How effective is nuclear propulsion?

[Josiah]

TL;DR

How effective is it

Hi
I was just wondering about the effectiveness of nuclear propulsion, specifically nuclear thermal propulsion.

 

[gmax137]

What have you looked at so far? Have you seen this:
https://www.energy.gov/ne/articles/6-things-you-should-know-about-nuclear-thermal-propulsion

 

[Josiah]

Hi I was wondering how fast a space shuttle could go, if it didn't have to first escape the earths gravity. As in if it had the rockets attached to it.

 

[Astronuc]

Hi I was wondering how fast a space shuttle could go, if it didn't have to first escape the earths gravity. As in if it had the rockets attached to it.

The space shuttle does have rockets attached to it! It has three main engines (SSME). The large fuel tank contains the liquid oxygen and liquid hydrogen that is pumped through piping between the large tanks and Space Shuttle. The booster rockets (SRBs) provide the propulsion for the large tank. The SRBs then separate when the shuttle gets high enough to atmospheric drag reduces to the point that the tank can travel with the shuttle without being torn away.

Nuclear thermal propulsion alleviates the need for the chemical reaction between hydrogen and oxygen; the hydrogen is simply passed through the reactor core and thermal energy is conducted from the reactor fuel elements into the hydrogen. Even with only hydrogen, a Space Shuttle would still require a tank (vessel) to hold the hydrogen, whether external or internal.

The space shuttle is design to return to earth much like an aircraft, and thus serves in low earth orbit. For venturing further out from LEO, one would require a different configuration, much like Apollo system or the successor, Artemis.

https://www.nasa.gov/humans-in-space/artemis/
https://en.wikipedia.org/wiki/Artemis_program

So far, only ground tests of NTR systems have been tested (NERVA). There have been no in situ deployments of NTRs, so it is not clear how effective such a system would be. Theoretically, the specific impulse of a nuclear thermal system is about twice that of a hydrogen-oxygen chemical propulsion.

 

[sbrothy]

https://projectrho.com/public_html/rocket/enginelist2.php

 

[sbrothy]

My personal favorite:

https://projectrho.com/public_html/rocket/enginelist2.php#nswr

That would result in an exhaust velocity of a whopping 4,725,000 m/s (about 1.575% c, a specific impulse of 482,140 seconds). In a ship with a mass ratio of 10, it would have a delta V of 3.63% c. Now you're talkin...

Although it might be pure scifi.

 

[Josiah]

The space shuttle does have rockets attached to it! It has three main engines (SSME). The large fuel tank contains the liquid oxygen and liquid hydrogen that is pumped through piping between the large tanks and Space Shuttle. The booster rockets (SRBs) provide the propulsion for the large tank. The SRBs then separate when the shuttle gets high enough to atmospheric drag reduces to the point that the tank can travel with the shuttle without being torn away.

Nuclear thermal propulsion alleviates the need for the chemical reaction between hydrogen and oxygen; the hydrogen is simply passed through the reactor core and thermal energy is conducted from the reactor fuel elements into the hydrogen. Even with only hydrogen, a Space Shuttle would still require a tank (vessel) to hold the hydrogen, whether external or internal.

The space shuttle is design to return to earth much like an aircraft, and thus serves in low earth orbit. For venturing further out from LEO, one would require a different configuration, much like Apollo system or the successor, Artemis.

https://www.nasa.gov/humans-in-space/artemis/
https://en.wikipedia.org/wiki/Artemis_program

So far, only ground tests of NTR systems have been tested (NERVA). There have been no in situ deployments of NTRs, so it is not clear how effective such a system would be. Theoretically, the specific impulse of a nuclear thermal system is about twice that of a hydrogen-oxygen chemical propulsion.

No, I mean if it had the rockets still attached to it once it had left the earths atmosphere. And if they still had the full amount of fuel. How fast could it go?

 

[Josiah]

My personal favorite:

https://projectrho.com/public_html/rocket/enginelist2.php#nswr



Although it might be pure scifi.

In response to "That would result in an exhaust velocity of a whopping 4,725,000 m/s (about 1.575% c, a specific impulse of 482,140 seconds). In a ship with a mass ratio of 10, it would have a delta V of 3.63% c. Now you're talkin..."
What does exhaust velocity mean? How does it translate to the speed of the rocket?

https://projectrho.com/public_html/rocket/enginelist2.php#nswr



Although it might be pure scifi.

