In my previous post ( The Case for Space V: Lightcraft ), I listed the positive and negative aspects of that mechanism for launching craft into space. Although this repeats some information already found in my very first blog entry in The Case for Space, I will include additional information including my subjective comparison of this technique to other techniques.
Light Gas Gun
1. Description
2. Advantages
3. Disadvantages
4. Economics
5. Summary
1. Description
The light gas gun is essentially a giant gun, analogous to that depicted in the movie A Trip to the Moon. However, there are some significant differences between the light gas gun works and fire arms. Here's a schematic of the light gas gun Light gas gun from Wikipedia.
Diagram of a light-gas gun
1 — Breech block
2 — Chamber
3 — Propellant charge (gunpowder)
4 — Piston
5 — Pump tube
6 — Light gas (helium or hydrogen)
7 — Rupture disk
8 — High pressure coupling
9 — Projectile
10 — Gun barrel
What you may notice is that a light gas gun uses 2 separate chamber. The first includes a combustible gas (probably hydrogen or methane plus oxygen). When combusted, this drives a plunger/driver into the second chamber which is filled with hydrogen from which the name light gas gun is derived. The hydrogen in this second chamber is not burned. Instead the driver (pushed by the combusting mixture) compresses the hydrogen enough to shoot the projectile out the launch tube. To further reduce the drag on the projectile, the launch tube in front of the projectile can be evacuated so that the projectile launches through a near vacuum.
A (long) video explaining how the light gas gun was developed.
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2. Advantages
Reusable launch infrastructure
The light gas gun, like all other methods I advocate can reuse most of its infrastructure. In fact the majority of the cost for the light gas gun will be sunk into the construction of its combustion chamber and launch tube. The chamber may see some erosion, however, since the chamber won't be carried with the vehicle, the designers can vastly over design the chamber and include plenty of margin for any chamber erosion. Because the launch tube will contain either vacuum or non-reacting hydrogen gas, the launch tube will experience very little wear.
This system will also require some infrastructure for generating hydrogen and oxygen. This too can remain on ground.
The only component of the system that will be expended will be the launch vehicle itself.
Doesn’t have to carry its propellant
The light gas gun, like all other methods I advocate except nuclear pulse propulsion, does NOT carry its own propellant. This makes the system much more efficient at launching because, as I've pointed out elsewhere, most of the weight of current launch systems (greater than 90%) is devoted to carrying its propellant.
Safe for nearby vehicles
A couple of the launch concepts (those using lasers and nuclear weapons) are unsafe for anything nearby. The light gas gun imposes none of those problems.
Can use cheap materials
Since the majority of the equipment required for this concept remains on the ground, very cheap construction materials (concrete and steel) can be used to build it. No advanced composites, no aircraft aluminum alloys, etc. The launch vehicle itself probably will use light weight materials so more payload mass can be carried into space.
Requires no technological breakthroughs
Some computational fluid dynamic studies would probably be required to ensure the system performed up to expectations, however, no technological breakthroughs would be required to make this system work. In fact many light gas guns already exist and are used regularly for scientific investigation.
Space launch
Some propulsion systems I expect to write about (e.g. the NERVA concept) really don't have the ability to launch anything into space (the thrust to weight ratio is wrong). However, a light gas gun would be a system used for launching cargo into space from Earth only. It could not be used for interplanetary propulsion and better systems exist for launching cargo from the moon or satellites.
Note that the light gas gun can only accelerate the vehicle to about 7 km/s. Perhaps another 2 km/s may be required to get into orbit (to overcome the friction of the vehicle moving through the air). Also the light gas gun will require a small rocket motor to circularize its orbit once it achieves the correct altitude.
Cheap
This could be one of the premiere methods of launching indelicate cargo into space (e.g. bulk launching of water). At an estimated launch cost of $300 per pound, bulk materials like water, fuel, gases, and food would be MUCH cheaper to get into space in this manner.
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3. Disadvantages
This concept still suffers from a few difficulties; however, most of these have relatively simple and straight forwards solutions.
Substantial turn-around time
Unlike the lightcraft concept, each launch will require a significant amount of set up. Meaning, the three separate regions (the combustion chamber, propellant, and tube) need to be prepared. All three need to be evacuated, the propellant chamber must then be pumped full of hydrogen, and the combustion chamber needs to be pumped full of hydrogen and oxygen.
At best we could use a light gas gun facility about once per day.
Brutal acceleration
The light gas gun can not provide a tailored g-load environment. Stated another way, anything loaded into the cargo area better not be crush-able. The g-loads generated by this system will be tremendous, thousands of g's certainly. This means only certain types of cargo can be carried, bulk materials (like water), g-hardened electronics, etc.
