Starship Lite: from rapid Interplanetary to Interstellar

Elon Musk stated that a stripped-down SpaceX Starship could become an interplanetary boost vehicle able to push probes towards the farthest objects in our Solar System. 

What other missions could the Starship ‘Lite’ do, and how quickly?

Near SSTO

Rockets performance scales favourably with size. A larger rocket dedicates less mass to propellant tanks, engines and other equipment relative to the quantity of propellant it can hold. In technical terms, bigger rockets have better mass ratios.

The SpaceX Starship, planned to stand 55m tall, 9m wide and at 1350 tons on the launchpad, increasing to 118m and 4,400 tons once mounted on top of its giant booster stage, makes the most of its size.

 

Art by Charlie Burgess.

Despite being made of steel, the launcher manages a dry mass of 85 tons. The addition of landing legs, longer propellant tanks and large delta wings likely brings this closer to 90 tons. This gives it a mass ratio of (1350/90): 15. The current versions of the Raptor engines it uses have a sea-level Isp of 330s and a vacuum Isp of 360s. The average Isp over the course of a launch is about 350s.

Tsiolkovsky’ rocket equation gives us the deltaV we can expect from the Starship:

• DeltaV = ln(Mass ratio) * Isp * 9.81 

The deltaV is in m/s.

The mass ratio is the dimensionless ratio of full to empty weight.

Isp is in seconds, and multiplying it by 9.81 gives the exhaust velocity in m/s.

We find that it can produce 9.3km/s of deltaV. This is enough to reach Low Earth Orbit, and validates claims that it can act as a single-stage-to-orbit vehicle.

Art by Charlie Burgess.

However, these figures are for a Starship with no payload onboard except the vehicle itself, and no reserve propellant to perform a powered landing. Placing 100+ tons in LEO requires the help of the ‘Superheavy’ booster.

Starship Lite

Elon Musk presented two versions of the Starship back in 2017: a crewed version and an uncrewed tanker or cargo-carrier version.

The 85-90 ton figures are for the crewed version. It has to have a large habitable volume, life support systems and other contributors to a larger dry mass.

The uncrewed version can dispense with all that. Its dry mass is reported to be 60-75 tons. The mass ratio increases to 18-22, as good as that of the Falcon 9 booster stage.

This tweet from Elon Musk introduces what we’ll be calling the Starship Lite – a stripped-down version with no features meant for re-entry, recovery or holding a payload. It would be a naked steel tank with an engine at the bottom and used solely in space. 

Starship Lite has a mass ratio of 30, from a wet mass of 1200 tons and a dry mass of 40 tons. It is unknown why the wet mass is lower than previously stated. The engines can be optimized for the vacuum environment – the addition of huge nozzles increases their Isp to 380s.

Going through the deltaV equation again, we find a value of 12.7km/s.

It will likely resemble the vehicle on the right. Art by 'teamonster'.

The vehicle could start out sitting in Low Earth Orbit, fuelled and ready to go. It could be a regular Starship that was converted in space instead of returned to Earth. Filling it up would take about 12 tanker launches.

Alternatively, it could be boosted into an extremely elliptical orbit, reaching out to beyond the Moon in apoapsis (400,000km) and just above the atmosphere in periapsis (200km). Tankers would struggle to match its orbit and deliver more fuel, increasing the number of launches required to fill it up to 70 (!).

For the following sections, we’ll attach various payloads to the Starship Lite and work out which missions can be carried out and how quickly they can get to their destination.

Ultima Thule and beyond

In that same tweet, Elon Musk talks about Starlink satellites converted into probes. They would have a solar-electric propulsion system with an Isp of 1600s, so with the mass ratio of 2, they’d have a deltaV of 10.9km/s. 

Between the elliptical orbit giving some starting velocity, a fully fuelled Starship Lite and the probes with their efficient engines, we can look forwards to some pretty extreme missions.

Adding up the deltaV amounts, we can already tell that the probes can be put into trajectories that escape the Solar System. This is what probes Voyager I and II accomplished.

Let’s look for the time required to reach the original goal: 2014 MU69 ‘Ultima Thule’.

The asteroid orbits at a distance of 44.5 AU from the Sun on average. Because we don’t have a launch date, and we can assume that the launch will be optimally timed and won’t need an inclination adjustment, we can do some simple calculations.

