7 ms·

An electromagnetic launcher seems to be the way to go. Why is everyone spending time on rockets?

Symmetry 10y ago
In theory you need to apply 7.8 km/s of deltaV to an object to put it into orbit. In practice, you also need to use more to overcome to factors. There's gravity drag, the action of gravity on your rocket while it's speeding up to orbital velocity. There's also atmospheric drag, the resistance of the atmosphere to your rocket when it's traveling up to space. The faster you accelerate the less gravity drag but the more atmospheric drag you experience since you spend less time accelerating but are traveling faster lower in the atmosphere, and drag is proportional to the square of the velocity. As you can see, using an electromagnetic cannon where you are at full speed leaving the barrel of the gun, you will have zero gravity drag but an immense amount of atmospheric drag. This means both that you will have to apply much more deltaV and that the heating caused by the passage of your capsule through the atmosphere will be enough to destroy any known material. Finally, if you launch at full velocity from the surface of the Earth then the rules of orbital mechanics mean that, if you are going into an elliptical orbit, your orbit will intersect the launch point eventually and your capsule will lithobrake on the ground, also destroying it. Neither of these problems apply to launching from the Moon back to Earth, for the record. The moon has no atmosphere to apply drag and heating. And if you're leaving the Moon's orbit for Earth you're on a hyperbolic orbit rather than an elliptical one and won't be coming back around in any case. So for getting off of airless Moons into orbit around their primary electromagnetic launchers are an excellent idea.
throwanem 10y ago
Because it's easier to build a reusable rocket than a reusable railgun, especially a reusable railgun that's hundreds of kilometers long.
- jsprogrammer 10y ago
  Easier solutions aren't necessarily good. Rockets are extremely dangerous and expensive. The ~200km required is daunting, but imminently possible.
  - throwanem 10y ago
    Many things are possible. But military experiments on a scale four orders of magnitude smaller have revealed many previously unsuspected engineering difficulties with putting the technology into production. Solving those problems at the 20m scale would seem a necessary prerequisite to successfully building a space launcher at the 200km scale.
  - Someone 10y ago
    Pointing your rail gun up is best for getting out of the atmosphere ASAP, but how do you build a gun that goes, say, 20 km high into the air? A mostly (the curvature of the earth will make it point up) horizontal gun will have the problem that the projectile will leave the gun at escape velocity, at around 1 atmosphere of air pressure (either after accelerating through 1 atmosphere of air pressure, which takes lots of extra energy, or leaving a vacuum, which makes exiting the gun barrel quite a bang) Also, your barrel will have to extremely straight. I don't think either is 'imminently' possible.
    - stcredzero 10y ago
      Pointing your rail gun up is best for getting out of the atmosphere ASAP, but how do you build a gun that goes, say, 20 km high into the air? It's physically possible for us to build a 100km tall tower with current technology. (Columns formed from many highly pressurized tanks made from boron.) It's not really economically feasible, however.
      - throwanem 10y ago
        I wonder how well such a tower would withstand the forces involved in also launching projectiles at escape velocity.
        stcredzero 10y ago
        As of around the year 2000, I remember a figure of 11 miles as the height limit for a fairly conventionally built skyscraper. If we used lightweight truss construction, I suspect we could build a load bearing tower. If we used aerospace grade materials, we could do even better. (Yes, it would be quite expensive.)
dogma1138 10y ago
An electromagnetic launcher isn't the way to go not from earth, especially if you want to put people into orbit. Forget about the energy required, forget about the electromagnetic field and what it can do the payload even non-biological one. An electromagnetic launcher is effectively a canon, calculate the acceleration needed to achieve orbital velocity not to mention escape velocity and you'll get to a point where you can't have anything surviving the launch. Not to mention that achieving Max-Q effectively on the launch pad is both the least effective thing you can do as well as the most dangerous one.
- jsprogrammer 10y ago
  Acceleration over 200km will get you to orbit. The electricity required for a single launch is measured in US pennies. An aerodynamic vehicle could be extremely safe; in the event of a failure, you could just glide back to the ground. The possibility of violent explosion would be practically non-existent.
  - ubercore 10y ago
    Have a source on the electricity costing pennies?
    - jsprogrammer 10y ago
      Please see my response to kragen, where I derive the cost.
    - dogma1138 10y ago
      It's not going to cost pennies, it's an easy calculation treat energy efficiency as 100% efficient and calculate how much watts you would need to put say a 10,000KG payload in orbit and then calculate how much it would cost you in terms of electricity. F (d)x/(d)t = F * v = m * a * v = joules per second = watts
  - dogma1138 10y ago
