Antigravity

Theemile wrote:The tower in the Patent is ~ 20 KM tall, which would reduce the fuel use of LEO orbital launches by ~30%.

Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

Grashtel wrote:

Theemile wrote:The tower in the Patent is ~ 20 KM tall, which would reduce the fuel use of LEO orbital launches by ~30%.

Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

You actually get a small increase in velocity by being lifted 20 km and forced to stay above one spot on the Earth. And fuel use is logarithmic--the fuel has to lift the capsule, and all the rest of the fuel. So a little bit of savings helps quite a bit.

That said, I'm a little surprised the savings are that large, myself. I'll have to check it out.

Theemile wrote:The tower in the Patent is ~ 20 KM tall, which would reduce the fuel use of LEO orbital launches by ~30%.

Grashtel wrote: Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

SWM wrote: You actually get a small increase in velocity by being lifted 20 km and forced to stay above one spot on the Earth. And fuel use is logarithmic--the fuel has to lift the capsule, and all the rest of the fuel. So a little bit of savings helps quite a bit.

That said, I'm a little surprised the savings are that large, myself. I'll have to check it out.

Another factor is higher efficiency at high altitude. For instance, SpaceX's Merlin engine has 654 kilonewtons of thrust in sea-level air, but 740 kN in vacuum. At 20km, you aren't quite in vacuum, but....

At liftoff, the Saturn 5 was getting 7 inches per gallon.

Bill Woods wrote:

Theemile wrote:The tower in the Patent is ~ 20 KM tall, which would reduce the fuel use of LEO orbital launches by ~30%.

Grashtel wrote: Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

SWM wrote: You actually get a small increase in velocity by being lifted 20 km and forced to stay above one spot on the Earth. And fuel use is logarithmic--the fuel has to lift the capsule, and all the rest of the fuel. So a little bit of savings helps quite a bit.

That said, I'm a little surprised the savings are that large, myself. I'll have to check it out.

Another factor is higher efficiency at high altitude. For instance, SpaceX's Merlin engine has 654 kilonewtons of thrust in sea-level air, but 740 kN in vacuum. At 20km, you aren't quite in vacuum, but....

At liftoff, the Saturn 5 was getting 7 inches per gallon.

The engine efficiency changing with altitude is IIRC the effect of using a fixed expansion bell engine. You have to tune the bell size for a desired external air pressure. Efficient in vacuum means you're under-expanding and being inefficient at sea level.
Getting up to 20km altitude should let you use a fixed bell tuned for low- to no-pressure without the efficiency impact you'd get using that same engine/bell convo from sea level.


Otoh, in theory you can get the same efficiency gain using an aero-spike engine; and quite a lot of it with a telescoping bell disign. However neither of those are well tested.

Grashtel wrote:

Theemile wrote:The tower in the Patent is ~ 20 KM tall, which would reduce the fuel use of LEO orbital launches by ~30%.

Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

It's not an exact representation, but you might want to try out Kerbal Space Program. The aerodynamics in the game have been tweaked a bit over the past couple years, and they seem to be narrowing things down. The issue is that they have basically Earth-scale atmosphere (and gravity, for that matter) on a planet thats roughly 600 km in diameter. There *are* mods that replace Kerbin with an Earth-sized planet (or Earth itself), so that at least you are somewhat closer to our reality as far as getting into orbit or moving off to one of Kerbins moons, or other planets in the system.

The isp of each engine in the parts menu is given for both atmo and vac, and they are designed to allow the gamer to pick which to use based on where it will be used most, and have the best efficiency. Of course, that doesn't stop folks from trying the "moar boosters!" method of getting a payload to orbit.