This experiment applied approximately 2 BILLION gravities of acceleration to the test article. Although the gun could be tuned somewhat to reduce the acceleration of the projectile, most methods of reducing the acceleration would also reduce the top velocity of the exit velocity of the projectile. Even if researchers tune the system to reduce the acceleration experienced by the projectile the launch accelerations will remain higher than that experienced by an artillery projectile.
People will never be able to be launched into space in a light gas gun.
Light payloads
One desperate need for space launch is the ability to launch large and heavy structures (e.g. large hunks of required space infrastructure). Although it would be possible to build a super-size light gas gun to launch huge payloads into space, I find it unlikely we'll do so in the near future - especially since we haven't even bothered to build a small one for space launch.
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4. Economics
Most costs for this launch system come from the infrastructure cost of building the gun and then operation costs of hydrogen generation and replacing the (disposable) launch hardware.
According to the "Next Big Thing", it would cost about $500 million to build the facility. Once built it could be used at most once per day at an operating launch cost of $2 million per launch.
Of course this system could launch a 7,200 lb projectile into a 185 km orbit (at high inclination) so the cost per pound come to less than $300 / lb into LEO. This is a HUGE bargain. Some analysts feel that the system could pay for itself after 50-100 launches. In order to recoup the infrastructure costs in this time frame, an additional $1 million per launch would be tacked on, raising the per pound launch costs to about $450 per pound - which is still a fantastic bargain compared to current launch costs of ~$10,000 per pound on the space shuttle.
The only remaining question is whether a system which generates such high g-loads on the payload would get that kind of usage.
One newly formed company is attempting to turn this idea into a commercial operation:
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5. Summary
Light Gas Gun
1. Description
2. Advantages
3. Disadvantages
4. Economics
5. Summary
1. Description
The light gas gun is essentially a giant gun, analogous to that depicted in the movie A Trip to the Moon. However, there are some significant differences between the light gas gun works and fire arms. Here's a schematic of the light gas gun Light gas gun from Wikipedia.
 |
| Schematic of a Light Gas Gun |
What you may notice is that a light gas gun uses 2 separate chamber. The first includes a combustible gas (probably hydrogen or methane plus oxygen). When combusted, this drives a plunger/driver into the second chamber which is filled with hydrogen from which the name light gas gun is derived. The hydrogen in this second chamber is not burned. Instead the driver (pushed by the combusting mixture) compresses the hydrogen enough to shoot the projectile out the launch tube. To further reduce the drag on the projectile, the launch tube in front of the projectile can be evacuated so that the projectile launches through a near vacuum.
A (long) video explaining how the light gas gun was developed.
Back to the top
2. Advantages
Reusable launch infrastructure
The light gas gun, like all other methods I advocate can reuse most of its infrastructure. In fact the majority of the cost for the light gas gun will be sunk into the construction of its combustion chamber and launch tube. The chamber may see some erosion, however, since the chamber won't be carried with the vehicle, the designers can vastly over design the chamber and include plenty of margin for any chamber erosion. Because the launch tube will contain either vacuum or non-reacting hydrogen gas, the launch tube will experience very little wear.
This system will also require some infrastructure for generating hydrogen and oxygen. This too can remain on ground.
The only component of the system that will be expended will be the launch vehicle itself.
Doesn’t have to carry its propellant
The light gas gun, like all other methods I advocate except nuclear pulse propulsion, does NOT carry its own propellant. This makes the system much more efficient at launching because, as I've pointed out elsewhere, most of the weight of current launch systems (greater than 90%) is devoted to carrying its propellant.
Safe for nearby vehicles
A couple of the launch concepts (those using lasers and nuclear weapons) are unsafe for anything nearby. The light gas gun imposes none of those problems.
Can use cheap materials
Since the majority of the equipment required for this concept remains on the ground, very cheap construction materials (concrete and steel) can be used to build it. No advanced composites, no aircraft aluminum alloys, etc. The launch vehicle itself probably will use light weight materials so more payload mass can be carried into space.
Requires no technological breakthroughs
Some computational fluid dynamic studies would probably be required to ensure the system performed up to expectations, however, no technological breakthroughs would be required to make this system work. In fact many light gas guns already exist and are used regularly for scientific investigation.
Space launch
Some propulsion systems I expect to write about (e.g. the NERVA concept) really don't have the ability to launch anything into space (the thrust to weight ratio is wrong). However, a light gas gun would be a system used for launching cargo into space from Earth only. It could not be used for interplanetary propulsion and better systems exist for launching cargo from the moon or satellites.
Note that the light gas gun can only accelerate the vehicle to about 7 km/s. Perhaps another 2 km/s may be required to get into orbit (to overcome the friction of the vehicle moving through the air). Also the light gas gun will require a small rocket motor to circularize its orbit once it achieves the correct altitude.
Cheap
This could be one of the premiere methods of launching indelicate cargo into space (e.g. bulk launching of water). At an estimated launch cost of $300 per pound, bulk materials like water, fuel, gases, and food would be MUCH cheaper to get into space in this manner.
