Comments on Next Big Future: Railgun on track for 2012 deployment trials on US ...

Since the individual magnetic fields created in th...

2008-02-07T13:46:00.000-08:00

Since the individual magnetic fields created in the rails are repulsed by one another, a tremendous strain is placed on the rails as they try to push away from one another. While rail guns do not suffer from the traditional recoil forces associated with conventional expanding gas weapons, this repulsive effect can be equally destructive if not properly compensated for.

So instead of recoil backwards from a forward shot the pressures are pushing out against the rails.
There is still backwards force but less because of higher railgun efficiency versus expanding gas.

This patent discusses how to make the mobile railgun lighter and stiffer

This patent also discusses designing around the forces involved on a railgun

The bigger and more powerful you want to make your flying railgun then the more engineering challenges there will be to stop it from falling apart or getting worn out.

Sticking it on a mountain top or space pier could allow for bigger construction with more reinforcement. A flying or orbital railgun could be done but how big could they make it ?

There has been speculative designs of really big lighter than air platforms. 1000 ton cargo Walrus airship design

Back in 1997, Mike deGyurky, a program manager at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., had a design for a giant blimp, perhaps a mile in length. With a capacity of 50,000 tons or more

Mike had the skytrain concept

I have two questions:1) How do you deal with the r...

2008-02-07T11:43:00.000-08:00

I have two questions:

1) How do you deal with the recoil of such a rail gun from an airborne platform ?
2) Supposing you have an answer for the first question, could this be launched from a lighter-than-air platform and thus dramatically decrease the launch cost?

Maybe the answer for both questions would be a earth-tethered LTA platform using something like the proposed space cable.

I can't imagine the jolt you would get from the recoil when firing something like this from a scramjet or any other airborne platform. The projectile still has to push against something. You could compensate for the recoil with larger mass from the launching platform but that would dramatically increase the cost to launch. "Padding" the recoil would decrease the muzzle velocity. I mean, look at how current cannon technology rocks a huge chunk of metal like a destroyer.

Just curious.

bsrwnr

2008-02-07T11:32:00.000-08:00

bsrwnr

Anonymous: The space shuttle is not a scramjet. th...

2008-02-07T09:01:00.000-08:00

Anonymous: The space shuttle is not a scramjet. the Space shuttle is a multi-stage partially resusable rocket. A scramjet is a jet engine with supersonic airflow going into the engine. A ramjet is a jet with subsonic airflow which can power supersonic flight. Scramjets are more complicated because of the difficulty in modeling and handling the supersonic airflow. If you can handle that airspeed then the performance of a jet engine is superior to a rocket, because as commenter Lowell mentions you do have to carry 85% of the fuel weight which is oxygen. Oxygen can be taken from the atmosphere.

I detail out some recent hypersonic/scramjet design and testing progress.

A good scramjet design good achieve 3000 ISP. The shuttle has about 450 ISP. ISP is like fuel efficiency for rockets instead of mpg for cars. More fuel efficient means you can take more cargo up instead of fuel.

Bc3tech: The energy to fire was indicated at 10.6 megajoules. This was mentioned in the article. The current system being tested can go up to 32 megajoules

This site has some energy unit conversion stats and an indication of how energy is in different fuels and batteries

A lithium ion battery has about 400-580 kilojoules per kilogram. So you would have to completely empty 20 kilograms or 44 pounds of lithium ion batteries in a second to power the 10.6 megajoule test shot. Lithium ion batteries cannot be emptied like that (15 minutes to hours to discharge). New ultracapacitors can discharge that fast (1-30 seconds). New ultracapacitors have better energy density but still less than lithium ion batteries (usually one tenth or less, so you need ten times as much or 440 pounds). 1300 pounds to power the full 32 megajoule shot. There are some higher energy density ultracapacitors in labs, most famously from the secretive EESTOR. They could rival lithium ion energy densities.

Supercapacitors at wikipedia

So, how much energy does it take to FIRE the thing...

2008-02-07T08:29:00.000-08:00

So, how much energy does it take to FIRE the thing? Unless i missed it somewhere in the article (or associated links)

I'm no expert but I believe that the advantage of ...

2008-02-07T07:58:00.000-08:00

I'm no expert but I believe that the advantage of scramjet over the space shuttle is a a signficant cost reduction. The scramjet's fuel does not have to contain oxygen as it can get that from the air. The space shuttle fule will thus be about twice as heavy requiring twice as much of it. Scram jets can probably be powered by hydrocarbon fuels instead of the solid rocket fuel used by the shuttle boosters as well.

Further, the scramjet can be a real airplane which the shuttle is not -- its a rocket/glider. Thus, the saftey factor should be much higher.

"So a nice big scramjet that could fly at Mach 10-...

2008-02-07T00:26:00.000-08:00

"So a nice big scramjet that could fly at Mach 10-12 could..." ???Uhh,say something like the space shuttle? The whole reason for the alternative development?

This 10 page IEEE pdf talks about the requirements...

2008-02-06T10:33:00.000-08:00

This 10 page IEEE pdf talks about the requirements for orbital launch

The extension of this technology to the muzzle velocities ( 7500 m/s) and energies ( 10 GJ) needed for the direct launch of payloads into orbit is very challenging, but may not be impossible. For launch to orbit, even long launchers ( 1000 m) would need to operate at accelerations 1000 gees to reach the required velocities, so that it would only be possible to launch rugged payloads, such as
fuel, water, and material. Estimated launch costs could be attractively low ( $600/kg) compared with the Space Shuttle ( $20 000/kg), provided that acceptable launch rates can be achieved.

So triple the muzzle speed and increase power by 1000 times the current test level or 330 times the current 32 MJ system.

A disadvantage of gun launch is that the launch package has to
leave the gun barrel at a very high velocity ( 7500 m/s) through
the Earth’s atmosphere, leading to a very high aerothermal load
on the projectile.

However, the current system is only about the size of a truck. So a nice big scramjet that could fly at Mach 10-12 could use a moderately scaled up version of the rail gun current system to fly above most of the atmosphere and then fire hardened payloads into orbit. Then less heat shielding would be needed.

To provide 500 tons/year to orbit would require 2000 launches/year—a little over five per day on average.

Interesting possibilities for orbital and sub-orbi...

2008-02-06T10:18:00.000-08:00

Interesting possibilities for orbital and sub-orbital applications.

It would work much better in a vacuum, or low atmospheric pressure environment.

No wonder the Navy is investing in unconventional power generation.