My personal favorite:

https://projectrho.com/public_html/rocket/enginelist2.php#nswr



Although it might be pure scifi.

 

[Janus]

No, I mean if it had the rockets still attached to it once it had left the earths atmosphere. And if they still had the full amount of fuel. How fast could it go?

For simplicity's sake, we will assume a shuttle in orbit with a fully fueled external tank. IOW, we won't factor in the Solid fuel boosters. The Shuttle itself has a mass of 75,000 kg. The external tank has a empty mass of 35,500 kg and holds 718,500 kg of fuel and oxidizer. This puts the fully fueled mass at 829,000 kg and the total "empty"mass at 110,500 for a mass ratio of ~7.5. The main engines of the shuttle have an exhaust velocity( Ve) of of ~4400 m/s. Delta v ( change in velocity_ can be found by Dv = Ve * ln(mass ratio) . So plugging the above numbers in, we get Dv = 4400 m/s *ln(7.5) = 8.87 km/sec. That's for a empty shuttle. If it had its full payload of 29,500 kg, this drops to 7.83 km/sec. ~3.2 km/sec of that will be used up just in achieving escape velocity from orbit. If we add in the solid boosters, this increases by about 4.7 km/sec

 

[Josiah]

For simplicity's sake, we will assume a shuttle in orbit with a fully fueled external tank. IOW, we won't factor in the Solid fuel boosters. The Shuttle itself has a mass of 75,000 kg. The external tank has a empty mass of 35,500 kg and holds 718,500 kg of fuel and oxidizer. This puts the fully fueled mass at 829,000 kg and the total "empty"mass at 110,500 for a mass ratio of ~7.5. The main engines of the shuttle have an exhaust velocity( Ve) of of ~4400 m/s. Delta v ( change in velocity_ can be found by Dv = Ve * ln(mass ratio) . So plugging the above numbers in, we get Dv = 4400 m/s *ln(7.5) = 8.87 km/sec. That's for a empty shuttle. If it had its full payload of 29,500 kg, this drops to 7.83 km/sec. ~3.2 km/sec of that will be used up just in achieving escape velocity from orbit. If we add in the solid boosters, this increases by about 4.7 km/sec

How many boosters would you need to go to the nearest star, in a human life time, or 100 years, again assuming there is no need to escape earths gravity.

 

[Josiah]

What if you added 2 NTP rockets to the boosters, and shuttle how fast would it go?

 

[Janus]

How many boosters would you need to go to the nearest star, in a human life time, or 100 years, again assuming there is no need to escape earths gravity.

To reach Alpha Centauri in 100 years, you would need to reach a speed of least 0.043 c (12900000 meters/sec) Given the exhaust velocity of even the main shuttle engines, this works out to e^2931 kg of fuel per kg of payload mass.( here e is Euler's number) This is more mass than contained by the observable universe. Chemical rockets simply aren't up to the task. Even our best Ion drives on the drawing board would require 1.47x 10^57 kg of fuel per kg to reach that speed (Still trillions of times the mass of our galaxy )

 

[Janus]

What if you added 2 NTP rockets to the boosters, and shuttle how fast would it go?

Given that this type of engine have about twice the exhaust velocity of the Shuttle's main engines, then if you were able to substitute, then about twice as fast as given above.

 

[Ken Fabian]

Given that this type of engine have about twice the exhaust velocity of the Shuttle's main engines, then if you were able to substitute, then about twice as fast as given above.

Which is not as much as I would have expected. Also I would expect higher 'dry' mass for a nuclear rocket, so less than twice for similar payloads, so not nearly as much gained by using them as comparing exhaust velocities suggest. Dry mass (is that the correct term?) is an issue for ion rockets too if I understand correctly.

Doubling rocket efficiency is significant but for the hoped for applications - cost effective trade within the solar system or for missions beyond - I think much more, orders of magnitude more, are needed and I am not seeing where those kinds of gains can be achieved.

 

[gmax137]

this works out to e^2931 kg of fuel per kg of payload mass.( here e is Euler's number) This is more mass than contained by the observable universe.

well, that tears it! No space operas without warp drives