Back to the top
3. Disadvantages
This concept still suffers from a few difficulties; however, most of these have relatively simple and straight forwards solutions.
Substantial turn-around time
Unlike the lightcraft concept, each launch will require a significant amount of set up. Meaning, the three separate regions (the combustion chamber, propellant, and tube) need to be prepared. All three need to be evacuated, the propellant chamber must then be pumped full of hydrogen, and the combustion chamber needs to be pumped full of hydrogen and oxygen.
At best we could use a light gas gun facility about once per day.
Brutal acceleration
The light gas gun can not provide a tailored g-load environment. Stated another way, anything loaded into the cargo area better not be crush-able. The g-loads generated by this system will be tremendous, thousands of g's certainly. This means only certain types of cargo can be carried, bulk materials (like water), g-hardened electronics, etc.
 |
| Velocity vs Time for a light gas gun experiment |
This experiment applied approximately 2 BILLION gravities of acceleration to the test article. Although the gun could be tuned somewhat to reduce the acceleration of the projectile, most methods of reducing the acceleration would also reduce the top velocity of the exit velocity of the projectile. Even if researchers tune the system to reduce the acceleration experienced by the projectile the launch accelerations will remain higher than that experienced by an artillery projectile.
People will never be able to be launched into space in a light gas gun.
Light payloads
One desperate need for space launch is the ability to launch large and heavy structures (e.g. large hunks of required space infrastructure). Although it would be possible to build a super-size light gas gun to launch huge payloads into space, I find it unlikely we'll do so in the near future - especially since we haven't even bothered to build a small one for space launch.
Back to the top
4. Economics
Most costs for this launch system come from the infrastructure cost of building the gun and then operation costs of hydrogen generation and replacing the (disposable) launch hardware.
According to the "Next Big Thing", it would cost about $500 million to build the facility. Once built it could be used at most once per day at an operating launch cost of $2 million per launch.
Of course this system could launch a 7,200 lb projectile into a 185 km orbit (at high inclination) so the cost per pound come to less than $300 / lb into LEO. This is a HUGE bargain. Some analysts feel that the system could pay for itself after 50-100 launches. In order to recoup the infrastructure costs in this time frame, an additional $1 million per launch would be tacked on, raising the per pound launch costs to about $450 per pound - which is still a fantastic bargain compared to current launch costs of ~$10,000 per pound on the space shuttle.
The only remaining question is whether a system which generates such high g-loads on the payload would get that kind of usage.
One newly formed company is attempting to turn this idea into a commercial operation:
Back to the top
5. Summary
Like many other ideas I've mentioned in my blog, this concept provides most of the launch velocity to the launch vehicle without requiring rocket engines. Because most of the hardware remains on the ground, the system doesn't have to pay launch costs for that hardware. A system like this should be reserved for acceleration tolerant payloads such as fuel, oxygen, water, g-hardened electronics, and anything else that won't be damaged by the environment.
At best this concept will be a system to supplement other launch infrastructure. We could not build our space infrastructure with only light gas guns. However, we could use it to reduce the costs of launching the materials identified above.
Positive attributes of
craft
|
LC
|
LGG
|
RA
|
NPP
|
NR
|
FFR
|
CR
|
Most equipment reused?
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
Doesn't carry its
propellant?
|
Yes
|
Yes
|
Yes
|
Yes1
|
No
|
Yes1
|
No
|
Rapid turn around?
|
Yes
|
No
|
No
|
Yes
|
No
|
Yes4
|
No
|
Safe for nearby craft?
|
No
|
Yes
|
Yes
|
No
|
Yes
|
Yes
|
Yes
|
Can use cheap
materials?
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
No
|
No
|
Requires no
breakthroughs?
|
Yes
|
Yes
|
No
|
Yes
|
Yes
|
No
|
Yes
|
Gentle acceration3
|
Yes
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
Large payloads (>50
tons)
|
No
|
No
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
Interplanetary flight
|
No
|
No
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
Space launch
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
No
|
Yes
|
Estimated launch cost
per pound
|
<$500
|
<$500
|
$500
|
<$5002
|
N/A
|
N/A
|
>$10,000
|
Score
|
7
|
6
|
6
|
9
|
6
|
7
|
6
|
|
- Nuclear pulse propulsion has such good performance that this really isn't a drawback for that concept.
- I'm making a WAG (wild @$$ed guess) at this because I have no idea how much nuclear bombs cost.
- This is a measure of whether the system could be used for manned vehicles. Many of these systems can be tuned to provide different accelerations. However, the light gas gun would leave people as squished jelly in the bottom of the launch vehicle.
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Proceed to my next space blog, The Case for Space VII: Ram accelerator.
Return to my previous space blog, The Case for Space V: Lightcraft
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