang for your buck. And even if you go up at the equator you face problems.
[Max...] I saw your previous annotation to n-prize, and included the anomaly of the 103 kilometer tower for show. The idea being for a totally unreasonable cost you could launch for *free*, according to the prize rules.-- 4whom, May 03 2009 Who cares about the energy needed to build/maintain the thing? The rocket certainly doesn't.-- simonj, May 03 2009 [simon...] That's analagous to, in your terms, facebook being ambivalent about who makes money off their platform. Jesus, does the global meltdown mean that we explain capatalism all over again?-- 4whom, May 03 2009 [4whom]No it isn't, it's like not inventing the internal combustion engine because there are no petrol stations. The two are unrelated.-- simonj, May 04 2009 [simon...] that philosophy got us into this energy mess. UUustUrgg.
Ok, besides the fuel that an engine will combust, there is energy used in the creation of the engine. And energy required in the aquisition of fuel. ICE blocks got us so far, Solid boosters got us a little further. Liquid boosters even further.
A 1000m, or 500m, verticle tube will get us how much further (remember this will be part of the engine). I have a small hat here, together with utensiles, and some flavorants. I will eat this particular hat, if you can conclusively prove, that your engine (please take into consideration construction and maintanence (of no significant value to you) costs) will beat out a solid fuel booster and liquid booster engine, for the first ten kilometers. I don't think anyone is arguing the first two kilometers.
My hat, your arse...-- 4whom, May 04 2009 You're missing the point. You can still use the same solid booster as in your preferred configuration, only now you have an added boost via the launch tower. Would you like some salt with that hat?-- simonj, May 04 2009 The major advantage of a ground based system is simply that it doesn't reach the point of diminishing returns on additional lift capacity. At some point adding another SRB to a ship doesn't add any net thrust to the system. Adding the equivalent in thrust from a ground based system always does. Other factors such as frequency of use (daily amortizes maintenance a lot more cheaply than monthly) also play a role. As such, I would suggest it is impossible to say that a catapult system can't pay for itself (or even the original idea, partial lift refueling). Since it makes possible something that is not presently possible, it can't be an apples to apples comparison. I also have to clarify that, at present, space travel is going to be an economic and energetic sink. Until such time as we have space based industry it will remain so. That being said, if we don't sink the cost now, we never will see a return. Like any other activity, a certain development time is required. Unfortunately due to the complexity and handling of space flight, that development time has been on the long side.-- MechE, May 04 2009 The argument over economics and environmental impact is fine, but (at least for me) it's not the point. Nobody is envisaging launching the Shuttle this way.
In terms of an N-Prize launch, a gun-type launcher can shift the cost partly away from the rocket and into a ground-based facility, which is desirable in this context. Nor is the overall cost that great for a 100m "spud gun" of the type considered here.
Whether the advantage of a "free" (in this context) 1/3rd of a kilometer per second is worth the hassle for the N- Prize, I'm not sure. But it's a possibility.-- MaxwellBuchanan, May 04 2009 If we're building a 1km tall refuelling station, we might as well build a 1km tall launch station, that propels the rocket through other means.-- kolinko, Feb 22 2015 if you're going to build a 1 km tall tower to continuously fuel the rocket through the first 1 km, why not just put the rocket on the top of the tower?
Anyhow, with regard to all the other compressed air launch jiggery pokery, I believe the Soviets were fond of shooting missiles out of silos with compressed air.
The Soviets were fond of many things.-- bs0u0155, Feb 24 2015 Vodka seemed to be the main one ...
// why not just put the rocket on the top of the tower? //
It's because of this thing called "physics".
If the rocket starts from rest at the top of a 1km tower, it needs to accelerate to orbital velocity v.
If, however, the rocket leaves the tower with a departure velocity u, all its fuel can be expended to to achieve a final velocity of u + v for the same fuel load, or for a given value of v, a bigger payload because less fuel is needed.
Because the vertical acceleration of a rocket under gravity is a second-order differential equation (because as the rocket accelerates, its mass also decreases, giving a greater acceleration for the same thrust) a very high proportion of the fuel is consumed imparting the initial acceleration to the large amount of fuel needed to lift a large amount of fuel ... hence the problems of building a practical SSTO launcher with a useful payload.
In effect, the first stage of a multistage rocket acts as the "tower" in this idea, lifting the second stage and above to a high altitude and a greatly increased velocity before the second stage begins its work.
A "V-1" type launch ramp with a parabolic profile and a rocket-sled or maglev carrier would appear attarctive in terms of imparting initial momentum, but as [MechE] points out, at the end of the day you're going to bankrupt yourselves hauling stuff out of Earth's gravity well using primitive reaction engines. Serious orbital shuttles have about as much resemblance to an SRB as your space shuttle had to a first-century Roman Trireme.-- 8th of 7, Feb 24 2015 I think that this Idea comes from the feeling that the rocket has to become "unstuck" to get moving.-- FlyingToaster, Feb 24 2015 In a very minimal way, it does, as the retarding gravitational force diminishes as the rocket moves away from the planetary surface - but on a macroscopic scale, that effect is small compared to, for example, air resistance, or forgetting to cut the string that holds the rocket to the launch pad.-- 8th of 7, Feb 24 2015 random, halfbakery