    The electricity and cost of the rail gun isn't measured in pennies. An Aerodynamic vehicle could be safe, but it would also mean it would generate considerable drag on the way up requiring more power, it also would be safe only after almost reaching orbit because it would be hypersonic out of the launch pad, if you can design a hypersonic glider NASA would like hear about it. Building a launch pad over 200KM is also not a simple feat, if you ask why people are building rockets it's because we have no clue how to build EM and by all accounts it's not sustainable for earth. EM launchers for the moon and even mars as well as large asteroid bases are considerably more sensible.
deleted 10y ago
[deleted]
daedalus_j 10y ago
Using a mass driver to launch, if youkeep the G forces down to the levels NASA wants I think you need something like 100 miles of track? You can build them much shorter, but they could never be used to launch living cargo. My guess would be that we'll see more investment in them once we have a big rocket (BFR/New Armstrong) capable of launching larger habitation units into higher orbits. Once you have more than 10 humans living up there you start having to think about how to supply them, and then a rail gun looks pretty good, but in the meantime we need to be able to lift large (dimension) things as well as humans, and rockets are actually a pretty good choice for that.
- willismichael 10y ago
  Don't rocket-based vehicles end up burning a significant amount of fuel just to lift the weight of other fuel? What if there was a hybrid approach of using a mass driver to launch fuel containers into orbit, and then the rocket vehicle carrying people (and other more delicate cargo) could rendezvous with and pick up the previously launched fuel containers?
  - usefulcat 10y ago
    Would certainly make for some interesting failure scenarios to consider. Such as, what happens if the engines fail to start? Presuming they are intended to start after the vehicle is some distance down the rail. Edit: oops, misread your comment.. sorry for the non-sequitur.
  - stcredzero 10y ago
    How about launching the personnel carriers this way? https://en.wikipedia.org/wiki/Laser_propulsion#Laser_Thermal_Rocket_.28Heat_exchanger_.28HX.29_thruster.29 https://en.wikipedia.org/wiki/Laser_propulsion#Laser_Thermal... Then send cargo up that's hardened for 30g in a vehicle launched from a 20 mile long mass driver that's tilted 45 degrees. We should be able to build such a structure by starting several miles underground, then building up the side of a mountain. 30g over 20 miles yields a final speed of about 9735 miles per hour. With a boost like that, the vehicle wouldn't need a 1st stage and could probably get to orbit without staging.
jacquesm 10y ago
Because: (1) Rockets are proven technology, nobody has yet been able to put anything into LEO using a railgun (2) Rockets have significant freedom when it comes to choosing the final orbit to deploy to (3) Rockets are relatively cheap on the 'capex' front, a 200 km long electromagnetic launcher up the side of a mountain will be extremely expensive and may not even work... Of course if you're so sure about this then maybe start a company to create an electromagnetic launcher, it should be easy to raise money assuming you are right.
- personjerry 10y ago
  > Of course if you're so sure about this then maybe start a company to create an electromagnetic launcher, it should be easy to raise money assuming you are right. Could you explain how it would be easy? In my mind it seems hard to raise several billion dollars, no matter how you put it...
  - bbctol 10y ago
    It's a joke: if it really was clear that a railgun was "the way to go," companies would be doing it.
    - jddjdbdbd3 10y ago
      Wht? That's some seriously misplaced faith in capatilism there. I can think of 5 examples off the top of my head where group think delayed the uptake of more optimal solutions.
  - beamatronic 10y ago
    It's apparently not hard to raise several billion dollars if you're trying to sell pet food online, create an app that dispatches taxis, or post extremely short messages on a web site.
    - whamlastxmas 10y ago
      >It's apparently not hard to raise several billion dollars if you're trying to sell something that makes hundreds of billions of dollars
hga 10y ago
Laser propulsion is thought to be a better way for launching from the Earth, and has been tried with small models. You build a powerful laser ground laser system, and through some method I'm not clear on, heat air in pulses? in the bell of the rocket chamber. Once that gets too thin, the top layer of the chamber is ablated to provide the final thrust needed. Supposedly can be gentle enough for humans, see some of Jerry Pournelle's fiction and non-fiction for examples of this. Or, if you're in a hurry, there's always Orion (see Niven and Pournelle's Footfall): https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
- jsprogrammer 10y ago
  Laser propulsion could work. I'll need to learn more about the physics of it.
Animats 10y ago
It's not impossible. After all, a "space gun" reached 117 miles in altitude in 1967.[1] A small satellite launcher is probably possible, especially with a rocket upper stage. In recent years, military work on electromagnetic launchers and weapons has increased substantially. The US Navy has a railgun in test, and plans to mount it on a ship (probably the USS Zumwalt) next year. The new generation of carriers use electromagnetic catapults. Many of the components thus already exist. The acceleration in a gun-type launch would be huge. All the acceleration has to take place in the length of the gun or track. Hardware, yes; humans, no. Think in terms of bulk launching of small satellites for mobile/Internet access. [1] http://www.atlasobscura.com/places/project-harp-space-gun-barbados http://www.atlasobscura.com/places/project-harp-space-gun-ba...

