Nuclear Thermal Rockets Versus SpaceX Starship

A nuclear thermal rocket is absurd. Our chemical rockets work mostly because they literally throw all that thermal energy out the back  A nuclear rocket has to be just as hot and then depend of the propelent to carry off all that heat.  And of course there is the mass and can this work ar0und our present engine design.,

After all Space X uses over 30 engines.  That happens to be what a design limitation looks like.

We need our Plasma fusion engine working with the enegy takeoffr in the form of direct electric power able to produce a plasma drive.  That would work in space at least to continously drive a payload both ways.

Before we have gravity management, we are stuck with chemical boosters to get into space.  The Space X approach is the practical path. It also move sub orbital cargos point to point as well.

Nuclear Thermal Rockets Versus SpaceX Starship

January 23, 2025 by Brian Wang

https://www.nextbigfuture.com/2025/01/nuclear-thermal-rockets-versus-spacex-starship.html

Angry Astronaut and Nextbigfuture commenter are making the case that SpaceX and Elon Musk must switch to nuclear thermal rockets to colonize Mars. I will review that the nuclear thermal rocket program is taking and will take far longer. Also, triple the ISP does not cut the travel time by one third. There is no discussion by supporters of nuclear thermal like Angry Astronaut about the cost completed systems that match or get close to Starship thrust or even the cost of a production system. There is no complete analysis of scaling up the supply chain. Angry Astronaut and NASA missed the problem of no 

Angry Astronaut describes a hypothetical nuclear thermal rocket assembled in orbit. He assumes a nuclear thermal rocket will be working by the end of 2030. He says the SpaceX approach to building a lot of rockets and doing 11 launches for refueling is more complicated than getting a nuclear thermal rocket working. Also, his plan is only for one nuclear thermal rocket and not mass producing them by the thousands as planned by SpaceX. Angry Astronaut has made his own crude graphics of his proposal for nuclear thermal rockets. This is not coming from Lockheed Martin, NASA, DARPA, General Atomics or BWXT. None of the agencies or companies involved are pushing to rapidly scale nuclear thermal rocket technology.

Basically, Angry Astronaut is saying everything will work so much better and safer if we pull nuclear Excalibur from the stone. Pulling nuclear Excalibur is clearly easier than building and mass producing thousands of larger chemical rockets. He does not mention it but maybe we could also make nuclear passenger jets to take people anywhere on earth in ten minutes instead of using 30,000 commercial passenger jets. Nuclear has better ISP and thrust specs so we should use it everywhere.

Jim Shoemaker, DARPA’s second DRACO program manager, says once a DRACO demonstration proves successful, it could take another 10-15 years before the technology is used on an operational basis. So youtuber Angry Astronaut says we should depend upon a nuclear thermal rocket that is not yet into its full design phase and has no actual specs (ISP or thrust). However, one of the program managers says it will 10-15 years after a demo in 2028 or so before nuclear thermal rockets could first be operationally used. This means 2028-2033. Rocket programs that depend upon completely new technology and that technology has to be scaled in size (millions of newtons of thrust or clusters of 250,000 newton engines) and mass produced by the thousands are far more risky.

A nuclear thermal rocket could potentially achieve double or triple the Delta-V of a SpaceX Starship with just the full orbital refueling. But to move large payloads of 100-200 tons, then the thrust must be increased far beyond the potential demonstration. There will also need to be larger hydrogen tanks.

NASA, Defense Advanced Research Projects Agency (DARPA) and Lockheed Martin (prime contractor) are design, build, and testing of NASA and DARPA’s nuclear-powered rocket demonstration, in collaboration with other industry partners. The Demonstration Rocket for Agile Cislunar Operations (DRACO) program will test a nuclear-powered rocket in space as soon as 2027. However, the 2027 launch date for the Demonstration Rocket for Agile Cislunar Operations (DRACO) is on indefinite hold.

UPDATE: Angry Astronaut also realized that the nuclear thermal rocket project has been delayed with lack of test facilities.

The DRACO program has not yet entered the implementation phase as originally planned. According to NASA’s FY 2025 Budget Estimate document, the project aimed to begin the implementation phase in September 2024. However, this target has been missed, and the program is still in earlier stages of development. A cold flow ground test of the reactor is planned for 2025 by Lockheed Martin and BWXT. The program’s design phase started two years ago and the DARPA-NASA management team has encountered the challenges inherent in sending a nuclear reactor into space for the first time in more than 60 years.