First of all, the Starship Lite is loaded with a couple of modified Starlink satellites. Let’s suppose 4 of them fit within a 1 ton payload. Mass ratio is reduced to 29.3

To escape Earth, the loaded vehicle burns all of its propellant at periapsis. It is already travelling at 10.9km/s, to which it adds 12.6km/s of deltaV. This gives it an initial velocity relative to Earth of 23.5km/s.

The Oberth effect is significant. Even after gravity slows down the Starship Lite, we expect it to shoot away into interplanetary space at a whopping 20.9km/s.

Earth orbits at 1 AU from the Sun at 29.7km/s. The escape velocity from the Sun at Earth’s orbit is 42km/s. Our Starship Lite leaves Earth and enters interplanetary space 50.7km/s. Another way of putting it is that the Starship is going faster than the Sun’s escape velocity… so it will continue travelling beyond the Solar System and go interstellar. After millions of years, it will meet another star system. A true star ship.

Kerbal Space Program, modified to represent the real Solar System, can give decent approximations of the trajectories possible. If the screenshots taken look too small to read on your screen, right click and open them to full size in a new tab. We position a target in Ultima Thule’s rough orbit and send off a model of the Starship Lite to meet it.

We find that Ultima Thule can be intercepted after about 6 years and 10 months. Our Starship Lite would pass the asteroid by at a blistering 28.6km/s!

Let’s add the deltaV from the probes’ electric engines on top. They can raise the velocity at which they escape Earth by another 10.9km/s, allowing for a total of 31.8km/s relative to the Earth, or an incredible 61.6km/s relative to the Sun.

The increased velocity shortens the travel time to 4 years and 7 months and the modified Starlinks cross the asteroid’s path going even faster. The biggest challenge would be resolving the asteroid in the probe’s cameras before it is out of sight again!

To the planets, quickly

There is plenty left to explore in the Solar System despite decades of probes and dozens of robotic missions. Scientists would love to be able to send a heavy probe loaded with instruments, RTGs, propellant and radiation shielding for long-duration missions to places such as Mercury or Uranus.

The Cassini-Hyugens mission put a lander on Titan and orbited Saturn for 13 years. It represented a 5.7 ton payload. Using the payload capacity of the Starship Lite, we can put together a bigger, heavier and more capable probe. Since we want the probe to spend a long time doing science instead of flying past like at Ultima Thule, we need to have a way to brake and insert the probe into an orbit around its destination. This means that the probe needs propulsion capability.

Now, working out the optimal probe mass ratios, power densities, ion engine endurance and all the other factors that go into proper mission design would take weeks of work and accurate simulation tools. ToughSF does not have access to those resources… so we will cut short the work by fixing the probe mass at 25 tons.

Depending on the mission parameters, those 25 tons could be nearly entirely dedicated to scientific equipment (24 tons dry mass, 1 ton propellant), entirely filled with propellant (1 ton dry mass, 24 tons propellant) and anything in between.

The exact propulsion type is left open. A hypergolic-fuel system with 320s Isp, where a lightweight 2 ton probe carries along 23 tons of propellants would have a deltaV capability of 7.9km/s would be ideal for a rapid gravity assist maneuver deep in Jupiter’s gravity well, where the radiation environment makes solar power tricky at the very least. A Starlink-like electric engine would work best when braking into orbit around Venus or Mercury, where abundant sunlight allows for decent acceleration. Going further, we could even expect a nuclear-electric power system and a HiPEP-derived 6000s Isp engine slowly accumulating velocity in the Outer Solar System; with a mass ratio of just 1.5, it would have a whopping 23.8km/s to perform a braking maneuver at Uranus or Neptune.

Furthermore, we won’t be using the complicated and expensive elliptical orbit as a starting point. While it might be worth it for a once-in-a-lifetime opportunity to visit an interstellar asteroid leaving the Solar System, it would be too expensive of an option for the exploration of our planets. Instead, we will assume a straightforward and cheaper 1,000km starting altitude.

We can go ahead and focus solely on the outbound trajectory from Low Earth Orbit. Where could Starship Lite position this 25 ton probe and how quickly could we get there?

A 25 ton payload increases the Starship Lite’s dry mass to 65 tons. This decreases the mass ratio to 18.46, and its deltaV capability to 10.9km/s.

From Hohmann trajectory data, we know that this is enough to reach every single body in the Solar System. 

