Space Elevator or Scramjet?

Tom Kalbfus · 2007-11-17 10:04:38

Yes, a space elevator is basically a "building" you go up while a scramjet is a vehicle. A scramjet may make access to space cheaper, but that is the same thing they used to say about the Space Shuttle. Scramjets will no doubt require maintenance between flight, the open question is how much maintenance will it require, it turns out that the Space Shuttle actually requires alot more maintenance than was advertised by NASA. It was in fact the maintenance that maid the Shuttle expensive. Now what does the scramjet advertise? It is a single stage to orbit vehicle, no large parts need to be manufactured for every flight, for the Shuttle, the only large part that needed to be build and discarded was the external tank, and NASA said external tanks were no big deal anyway. The worst case scenario for the scramjet is that it is a maintenance hog, that it will require thousands of workers to swarm over it between flights, to test and retest its components to make sure they are flight ready. Another nightmare is that the scramjet might be allergic to bad weather, and that on the runway, the launch crew will keep on finding technical glitches that will scrub the launch, and then have to wait for the next window of clear weather at both the launch site and the emergency landing site should something go wrong during ascent.

A space elevator car is the vehicle that might need maintenance, it is basically an elevator car, it doesn't travel at hypersonic velocities through an atmosphere, it must have a functioning life support system of course if its to carry passengers, but it doesn't undergo as much stress as a scramjet does with each journey, so I think maintaining the cars will be less expensive than maintaining a scramjet. We could probably start building prototype space elevator cars right now. What would be really interesting would be a maglev elevator car, that would be a car that requires no cables and that magnetically levitates up a verticle track. If we could build such a thing, we can go up a space elevator in hours instead of days. What I've seen liftport propose is a rollers on ribbon design, this thing would take about a week to ascend the full height of the space elevator, and that would be one week the space elevator would not be available for lifting other things, as it can only lift so much at any one time. A maglev elevator would be a great asset, and could make use of centrifugal force to fling payloads to Saturn if so desired.

Terraformer · 2007-11-17 11:05:40

There's one technology we have right at this very moment that would be really cheap to develop, we already have the craft needed, the only thing to build is the base. Can't guess it? It's a suborbital helium (or hydrogen, seeing as there's no oxygen to burn) space station. It would float on top of the atmoshpere. Suborbital craft could fly up, dock, and the passengers could transfer to an orbital craft. If the calculations are worked out an orbital craft could land without needing loads of heat shielding (it wouldn't be going through the atmosphere.) Assuming it costs 5 million (probably an overestimate) to mass produce a suborbital craft that could carry, say, either 25 passengers or a cargo pod; and 50 million (again probably an overestimate) to build the station, you just need to build the orbital craft, get them up there, and we can start to get to work on missions to the Moon and other planets. Personally I'm getting bored of sitting at my computer talking to you random people when I should be doing something.

Tom Kalbfus · 2007-11-17 12:55:56

Balloons don't float on top of the atmosphere, they float within the atmosphere.

Terraformer · 2007-11-17 13:17:43

Well yes, but it would still be floating just within the atmosphere and it would only be hydrogen and helium outside. Radiation protection is a must.

Terraformer · 2007-11-26 10:20:12

Which in my design would mostly be provided by the hydrogen in the balloon.

samy · 2007-11-26 16:12:42

Let's assume an ideal best case scenario: a balloon of mass zero, containing a vacuum (the best lifting "gas"). So, neither the balloon canvas nor the balloon contents add any additional weight to the system (as I said, idealized).

International Space Station mass is approximately 230 000 kg.

In order to lift an ISS with a balloon, you would need buoyancy B that is equal to the station's weight W.

