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Description
Physical Specifications:

Dimensions: 30 m diameter bottom, 85 m height

Composition: carbon-fiber reinforced silicon carbide aeroshell, boron nitride aerogel insulation, stainless steel tankage

Specific Impulse: 401 to 818 s (sea level), 492 to 919 s (vacuum)

Dry Mass: 430 tons

Propellant Mass: 1550 tons liquid hydrogen, 3100 tons liquid oxygen

Payload to LEO: 845 tons

Thrust: 22.25 to 98.19 MN (sea level), 25 MN to 120.5 MN (vacuum)

Thrust/Weight: 0.383 to 1.69 (wet, full payload), 5.33 to 23.3 (dry, no payload) (sea level)

Maximum Delta-V: 16.0 km/s

Powerplant: 2 MW thermionic generator

This project started as an attempt to develop an updated version of the Vehement - a single stage to orbit reusable nuclear shuttle - that was both more powerful and more practical. Such proved to be much easier said than done, with several spinoffs resulting over the course of months that never quite made the grade, and even this iteration required nearly three weeks straight effort, three weeks after I'd developed code to assist in the process. It was an incredibly demanding process, but well worth it, as I would say this craft exceeded my wildest expectations.

Until SpaceX proved the cost savings potential of simply refurbishing spent rocket stages, space agencies sought to reduce launch prices by developing a fully reusable Single Stage to Orbit (SSTO) vehicle. The trouble herein lies in modern chemical rockets' low specific impulse (read, fuel efficiency), meaning any such craft has to be so many parts propellant to actually get into orbit that there's very little room left for actual rocket or payload. Nuclear Thermal Rockets (NTR's) can achieve much higher specific impulse, which should give much more room to work with. However, NTR's suffer relative to chemical rockets due to their comparatively low thrust to weight ratio, which is a problem when you're fighting Earth's gravity every step of the ascent. A pure NTR ship that can actually take off from Earth would thus require massive engines, cutting out most of the benefit you would have gotten.

You might have noticed that each system seems to excel where the other falls short. So, somebody got the bright idea, why not use the two together where they work best?

For NASA proposals, this has generally meant using a chemical lower stage (where thrust is important) and a nuclear upper stage (where thrust isn't important so you can focus on saving propellant). However, a much more fundamental synergy is possible. Enter LANTR, the Liquid Oxygen Augmented Nuclear Thermal Rocket, which is essentially a nuclear thermal rocket with a chemical afterburner. When you want high fuel efficiency, you run it in nuclear mode, and when you want thrust, you turn on the afterburner. For intermediate performance, you can change the amount of oxidizer thrown in to get in any value inbetween.

This is the basis of the Defiant. Taking advantage of the high thrust afterburner mode allows it to get by with much smaller engines, freeing up mass for other systems. Its wet mass is equivalent to SpaceX's Starship, yet in spite of having half again the dry mass, it can haul eight times more payload, very nearly twice Defiant's own dry mass. Moreover, this is assuming very conservative nuclear specific impulse - this is nearly on the lower end of what's possible for such engines, a more advanced ship could take push this further - and a number of design decisions that add mass but make the ship much hardier.

Further detail of ship systems and functioning as well as references are available if desired. I must thank the Atomic Rockets website and ToughSF server for their invaluable contributions to the above, and hope it merits their seals of approval.

Description
Physical Specifications:

Dimensions: 30 m diameter bottom, 85 m height

Composition: carbon-fiber reinforced silicon carbide aeroshell, boron nitride aerogel insulation, stainless steel tankage

Specific Impulse: 401 to 818 s (sea level), 492 to 919 s (vacuum)

Dry Mass: 430 tons

Propellant Mass: 1550 tons liquid hydrogen, 3100 tons liquid oxygen

Payload to LEO: 845 tons

Thrust: 22.25 to 98.19 MN (sea level), 25 MN to 120.5 MN (vacuum)

Thrust/Weight: 0.383 to 1.69 (wet, full payload), 5.33 to 23.3 (dry, no payload) (sea level)

Maximum Delta-V: 16.0 km/s

Powerplant: 2 MW thermionic generator

This project started as an attempt to develop an updated version of the Vehement - a single stage to orbit reusable nuclear shuttle - that was both more powerful and more practical. Such proved to be much easier said than done, with several spinoffs resulting over the course of months that never quite made the grade, and even this iteration required nearly three weeks straight effort, three weeks after I'd developed code to assist in the process. It was an incredibly demanding process, but well worth it, as I would say this craft exceeded my wildest expectations.

Until SpaceX proved the cost savings potential of simply refurbishing spent rocket stages, space agencies sought to reduce launch prices by developing a fully reusable Single Stage to Orbit (SSTO) vehicle. The trouble herein lies in modern chemical rockets' low specific impulse (read, fuel efficiency), meaning any such craft has to be so many parts propellant to actually get into orbit that there's very little room left for actual rocket or payload. Nuclear Thermal Rockets (NTR's) can achieve much higher specific impulse, which should give much more room to work with. However, NTR's suffer relative to chemical rockets due to their comparatively low thrust to weight ratio, which is a problem when you're fighting Earth's gravity every step of the ascent. A pure NTR ship that can actually take off from Earth would thus require massive engines, cutting out most of the benefit you would have gotten.

You might have noticed that each system seems to excel where the other falls short. So, somebody got the bright idea, why not use the two together where they work best?

For NASA proposals, this has generally meant using a chemical lower stage (where thrust is important) and a nuclear upper stage (where thrust isn't important so you can focus on saving propellant). However, a much more fundamental synergy is possible. Enter LANTR, the Liquid Oxygen Augmented Nuclear Thermal Rocket, which is essentially a nuclear thermal rocket with a chemical afterburner. When you want high fuel efficiency, you run it in nuclear mode, and when you want thrust, you turn on the afterburner. For intermediate performance, you can change the amount of oxidizer thrown in to get in any value inbetween.

This is the basis of the Defiant. Taking advantage of the high thrust afterburner mode allows it to get by with much smaller engines, freeing up mass for other systems. Its wet mass is equivalent to SpaceX's Starship, yet in spite of having half again the dry mass, it can haul eight times more payload, very nearly twice Defiant's own dry mass. Moreover, this is assuming very conservative nuclear specific impulse - this is nearly on the lower end of what's possible for such engines, a more advanced ship could take push this further - and a number of design decisions that add mass but make the ship much hardier.

Further detail of ship systems and functioning as well as references are available if desired. I must thank the Atomic Rockets website and ToughSF server for their invaluable contributions to the above, and hope it merits their seals of approval.