The DRACO team is working to maximize ground-based component and subsystem testing that can be done with existing capabilities, a subset of which will take place at Marshall Space Flight Center.

Even after the demonstration, DRACO’s challenges will not be behind it. Long-term storage of cryogenic hydrogen for the follow-on propulsion system remains a key challenge for the scientific community.

The engine would consist of a 1-m-long (3.2-ft.), ultra-high-temperature, high-assay low-enriched uranium-fueled, flow-through nuclear reactor. The fission reaction is harnessed to generate heat to energize a helium gas propellant, which is exhausted to produce thrust. A follow-on operational engine would replace helium with more energetic liquid hydrogen fuel.

The DRACO team has not signed off on a design for BWXT’s reactor. The reactor is at a “PDR level of maturity,” DARPA says it is examining design refinements meant to improve ground processing safety and enhance on-orbit data collection.

BWXT hasn’t recently completed any new reactors, the company has a long history of manufacturing nuclear components. BWXT has been manufacturing naval nuclear components and reactors since the 1950s, including components for the USS Nautilus, the world’s first nuclear-powered submarine. The company continues to manufacture reactor components for U.S. Naval submarines and aircraft carriers.

NASA is committing up to $499 million toward the DRACO partnership. This includes up to $250 million in costs for the design and development agreement for the nuclear-powered engine, as well as technical oversight and expertise from agency personnel. The U.S. Space Force will provide the DRACO launch and launch site support.

We do not know the exact thrust value for the General Atomics DRACO engine in 2025. It will probably be in the range of 100,000 to 250,000 newtons. Most of it has not been built yet. It will likely have 800-900 ISP in the first version.

How Fast Can SpaceX Starship Reach Mars -One Way?

We can calculate the travel times for the SpaceX Starship to reach Mars. It is relatively easy to get 90 day trips each way with SpaceX Starship. This is faster than the usual 180-270 one-way travel times. This can be faster because we will have a lot more fuel to enable more direct routes to Mars. We could catch up Mars in 1/6th of an orbit instead of half of an orbit around the Sun.

There are ways to use extra expandable Starship tankers that fly with the main Starship and then transfer the extra fuel for deceleration from higher speed.

If there is more things built and working in orbit around the Earth, then this can be used to enable more ways to save fuel for faster or bigger missions. This can be done with reusable tugs to move a fully fueled Mars bound ship to higher orbits or even to escape velocity.

There is online calculator for the Lambert’s Targeting Problem (LTP) to produce launch and arrival v-infinity pork-chop plots between solar system targets selected by the user.

Scott Manley explains how these calculations work.

In terms of drastically reducing the time in days to get from Earth to Mars then it helps a lot to be able to go faster. The SpaceX Starship can enable a lot of extra fuel and more powerful chemical engines. Extra fuel can come from orbital refueling of the Starship. Other spacecraft and the Starship might only have 10% of the starting fuel because they used most of it getting to orbit.

There is spreadsheet (not in editable form) showing calculations of the Delta-V (change in velocity) that is possible for a SpaceX Starship.

The performance of the SpaceX Raptor engines is already very good but SpaceX is working on better engines. LEET 1337 will have even higher chamber pressure which will enable more thrust. The SpaceX LEET 1337 engines will be simpler, lighter and cheaper. SpaceX will likely be able to build them at ten times the production volume from the same sized factory that will now make 4000 Raptor engines each year.

SpaceX is also developing improvements to Starship that could enable larger fuel tanks.

The travel time and fuel calculations are from a low earth orbit refueling location. This assumption and situation could be changed IF there was a reusable tug that moved the SpaceX Mars ship to near Earth escape velocity. This would let the Mars bound Starship get to a higher speed without using on board fuel. This would save fuel for deceleration and landing.

A lot of refueling and creating a reusable tug are things that can be done when the cost of ships and fuel to orbit are very low. SpaceX and Elon want to get the cost of fully reusable Starship down to less than $10 million for the upper stage. The mass produced rocket engines are targeting $250,000 each while the older Shuttle era RS25 engines cost about $100 million each. The methane fuel can be made from abundant natural gas or can be made in large quantities using solar power and factories using materials in Earth and Mars atmosphere.