W = B
m(ISS)*g = rho(air)*V(air)*g (the gs can be divided away)
V(air) = m(ISS)/rho(air)

If we take the displaced air in spherical shape

4/3*pi*r^3 = m(ISS)/rho(air)
r^3 = m(ISS)/rho(air)*3/(4pi)
cubic root that

Taking air densities at particular altitudes from http://en.wikipedia.org/wiki/Barometric_formula

20km altitude - 0.088 kg/m3 - 85m radius balloon
32km altitude - 0.013 kg/m3 - 161m radius balloon
47km altitude - 0.00143 kg/m3 - 337m radius balloon
51km altitude - 0.00086 kg/m3 - 400m radius balloon
71km altitude - 0.000064 kg/m3 - 950m radius balloon

In real conditions the radius would have to be somewhat larger to compensate for the weight of the balloon canvas and the gas contained within the balloon. But I think it's a close enough estimate to say that lifting an ISS to 70km altitude would require a balloon roughly 2km in diameter. (Or more sensibly of course, multiple balloons with the same volume, to avoid catastrophic failure.)

Incidentally, it would mean pretty much not being able to look (or launch) upwards at all, as pretty much everything above you would be taken up by the balloon. You could only look sideways or downwards. Looking up, anywhere you look is balloon.

cIclops · 2007-11-26 17:07:22

Even at an altitude of 71 kms a balloon has to withstand an external atmospheric pressure of about 4 N/m² or it will implode if it encloses a vacuum. Of course at sea level the pressure is about 100,000 times higher at 1 bar - a very strong therefore heavy material is needed to enclose a vacuum at sea level. Typically steel.

Vectran is used in high altitude balloons, it has a weight of about 0.08 kg/m² and has to contain the internal pressure of the gas, usually helium. The record for this of type balloon is over 50 kms.

Let's see how much a 1 km radius balloon made of Vectran will weigh: 1000*1000*4*pi*0.08 kg - that's about 1000 tons!

The calculation is similar for steel, but with a slightly higher weight/m²

samy · 2007-11-26 17:45:52

Antius · 2007-11-28 06:27:44

So to put it crudely, the higher up you go, the more air you need to displace in order to support any given mass from the balloon. This would appear to make balloons inpractical for reaching heights above 30-40km.

The next question is: how high do we need to go and how thin does the air need to be, in order to operate a mass driver capable of launching payloads to orbital speed? At Earth's surface it is obviously not feasible.

Would it be feasible at 32km?

Air resistance = K x air density x V(squared).

At 32km: air density = 0.013 kg/m3 (1% Earths surface).

Given that meteorites are fire balls at this altitude, it is probably safe to say that even at 30km, air resistence would create too much resistive heating for a mass driver to work. Air resistance would melt the barrel of the electromagnetic launcher before the projectile even left the launcher.

The only way around that would be to use a camera-shutter technique. The mass driver would be sealed and evacuated with vacuum pumps and a shutter would open microseconds before the projectile reached the end of it.

samy · 2007-11-28 06:47:23

The problem with a mass driver isn't really the air friction. You'll puncture through the thick part of the atmosphere so damn fast you won't really have time to heat up. At escape velocity you'll hit the stratosphere in a second and the mesosphere in less than 5 seconds. T+8 seconds you'll be out of the mesosphere and out of the danger zone. So if the heat shields can hold out for a mere 8 seconds, you're golden.

The real problem with mass drivers is the enormous g-forces required for them to work. There's just no feasible way to ever man-rate them on Earth, unless we manage to build a space elevator and run an acceleration rail along it.

Mass drivers will primarily be of use to deliver materials that are very resistant to high (200, 2000, even 20000 g's) accelerations. Meaning, stuff that doesn't have any appreciable internal structure to break. You could use mass drivers to deliver water, gas, raw metals to orbit. People? Never. Anything with a fine structure, such as electronics? Unlikely, but perhaps possible with durable enough components.

cIclops · 2007-11-28 08:19:48

The problem with a mass driver isn't really the air friction. You'll puncture through the thick part of the atmosphere so damn fast you won't really have time to heat up. At escape velocity you'll hit the stratosphere in a second and the mesosphere in less than 5 seconds. T+8 seconds you'll be out of the mesosphere and out of the danger zone. So if the heat shields can hold out for a mere 8 seconds, you're golden.