In theory you need to apply 7.8 km/s of deltaV to an object to put it into orbit. In practice, you also need to use more to overcome to factors. There's gravity drag, the action of gravity on your rocket while it's speeding up to orbital velocity. There's also atmospheric drag, the resistance of the atmosphere to your rocket when it's traveling up to space. The faster you accelerate the less gravity drag but the more atmospheric drag you experience since you spend less time accelerating but are traveling faster lower in the atmosphere, and drag is proportional to the square of the velocity. As you can see, using an electromagnetic cannon where you are at full speed leaving the barrel of the gun, you will have zero gravity drag but an immense amount of atmospheric drag. This means both that you will have to apply much more deltaV and that the heating caused by the passage of your capsule through the atmosphere will be enough to destroy any known material. Finally, if you launch at full velocity from the surface of the Earth then the rules of orbital mechanics mean that, if you are going into an elliptical orbit, your orbit will intersect the launch point eventually and your capsule will lithobrake on the ground, also destroying it. Neither of these problems apply to launching from the Moon back to Earth, for the record. The moon has no atmosphere to apply drag and heating. And if you're leaving the Moon's orbit for Earth you're on a hyperbolic orbit rather than an elliptical one and won't be coming back around in any case. So for getting off of airless Moons into orbit around their primary electromagnetic launchers are an excellent idea.

throwanem 10y ago

Because it's easier to build a reusable rocket than a reusable railgun, especially a reusable railgun that's hundreds of kilometers long.

jsprogrammer 10y ago
Easier solutions aren't necessarily good. Rockets are extremely dangerous and expensive. The ~200km required is daunting, but imminently possible.
- throwanem 10y ago
  Many things are possible. But military experiments on a scale four orders of magnitude smaller have revealed many previously unsuspected engineering difficulties with putting the technology into production. Solving those problems at the 20m scale would seem a necessary prerequisite to successfully building a space launcher at the 200km scale.
- Someone 10y ago
  Pointing your rail gun up is best for getting out of the atmosphere ASAP, but how do you build a gun that goes, say, 20 km high into the air? A mostly (the curvature of the earth will make it point up) horizontal gun will have the problem that the projectile will leave the gun at escape velocity, at around 1 atmosphere of air pressure (either after accelerating through 1 atmosphere of air pressure, which takes lots of extra energy, or leaving a vacuum, which makes exiting the gun barrel quite a bang) Also, your barrel will have to extremely straight. I don't think either is 'imminently' possible.
  - stcredzero 10y ago
    Pointing your rail gun up is best for getting out of the atmosphere ASAP, but how do you build a gun that goes, say, 20 km high into the air? It's physically possible for us to build a 100km tall tower with current technology. (Columns formed from many highly pressurized tanks made from boron.) It's not really economically feasible, however.
    - throwanem 10y ago
      I wonder how well such a tower would withstand the forces involved in also launching projectiles at escape velocity.
      - stcredzero 10y ago
        As of around the year 2000, I remember a figure of 11 miles as the height limit for a fairly conventionally built skyscraper. If we used lightweight truss construction, I suspect we could build a load bearing tower. If we used aerospace grade materials, we could do even better. (Yes, it would be quite expensive.)

dogma1138 10y ago

An electromagnetic launcher isn't the way to go not from earth, especially if you want to put people into orbit. Forget about the energy required, forget about the electromagnetic field and what it can do the payload even non-biological one. An electromagnetic launcher is effectively a canon, calculate the acceleration needed to achieve orbital velocity not to mention escape velocity and you'll get to a point where you can't have anything surviving the launch. Not to mention that achieving Max-Q effectively on the launch pad is both the least effective thing you can do as well as the most dangerous one.

jsprogrammer 10y ago
Acceleration over 200km will get you to orbit. The electricity required for a single launch is measured in US pennies. An aerodynamic vehicle could be extremely safe; in the event of a failure, you could just glide back to the ground. The possibility of violent explosion would be practically non-existent.
- ubercore 10y ago
  Have a source on the electricity costing pennies?
  - jsprogrammer 10y ago
    Please see my response to kragen, where I derive the cost.
  - dogma1138 10y ago
    It's not going to cost pennies, it's an easy calculation treat energy efficiency as 100% efficient and calculate how much watts you would need to put say a 10,000KG payload in orbit and then calculate how much it would cost you in terms of electricity. F (d)x/(d)t = F * v = m * a * v = joules per second = watts
- dogma1138 10y ago
  The electricity and cost of the rail gun isn't measured in pennies. An Aerodynamic vehicle could be safe, but it would also mean it would generate considerable drag on the way up requiring more power, it also would be safe only after almost reaching orbit because it would be hypersonic out of the launch pad, if you can design a hypersonic glider NASA would like hear about it. Building a launch pad over 200KM is also not a simple feat, if you ask why people are building rockets it's because we have no clue how to build EM and by all accounts it's not sustainable for earth. EM launchers for the moon and even mars as well as large asteroid bases are considerably more sensible.

