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What would be the specific impulse of a continuous nuclear fusion drive?
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Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) Let's also assume we managed to direct all products (and none of the substrates) of the fusion in one direction (through the nozzle). Plus matters of cooling, safety etc, all the engineering trivia.
Essentially, the drive reacts Deuterium and Tritium, converting them into Helium through nuclear fusion, and the newly created extremely hot helium is ejected through the nozzle, as reaction mass.
What would be the exhaust speed of such a drive - speed at which the atoms of helium would be ejected; the specific impulse of such a drive?
I tried to ballpark typical temperatures of fusion plasma into average speeds of particles, and got nowhere, as the gas equations don't really work with plasma. Could you give me a ballpark value?
advanced-propulsion specific-impulse
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SF.
30k899214
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up vote
2
down vote
favorite
Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) Let's also assume we managed to direct all products (and none of the substrates) of the fusion in one direction (through the nozzle). Plus matters of cooling, safety etc, all the engineering trivia.
Essentially, the drive reacts Deuterium and Tritium, converting them into Helium through nuclear fusion, and the newly created extremely hot helium is ejected through the nozzle, as reaction mass.
What would be the exhaust speed of such a drive - speed at which the atoms of helium would be ejected; the specific impulse of such a drive?
I tried to ballpark typical temperatures of fusion plasma into average speeds of particles, and got nowhere, as the gas equations don't really work with plasma. Could you give me a ballpark value?
advanced-propulsion specific-impulse
asked 2 hours ago
SF.
30k899214
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) Let's also assume we managed to direct all products (and none of the substrates) of the fusion in one direction (through the nozzle). Plus matters of cooling, safety etc, all the engineering trivia.
Essentially, the drive reacts Deuterium and Tritium, converting them into Helium through nuclear fusion, and the newly created extremely hot helium is ejected through the nozzle, as reaction mass.
What would be the exhaust speed of such a drive - speed at which the atoms of helium would be ejected; the specific impulse of such a drive?
I tried to ballpark typical temperatures of fusion plasma into average speeds of particles, and got nowhere, as the gas equations don't really work with plasma. Could you give me a ballpark value?
advanced-propulsion specific-impulse
asked 2 hours ago
SF.
30k899214
Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) Let's also assume we managed to direct all products (and none of the substrates) of the fusion in one direction (through the nozzle). Plus matters of cooling, safety etc, all the engineering trivia.
Essentially, the drive reacts Deuterium and Tritium, converting them into Helium through nuclear fusion, and the newly created extremely hot helium is ejected through the nozzle, as reaction mass.
What would be the exhaust speed of such a drive - speed at which the atoms of helium would be ejected; the specific impulse of such a drive?
I tried to ballpark typical temperatures of fusion plasma into average speeds of particles, and got nowhere, as the gas equations don't really work with plasma. Could you give me a ballpark value?
advanced-propulsion specific-impulse
advanced-propulsion specific-impulse
asked 2 hours ago
SF.
30k899214
asked 2 hours ago
SF.
30k899214
asked 2 hours ago
SF.
30k899214
asked 2 hours ago
SF.
30k899214
asked 2 hours ago
SF.
30k899214
30k899214
add a comment |Â
add a comment |Â
1 Answer
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3
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The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a neutron is 1.0087
Thus the net energy production is about 0.019 or very roughly 1/250 of the mass of the products, and in the perfect engine you describe, all of this ends up as kinetic energy in the exhaust.
So we get that $$1/2 mv^2 = 1/250 mc^2$$
From which we can quickly find $v$ to be about $c/11$.
Specific impulse is exhaust velocity divided by $g$, so we get about 2.7 million seconds as the $I_sp$.
edited 1 hour ago
SF.
30k899214
answered 1 hour ago
Steve Linton
4,6461531
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
accepted
The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a neutron is 1.0087
Thus the net energy production is about 0.019 or very roughly 1/250 of the mass of the products, and in the perfect engine you describe, all of this ends up as kinetic energy in the exhaust.
So we get that $$1/2 mv^2 = 1/250 mc^2$$
From which we can quickly find $v$ to be about $c/11$.
Specific impulse is exhaust velocity divided by $g$, so we get about 2.7 million seconds as the $I_sp$.
edited 1 hour ago
SF.
30k899214
answered 1 hour ago
Steve Linton
4,6461531
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
add a comment |Â
up vote
3
down vote
accepted
The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a neutron is 1.0087
Thus the net energy production is about 0.019 or very roughly 1/250 of the mass of the products, and in the perfect engine you describe, all of this ends up as kinetic energy in the exhaust.
So we get that $$1/2 mv^2 = 1/250 mc^2$$
From which we can quickly find $v$ to be about $c/11$.
Specific impulse is exhaust velocity divided by $g$, so we get about 2.7 million seconds as the $I_sp$.
edited 1 hour ago
SF.
30k899214
answered 1 hour ago
Steve Linton
4,6461531
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
add a comment |Â
up vote
3
down vote
accepted
up vote
3
down vote
accepted
The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a neutron is 1.0087
Thus the net energy production is about 0.019 or very roughly 1/250 of the mass of the products, and in the perfect engine you describe, all of this ends up as kinetic energy in the exhaust.
So we get that $$1/2 mv^2 = 1/250 mc^2$$
From which we can quickly find $v$ to be about $c/11$.
Specific impulse is exhaust velocity divided by $g$, so we get about 2.7 million seconds as the $I_sp$.
edited 1 hour ago
SF.
30k899214
answered 1 hour ago
Steve Linton
4,6461531
The easiest way is just to think in terms of energy. Using numbers from wikipedia, the mass of a deuterium nucleus is 2.014 daltons, that of a tritium nucleus is 3.016, helium 4 is 4.0026 and a neutron is 1.0087
Thus the net energy production is about 0.019 or very roughly 1/250 of the mass of the products, and in the perfect engine you describe, all of this ends up as kinetic energy in the exhaust.
So we get that $$1/2 mv^2 = 1/250 mc^2$$
From which we can quickly find $v$ to be about $c/11$.
Specific impulse is exhaust velocity divided by $g$, so we get about 2.7 million seconds as the $I_sp$.
edited 1 hour ago
SF.
30k899214
answered 1 hour ago
Steve Linton
4,6461531
edited 1 hour ago
SF.
30k899214
edited 1 hour ago
SF.
30k899214
edited 1 hour ago
SF.
30k899214
30k899214
answered 1 hour ago
Steve Linton
4,6461531
answered 1 hour ago
Steve Linton
4,6461531
answered 1 hour ago
Steve Linton
4,6461531
4,6461531
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
add a comment |Â
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
A month of specific impulse... That's damn respectable!
– SF.
1 hour ago
add a comment |Â
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