AFAIK air friction IS the problem. To retain an escape velocity of 11 km/sec or even an orbital velocity of 8 km/sec after passing through the atmosphere, a body initially requires far more energy and therefore speed. At these ridiculous hypersonic speeds every known material flashes into plasma. Chunks of rock or ice could survive after ablating their surface and exploding into many pieces. These pieces would have a wide range of trajectories, this would be extremely wasteful of energy and quite spectacular

Antius · 2007-11-28 09:03:54

The problem with a mass driver isn't really the air friction. You'll puncture through the thick part of the atmosphere so damn fast you won't really have time to heat up. At escape velocity you'll hit the stratosphere in a second and the mesosphere in less than 5 seconds. T+8 seconds you'll be out of the mesosphere and out of the danger zone. So if the heat shields can hold out for a mere 8 seconds, you're golden.
AFAIK air friction IS the problem. To retain an escape velocity of 11 km/sec or even an orbital velocity of 8 km/sec after passing through the atmosphere, a body initially requires far more energy and therefore speed. At these ridiculous hypersonic speeds every known material flashes into plasma. Chunks of rock or ice could survive after ablating their surface and exploding into many pieces. These pieces would have a wide range of trajectories, this would be extremely wasteful of energy and quite spectacular

Any object launched in this way would clearly need to be mechanically strong and protected from the heat by a thick ablative lining.

As samy pointed out, the mass driver is only likley to be useful in launching payloads that do not have fragile internal structures; ie, bulk materials such as carbon, water and metals.

A mass driver would be a potentially cheap(er) way of launching basic commodities into space that could not be derived from Lunar materials. It is likley to be useful in the early yeras of space manufacturing, before asteroid mining is developed on a significant scale.

cIclops · 2007-11-28 09:49:31

Yes everything would have to be encapsulated in a thick ablative coating. Then the problem is to accelerate these packages to absurd velocities (20 km/sec or higher) without destroying the launching device. Furthermore, gathering these packages in orbit would require a collector with propulsion.

John Creighton · 2007-11-28 13:14:34

I’m sure the cost and technical challenges and cost would be immense but if an object is accelerated 32 km and released 32 km above the ground as Antius suggested then would the G forces really be that large?

cIclops · 2007-11-28 13:45:02

An enormous amount of energy is needed to accelerate an object to 8km/sec, 1 kg would need 0.5*1*8000^2 or 3.2*10^7 Joules. It's about 100 times the energy needed to lift it to 32 km (1*32000*9.8 J)

If this is a mass driver it will need a long vacuum pipe with associated coils, vacuum pumps, shutters etc etc. To keep the structure to a feasible length, say 100m, it would need to accelerate the package in 0.025 secs at 200,000 G thus needing 800 kW of power assuming no losses due to friction. This device has to be suspended at 32 km together with its power supply and a collection system to receive and load the payload. Anyone want to bid for the contract?

John Creighton · 2007-11-28 14:22:35

An enormous amount of energy is needed to accelerate an object to 8km/sec, 1 kg would need 0.5*1*8000^2 or 3.2*10^7 Joules. It's about 100 times the energy needed to lift it to 32 km (1*32000*9.8 J)
If this is a mass driver it will need a long vacuum pipe with associated coils, vacuum pumps, shutters etc etc. To keep the structure to a feasible length, say 100m, it would need to accelerate the package in 0.025 secs at 200,000 G thus needing 800 kW of power assuming no losses due to friction. This device has to be suspended at 32 km together with its power supply and a collection system to receive and load the payload. Anyone want to bid for the contract?

I find the idea interesting from a technical point of view but I agree the economics aren’t there. All I need to do is consider how hard it is to justify the cost of a high speed train between two cities to see how dubious the economics would be for the device which is being proposed.

cIclops · 2007-11-28 14:32:10

There are many customers who will pay to ride a high speed train, how many people want to be accelerated at 200,000 G?