The main game changers are the making the ship and engines 100 to 1000 times cheaper and fully reusable. Things that would be wasteful or too costly become affordable.

There is more fuel that is needed to slowdown once a ship gets to Mars. There are ways to use the Martian atmosphere to aerobrake to land on Mars using no fuel. There is a maximum speed for aerobraking to work.

Background

The solution method is based on the works:

Approximate Analytical Solution of the Lambert’s Targeting Problem. Claudio Bombardelli, Juan Luis Gonzalo, Javier Roa. In Journal of Guidance, Control and Dynamics, Volume 41, Issue 3, 2018, pp. 792-801. https://doi.org/10.2514/1.G002887.

Approximate Analytical Solution of the Lambert’s Targeting Problem. Claudio Bombardelli, Javier Roa, Juan Luis Gonzalo. Paper AAS 16-212 in 26th AAS/AIAA Space Flight Mechanics Meeting, Napa, CA, USA, 14-18 February 2016.

Solar system bodies move on Keplerian orbits (all perturbations are neglected) and transfer arcs are Keplerian.

There are errors of up to 15-20% because it is computation of very inefficient transfer arcs (i.e. far from minimum delta-V conditions).

SpaceX Starship in 45 Days to Mars One Way

People believe that exotic new propulsion systems are needed to reduce the one way trip times from Earth to Mars from 180-270 days down to 45 days each way. The slower mission times are for chemical rockets where we barely get out of Earth orbit with a small rocket engine. SpaceX Starship can refuel after reaching orbit to enable faster orbits (straighter and less looping paths) to go to Mars. This makes 90 day times each way easy with chemical Starship and even more wasteful but still chemical rockets to Mars in 45 days each way.

This is calculated by Ozan Bellik.

In 2033 there are opportunities to do a high thrust ~45 day outbound transit with a ~10.5km/s TMI (trans Mars injection). If you refill in an elliptical orbit that’s at LEO+2.5-3km/s then the TMI burn requirement goes down to 7.5-8km/s. A SpaceX Starship with 1200 tons of fuel should be able to do with roughly 150 tons of burnout mass. This is enough for ship, residuals, and a crew cabin with enough consumables to last a moderately sized crew for the 45 day transit. The trouble is that once you get there, you are approaching Mars at ~15km/s.

Related Background

SpaceX Starship version 3 will be 150 meters tall. The current Super Heavy Starship is 120 meters tall (397 feet). It wil go from as tall as a 36 story building to 45 stories.

SpaceX is expanding the factory to make more Starships to make one every 72 hours.

SpaceX will scale to hundreds of Starship built per year by 2026.

Details on the transfer orbits between Earth and Mars.

Continuing the Details of Getting to Mars in 45 Days With Starships

One way to solve this would be to powerbrake to 8.5km/s and aerobrake from there. This needs an additional ~6.5km/s, and the most straightforward way to solve that is to send a fleet of expendable tankers alongside the crew ship and refill en route. This would call for 5 or more fully filled 2000 ton tankers in elliptical orbit. SpaceX could do it.

How can you refill an empty Starship either in Mars orbit or on the surface. Make the methane out of the Mars atmosphere or bring the return propellant from Earth. A slower and more fuel efficient 2000 tanker tanker topped off in elliptical Earth orbit can bring about 1200 tons of that fully propulsively to an elliptical Mars orbit.

A 7.5km/s burn from elliptical Mars orbit will top out at around a 60 day return time in our pretty optimal 2035 return window. We again need extra fuel to brake. Starship again must slow down and then aerobrake (15-16km/s vs. 12.5km/s ITS baseline). The same tanker fleet trick as for the outbound could credibly solve the return speed problem, and technically 45-day outbound + 60-day inbound does satisfy the 45-day transit is achievable with Starship claim.

More vehicle staging from elliptical Earth orbit for the outbound, and a cascade of tankers to deliver the requisite prop to HEO and HMO. This would enable a fully chemical orbital transfer vehicle setup that can do 45-day transit in both directions in the ’33-’35 mission window with a total of 25kt of payload to LEO and less then a dozen expended Starships.

SpaceX Starship has many ways with more ordinary refueling to enable 8.5 km/s delta-v which is far more than the 3.8-4.5 km/s for the low fuel and slower Hohmann orbital transfers that take 180 days.