It's a really cool game, and NASA even collaborated with the developers to create an asteroid redirect mission. The orbital physics are patched conics rather than strict Newtonian mechanics, so there are occasionally floating point errors when calculating trajectories for planetary rendezvous, but they're not too bad to work with in a game.

https://kerbalspaceprogram.com/en/?page_id=7

SWM wrote:

Grashtel wrote:Hmm, seems rather a lot of saving. I did not think that air drag inflicted that kinda of losses and otherwise don't see anything that lifting the launch site to 20km would give a big improvement, by far the largest cost in an orbital launch is getting up to orbital velocity, the altitude is relatively easy to do.

You actually get a small increase in velocity by being lifted 20 km and forced to stay above one spot on the Earth. And fuel use is logarithmic--the fuel has to lift the capsule, and all the rest of the fuel. So a little bit of savings helps quite a bit.

That said, I'm a little surprised the savings are that large, myself. I'll have to check it out.

I would think the log factor would be the primary effect. You are cutting off the largest, thus most expensive, part of the log curve. Rockets are horribly inefficient, since almost all the energy goes into the exhaust. That is why I suggested using fuels that can be raised to the platform as gases, even if they need final processing on the platform. BTW, is there any reason it would be restricted to 20 km, if proven to work? AND, could a catapult be built into the platform to get some initial speed without using expensive fuel? If the latter is tried, the tower would be sure to sway, so some sort of, perhaps dynamic, damping would need to be built into the system. Better yet, sway the tower before the launch in such a way that the launch gets extra velocity by fighting against said sway. Timing would be critical!!

As always, cutting running costs requires infrastructure. And INF. can have accidental or intentional damage. At least this design would not cause major damage in such a case, unlike the space tether types. Considering the other side of INF. , I really like being able to use the INF. supporting my refrigerator freezer to supply ice cubes in hot weather! When we replaced our old refrigerator, the extra efficiency chopped a third off of our electrical bill. Probably at present, efficiency improvements on things commonly used would save more than this space tower. Now if traffic to orbit happens more often . . ..

DDHvi wrote:

SWM wrote:You actually get a small increase in velocity by being lifted 20 km and forced to stay above one spot on the Earth. And fuel use is logarithmic--the fuel has to lift the capsule, and all the rest of the fuel. So a little bit of savings helps quite a bit.

That said, I'm a little surprised the savings are that large, myself. I'll have to check it out.

I would think the log factor would be the primary effect. You are cutting off the largest, thus most expensive, part of the log curve. Rockets are horribly inefficient, since almost all the energy goes into the exhaust. That is why I suggested using fuels that can be raised to the platform as gases, even if they need final processing on the platform.

From what other people have said, the difference in engine efficiency is pretty significant, too. Combined with the log factor, I can see now why the inventors think they can get that kind of improvement.

BTW, is there any reason it would be restricted to 20 km, if proven to work?

I imagine the biggest problem would be fact that it is essentially held up by balloons. There is a limit to how high balloons can go. Weather balloons can go much higher, but it requires an enormous volume of helium to lift a very small mass. I doubt that balloons could support a large platform at a higher altitude without being unreasonably big.

The problem with partial space towers, is that the cost of fuel is the miniscule part of the cost to LEO. The missile is the major cost, not the fuel. Fuel cost, by and large, is inconsequential. Now, if the number of launches dramatically increases and the number of rockets produced dramatically increases, dropping their price tag, then and only then will the price of fuel even make a small blip on the total $$$ to LEO.

Relax wrote:The problem with partial space towers, is that the cost of fuel is the miniscule part of the cost to LEO. The missile is the major cost, not the fuel. Fuel cost, by and large, is inconsequential. Now, if the number of launches dramatically increases and the number of rockets produced dramatically increases, dropping their price tag, then and only then will the price of fuel even make a small blip on the total $$$ to LEO.

But less fuel means a smaller rocket. Depending on the rocket design, essentially it could mean removing side mounted boosters or a stage from an existing design. That means a smaller, "simplified" rocket. Which also usually means safer, since every stage or booster is just another redundant set of problems bolted together.