Government subsidies for high speed trains are offset by the cost of new roads or other infrastructure.

Antius · 2007-11-29 04:33:35

There are many customers who will pay to ride a high speed train, how many people want to be accelerated at 200,000 G?
Government subsidies for high speed trains are offset by the cost of new roads or other infrastructure.

That is true. But the device that we are proposing is only useful for launching bulk materials, so costs do need to be reasonably low: probably no greater than $100/Kg launched into low orbit.

The power supply would be by far the heaviets component. I can think of a number of possibilities:

1) Beamed power by microwaves, with the surfcae of the balloon acting as a rectena type device;
2) A small, open cycle gas turbine;
3) A conducting tether;
4) Ultra-thin solar cells across the surfcae of the balloon;
5) a small unshielded high-temp nuclear reactor

samy · 2007-11-29 06:54:48

The power supply need not be heavy at all. In fact, theoretically (assuming a vacuum and superfluidic mass driver tube, so no friction losses), you can build a mass driver powered by a shaft cranked by a single human. Friction losses will give us a minimum level of power input necessary, but it should be trivial.

Something accelerated at as low as 0.01g will reach escape velocity after 31.2 hours of continuous acceleration. 0.0001g (a ten thousandth of a g) would reach escape velocity in about a third of a year of continuous acceleration. Since (by Newton) F=ma, then the force needed is directly proportional to the acceleration we use. By putting the acceleration suitably low, we can put something to escape velocity with very little force (and resultantly, watts) needed. It will just take time, and an acceleration track of infinite length (i.e. a ring, like particle accelerators use).

So the launch costs would be the construction of the launch ring in the first place, plus electricity costs.

The friction losses will be pretty extreme inside the launch tube at high speeds, you'll definitely want to have a spherical delivery system which can roll inside the tube for rolling friction, i.e. shooting giant marbles upwards.

Not taking into account the non-trivial friction losses, accelerating 1000kg of mass to escape velocity inside our circular accelerator would require 0.5mv^2 of energy. Let's say we have a solar power plant 200m by 200m for a total of 40000m2, each square meter giving us, say, 200W of power. That would give us 8 megawatts for the acceleration during peak daylight.

t = E/P = mv^2/2P ~= 2.1 hours

So you could launch several 1000kg loads during peak daylight, let's say 6-8 hours are good light, so we could fire off 3-4 loads a day with our 8MW solar power plant.

So your costs are the initial ring/tube construction, the solar power plant construction, and then just maintenance. Basically, once it's built, launches are free of cost.

Friction's the only thing that messes this up...air friction inside the tube can be pretty much eliminated by vacuuming the tube...but rolling friction can't really be eliminated and it's gonna add up at high speeds.

(Incidentally, at escape velocity and assuming 10000g centrifugal acceleration pressing the sphere against the launch ring outer wall, the rolling friction would be approximately 500 kN of force dragging on the sphere, or about 50g of tangential backward acceleration to beat. Whatever force we're using to accelerate the sphere inside the tube would have to be able to push at least 50g of tangential forward acceleration in order to maintain the sphere's escape velocity until launch.)

Antius · 2007-11-29 07:16:15

The capital costs of the device are fixed - the operating costs are linearly proportional to launch volume. One that basis, in order to minimise cost per kg launched, you want the highest throughput possible, ie, as many launches per day as you can. Ideally, you would want to operate the device in steady-stream mode.

The higher the acceleration of the mass driver, the smaller its size and the lower its mass. This is an important consideration in the economics of this proposal, so we would probably want the highest accelerations that we could get away with.