deleted 10y ago

[deleted]

daedalus_j 10y ago

Using a mass driver to launch, if youkeep the G forces down to the levels NASA wants I think you need something like 100 miles of track? You can build them much shorter, but they could never be used to launch living cargo. My guess would be that we'll see more investment in them once we have a big rocket (BFR/New Armstrong) capable of launching larger habitation units into higher orbits. Once you have more than 10 humans living up there you start having to think about how to supply them, and then a rail gun looks pretty good, but in the meantime we need to be able to lift large (dimension) things as well as humans, and rockets are actually a pretty good choice for that.

willismichael 10y ago
Don't rocket-based vehicles end up burning a significant amount of fuel just to lift the weight of other fuel? What if there was a hybrid approach of using a mass driver to launch fuel containers into orbit, and then the rocket vehicle carrying people (and other more delicate cargo) could rendezvous with and pick up the previously launched fuel containers?
- usefulcat 10y ago
  Would certainly make for some interesting failure scenarios to consider. Such as, what happens if the engines fail to start? Presuming they are intended to start after the vehicle is some distance down the rail. Edit: oops, misread your comment.. sorry for the non-sequitur.
- stcredzero 10y ago
  How about launching the personnel carriers this way? https://en.wikipedia.org/wiki/Laser_propulsion#Laser_Thermal_Rocket_.28Heat_exchanger_.28HX.29_thruster.29 https://en.wikipedia.org/wiki/Laser_propulsion#Laser_Thermal... Then send cargo up that's hardened for 30g in a vehicle launched from a 20 mile long mass driver that's tilted 45 degrees. We should be able to build such a structure by starting several miles underground, then building up the side of a mountain. 30g over 20 miles yields a final speed of about 9735 miles per hour. With a boost like that, the vehicle wouldn't need a 1st stage and could probably get to orbit without staging.

jacquesm 10y ago

Because: (1) Rockets are proven technology, nobody has yet been able to put anything into LEO using a railgun (2) Rockets have significant freedom when it comes to choosing the final orbit to deploy to (3) Rockets are relatively cheap on the 'capex' front, a 200 km long electromagnetic launcher up the side of a mountain will be extremely expensive and may not even work... Of course if you're so sure about this then maybe start a company to create an electromagnetic launcher, it should be easy to raise money assuming you are right.

personjerry 10y ago
> Of course if you're so sure about this then maybe start a company to create an electromagnetic launcher, it should be easy to raise money assuming you are right. Could you explain how it would be easy? In my mind it seems hard to raise several billion dollars, no matter how you put it...
- bbctol 10y ago
  It's a joke: if it really was clear that a railgun was "the way to go," companies would be doing it.
  - jddjdbdbd3 10y ago
    Wht? That's some seriously misplaced faith in capatilism there. I can think of 5 examples off the top of my head where group think delayed the uptake of more optimal solutions.
- beamatronic 10y ago
  It's apparently not hard to raise several billion dollars if you're trying to sell pet food online, create an app that dispatches taxis, or post extremely short messages on a web site.
  - whamlastxmas 10y ago
    >It's apparently not hard to raise several billion dollars if you're trying to sell something that makes hundreds of billions of dollars

hga 10y ago

Laser propulsion is thought to be a better way for launching from the Earth, and has been tried with small models. You build a powerful laser ground laser system, and through some method I'm not clear on, heat air in pulses? in the bell of the rocket chamber. Once that gets too thin, the top layer of the chamber is ablated to provide the final thrust needed. Supposedly can be gentle enough for humans, see some of Jerry Pournelle's fiction and non-fiction for examples of this. Or, if you're in a hurry, there's always Orion (see Niven and Pournelle's Footfall): https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

jsprogrammer 10y ago
Laser propulsion could work. I'll need to learn more about the physics of it.

Animats 10y ago

It's not impossible. After all, a "space gun" reached 117 miles in altitude in 1967.[1] A small satellite launcher is probably possible, especially with a rocket upper stage. In recent years, military work on electromagnetic launchers and weapons has increased substantially. The US Navy has a railgun in test, and plans to mount it on a ship (probably the USS Zumwalt) next year. The new generation of carriers use electromagnetic catapults. Many of the components thus already exist. The acceleration in a gun-type launch would be huge. All the acceleration has to take place in the length of the gun or track. Hardware, yes; humans, no. Think in terms of bulk launching of small satellites for mobile/Internet access. [1] http://www.atlasobscura.com/places/project-harp-space-gun-barbados http://www.atlasobscura.com/places/project-harp-space-gun-ba...