But even at this, you would need to add in the infrastructure costs per launch - and building, maintaining and keeping a 20km tall balloon erect is going to be expensive. The cost differential is probably going to require >>2 launches a month just to offset the current launch costs.

And given that any tower is a fixed launching point, like any other spaceport, it is only ideal for specific launch trajectories (without burning up hideous amounts of fuel, which would be antithema to such a launch technique). So it's only going to get a % of the global launch traffic, without building a 2nd tower and doubling infrastructure costs.

Relax wrote:The problem with partial space towers, is that the cost of fuel is the miniscule part of the cost to LEO. The missile is the major cost, not the fuel. Fuel cost, by and large, is inconsequential. Now, if the number of launches dramatically increases and the number of rockets produced dramatically increases, dropping their price tag, then and only then will the price of fuel even make a small blip on the total $$$ to LEO.

Fuel cost is trivial, but the need to devote a large fraction of the rocket's dry mass to fuel tanks is not.
From some Usenet discussions (quite a few years ago):

Geoffrey A. Landis wrote: Some of the advantages of high-altitude launch:
(1) You start with a fraction of the potential energy needed to get to orbit.

Blair Patric Bromley wrote: If you could launch at 10000 ft above sea level, you could reduce your velocity change to get into orbit by approx. 250 m/s. However, you need about 8000 m/s to get into orbit. A 3% improvement.

Landis wrote: But three percent is a *tremendous* improvement. A RL-10A has an Isp of about 450 seconds; thus, exhaust velocity Ve is about 4400 km/sec. Structure & payload mass fraction is exp[deltaV/Ve]; a RL-10A powered vehicle could achieve a maxium amount of structure plus payload to 8 km/sec of 16.3%. Typically about 5% of this is actually payload. A 3% decrease in delta-V to orbit increases this to 17.3%. This increases the *payload* to 6% of the gross lift-off mass -- a 20% increase in payload.

Landis wrote: (2) You start at a lower atmospheric pressure.
a. reduced atmospheric drag loss

Which wasn't commented on by Bromley or by Pat, but is a significant effect, at *minimum* equal to the potential energy gain.

b. vehicle can be designed with less attention paid to aerodynamics. Lower aerodynamic design penalty means higher performance designs (ie., smaller fineness ratio allows more efficient tanks)
c. More optimum trajectory possible; you can curve toward horizontal thrust much faster since you start out closer to out of the atmosphere
d. Max-Q occurs at a much lower pressure; lower aerodynamic stress on the system means vehicle can be designed lighter.

Pat wrote: I think Max-Q is going to be at the same altitude or lower depending upon tank fineness.

Landis wrote: To the contrary. Max-Q is the product of air density, the square of velocity, and a vehicle-dependent factor which depends on mach number. For a given acceleration profile, Max-Q occurs at the same altitude *above the launch site* independent of how high the launch site is. That is, the actual value of dynamic pressure will decrease linearly with the initial pressure.

Landis wrote: a. Higher performance out of the rocket nozzle at launch

Bromley wrote: Atm. pressure is about 10 psia at 10000 ft.
This would result in approx. a 2% increase in exhaust velocity, which, under certain assumptions would result in roughly a 3% increase in payload mass fraction.

Landis wrote: How do you figure? A 2% increase in exhaust velocity gives a 2% increase in Isp, Structure & payload mass fraction is exp[deltaV/Ve], or roughly exp[0.98 * 1.82]. I calculate this as a 3.6% decrease in *total* mass-ratio. Using the same [structure+payload] mass, that comes to an 11.8% increase in *payload* mass.

Adding this to the potential energy gain [factor of 20% payload increase] and the drag gain [quick estimate says another factor of 20%] gives (going back to the rocket equation, instead of adding the increases separately) an increase in [structure plus payload] to 18.9% instead of 16.3%. That increases the payload by 52%.

http://yarchive.net/space/exotic/tower_launch.html

[Yay -- promotion to the List!]