On the basis of what samy has calculated, it is clear that power supply imposes difficult limitations on the throughput of the device, limiting launch rates to several tonnes per day for solar powered devices. A larger (nuclear) power supply is possible, but this will also be heavier. On this basis I conclude that the cheapest and most efficient power supply is beamed power from a larger generator on the ground. Power density at the rectenna on the balloon surface could be as high as several KW/m2 and the rectenna would be no more substantial than conducting tin-foil on the surfcae ofthe balloon.

Docking with the balloon to transfer payloads provides an additional complication.

Terraformer · 2007-11-29 07:27:09

I suppose you'd want three mass drivers: one for launching durable payloads (high acceleration, 1000s of Gs); one for launching things like computer components (medium acceleration, 100s of Gs); and one for humans and other fragile stuff (low acceleration, 10s of Gs).

samy · 2007-11-29 07:38:40

There's no realistic way to get the gs down to merely tens with a mass driver. Even hundreds of gs is pushing it.

Maybe as a nationwide megastructure, barely.

For the record, here are the numbers:

For a 10 g force pressing you against the launch ring's outer wall, you will need a launch ring 1230km in radius (2470km in diameter).

100g, 123km radius.

1000g, 12.3km radius.

10000g, 1.23km radius.

I could see a 10000g mass driver being built, but beyond that is going to get real expensive, real fast.

And there's no reason to build multiple mass drivers. If you built a 10g one, you could also use it for the medium and bulk goods.

Terraformer · 2007-11-29 08:25:09

Only if the 10g one can go to 10000gs. The idea is you can launch bulk goods in quick succesion while the 10g one launches humans.

I thought the idea of a ring is that you go round it again and again, not just once.

samy · 2007-11-29 08:31:56

Only if the 10g one can go to 10000gs.

And what use would that be? Why would you want to accelerate something faster than you'd have to?

Are you sure you're not confusing centripetal acceleration and tangential acceleration?

Terraformer · 2007-11-29 09:39:03

So you can launch them quicker and so cram more launches in.

New Mars Forums

Announcement

#26 2007-11-17 10:04:38

Re: Space Elevator or Scramjet?

#27 2007-11-17 11:05:40

Re: Space Elevator or Scramjet?

#28 2007-11-17 12:55:56

Re: Space Elevator or Scramjet?

#29 2007-11-17 13:17:43

Re: Space Elevator or Scramjet?

#30 2007-11-26 10:20:12

Re: Space Elevator or Scramjet?

#31 2007-11-26 16:12:42

Re: Space Elevator or Scramjet?

#32 2007-11-26 17:07:22

Re: Space Elevator or Scramjet?

#33 2007-11-26 17:45:52

Re: Space Elevator or Scramjet?

#34 2007-11-28 06:27:44

Re: Space Elevator or Scramjet?

#35 2007-11-28 06:47:23

Re: Space Elevator or Scramjet?

#36 2007-11-28 08:19:48

Re: Space Elevator or Scramjet?

#37 2007-11-28 09:03:54

Re: Space Elevator or Scramjet?

#38 2007-11-28 09:49:31

Re: Space Elevator or Scramjet?

#39 2007-11-28 13:14:34

Re: Space Elevator or Scramjet?

#40 2007-11-28 13:45:02

Re: Space Elevator or Scramjet?

#41 2007-11-28 14:22:35

Re: Space Elevator or Scramjet?

#42 2007-11-28 14:32:10

Re: Space Elevator or Scramjet?

#43 2007-11-29 04:33:35

Re: Space Elevator or Scramjet?

#44 2007-11-29 06:54:48

Re: Space Elevator or Scramjet?

#45 2007-11-29 07:16:15

Re: Space Elevator or Scramjet?

#46 2007-11-29 07:27:09

Re: Space Elevator or Scramjet?

#47 2007-11-29 07:38:40

Re: Space Elevator or Scramjet?

#48 2007-11-29 08:25:09

Re: Space Elevator or Scramjet?

#49 2007-11-29 08:31:56

Re: Space Elevator or Scramjet?

#50 2007-11-29 09:39:03

Re: Space Elevator or Scramjet?

Board footer