However, relying on these trajectories means sometimes waiting for many decades for the probe to reach its destination. Instead, we will look at higher energy trajectories. We rely on Kerbal Space Program again to obtain approximations.

Let’s start with Mercury. It is a difficult planet to get to. The latest attempt, BepiColombo, has to perform multiple flybys of Earth, Venus and Mercury before it can enter Mercury’s orbit 7 years after lift-off. We won’t be so patient! We’ll make for a single transfer to Mercury.

We find that 10.9km/s is enough for a quick 55 day trip to Mercury.

Venus is closer. Earth’s sister planet was last visited by JAXA’s Akatsuki probe in 2010, where it failed to reach the desired orbit due to a malfunctioning engine. It had to wait 5 years before it could try again.

The ‘porkchop’ plots show us a way to get to Venus in a mere 30 days.

Mars has had a permanent robot population since 1971. Insight, the newest inhabitant, took about 7 months to get there.

According to our approximation, we could cut that down to 40 days with the Starship Lite.

Jupiter is far away. Juno had 4 years and 10 months of drifting through space to reach the gas giant.

A more energetic trajectory can carry a 25 ton probe to Jupiter in just under a year.

Saturn can be reached with a Hohmann trajectory lasting 73 months. Cassini-Hyugens took about this long to have a look at the Solar System’s most impressive rings.

If we had a Starship Lite sitting ready, we could have done it in just over 24 months.

Uranus and Neptune will always take a long time to get to, as they are 19.2 and 30.1 AU from the Sun respectively. Voyager 2, the only probe to visit both planets, took 8 years and 5 months to fly past Uranus and a full 12 years to pass Neptune.

Our faster trajectories mean a 4 year trip to Uranus and 7.5 years to Neptune. These long durations simply mean that chemical propulsion, even on the scale enabled by SpaceX vehicles, is not enough to cross over to the Outer Solar System in reasonable durations. It is much more likely that a good portion of the probe’s mass would be dedicated to electric rockets with high Isp that can shorten the trip and brake at the other end.   

Mars Express

Elon Musk’s dream is Mars. Just how quickly could we get to Mars using the Starship Lite?

It will depend of course on the payload we select for the mission. We also want to recover and reuse the Starship vehicle. Previous calculation assumed that once the payload separated from the vehicle, it would carry on into interplanetary or interstellar space, empty and discarded. Regular travel to Mars means that we would have to keep enough fuel in reserve to brake it into an orbit where it can be met by refuelling tankers.

One complication with using the Starship Lite instead of the regular Starship is that it does not have any features that allow it to aerobrake. No heatshield, no wings and no fairings means it must rely solely on its own propellants.

Let’s work out two scenarios: 10 ton fast, 10 ton staged and 10 ton ultrafast.

In the 10 ton fast scenario, we have as the name suggests a payload of 10 tons. The dry mass of the Starship Lite is therefore 40+10: 50 tons. We work out a mass ratio of 24.2 and a deltaV capacity of 11.9km/s.

When using 4.6km/s to leave a 1000km altitude Low Earth Orbit and 6.8km/s to brake into a ~31,000km altitude High Mars Orbit, we get a trip time of 120 days. The total deltaV is 11.5km/s.

In the 10 ton staged scenario, the payload is separated from the Starship Lite before it starts its braking burn. This allows the payload to perform an aerobraking or aerocapture maneuver while the Starship brakes while 10 tons lighter.

It is possible to accelerate by 5.36km/s leaving Earth to reach Mars in 95 days. After detaching from its payload, the Starship Lite can use its remaining 7.2km/s of deltaV to brake into a 170x800km Low Mars Orbit.

The 10 ton ultrafast trajectory consumes the Starship Lite, because we are entering the atmosphere. The payload stages and performs an aerobrake or aerocapture maneuver, just like in the staged scenario, but the Starship Lite burns up alongside it.

By not having to reserve any propellant for braking, we can take the most energetic trajectory possible. We find that using the full 11.9km/s deltaV capacity it has allows for a trip as short as 47 days.

The only caveat is that we must hope the payload’s heatshield can withstand an entry into the Martian atmosphere at over 19km/s!

Conclusion

The Starship Lite would be an amazing booster for sending off probes to all of the Solar System’s planets with much reduced travel times, or carrying significant payloads to destinations rapidly. In the most energetic trajectories, it proves itself to be a true Star ship.