Prediction: by year 2100 earth to mars in 19 days

This is an offshoot of the "System Ships" topic but my post is so long that I thought I should start a new topic.

It seems clear that a few hundred kilometers per second would be sufficient for getting around the inner solar system in a matter of days or weeks. So I decided to attack this problem from another angle. What velocities do we think they will achieve with technology available 100 years from now?

Imagine we have massive in-space infrastructure and don't have to worry about hauling everything up from earth in a small payload fairing. And imagine we have massive energy supplies. My in-system fast passenger ship would look like this: The propellant would be solid hydrogen stored in a thin-walled tank at its triple point, temperature 13.84K, and pressure 7040Pa. The engine would be a thermal engine where the hydrogen is heated by an external energy source rather than being combined chemically with an oxidizer. The energy source could be solar, fission, fusion, antimatter, or beamed to the ship with lasers. The hydrogen could be mined from icy comets and moons in the outer solar system. You could eventually mine it from the gas giants if you needed that much, but the gravity well is deeper. The hydrogen would be refined and frozen into fuel pellets which would be shipped on a slow freighter to where the people are.

What kind of velocities would this ship achieve? The figure of merit for fuel tanks is tank pressure times tank volume per tank mass. A reasonable value for current composite tanks is 2.5e5Pa*m^3/kg. I'll posit a one order of magnitude increase for new technologies like carbon nanotubes. A thousand cubic meters of solid hydrogen masses 88 tons. Its tank would mass:

7040Pa * 1000m^3 / 2. 5e6Pa*m^3/kg = 2.8kg

By making our fuel tank huge we could make the engine and payload masses vanishingly small. Such a rocket would have a mass ratio of 31,500 for a delta V of 10.4 times our exhaust velocity. With a chamber temperature around 3000K, pressure of 1000atm, and nozzle ratio of 1000:1 we would have an exhaust velocity of about 10km/s making a total delta V of 104km/s. With a chamber temperature around 6500K and nozzle ratio of 10,000:1 exhaust velocity would be 20km/s for a delta V of 208km/s, one hundred to accelerate and one hundred to decelerate. It looks promising, but let's add more details.

Of course you need to accelerate and decelerate fast too. It makes no sense to travel at a speed that will get you to your destination in a day if it takes you a year to accelerate to that speed. If you want to use up your 88 tons of propellant in 88,000 seconds (about a day) you will need a mass flow rate of 1kg/s. At an exhaust velocity of 10km/s that would make a thrust of 10,000 Newtons. Current engines that use low density propellants like hydrogen might get a thrust to weight of 50. Of course, this is in units of kilograms force per kilogram mass. I'll multiply by gravity to get 500 Newtons per kilogram. And I'll posit a one order of magnitude improvement here too. So the engine masses:

10,000N / 5000N/kg = 2.0kg

That's nicely within the same order of magnitude as the fuel tanks. It brings our mass ratio to 18,300 and delta V to 9.8 times exhaust velocity. But at the end of the burn our acceleration would be over 200g, not good for a passenger ship. We could add 5.2kg of payload and taper off thrust to maintain no more than 1g acceleration. Our mass ratio would be 8801 and delta V would be 9.1 times our exhaust velocity of 20km/s: 91km/s for acceleration and 91km/s for deceleration. To travel 1AU would take:

1.5e8km / 91km/s = 1.6e6s = 19 days

To calculate acceleration and deceleration time without calculus I'll assume the engine thrusts at 10,000N until the spacecraft mass reaches 1000kg and then thrusts at 1000N until the spacecraft mass reaches 100kg and then thrusts at 100N thereafter. We will use 87010kg of propellant at 1kg/s plus 900kg at 0.1kg/s plus 90kg at 0.01kg/s for a total of 105,000s or 29 hours which is nicely less than our trip time.

Of course, 5.2kg of payload isn't enough to hold a person, but if you want 5.2 tons of payload just scale everything up to 88,000 tons of hydrogen. I predict that at the end of the 21st century people will be traveling around the solar system with trip times in the range of weeks using ships similar to what I have just described.

Caveats:

Since I'm making wild guesses about technology one hundred years form now I am assuming an order of magnitude decrease in the weight of tanks and engines and assuming engine materials that can withstand hydrogen at 6500K and 1000atm. I'm also assuming an energy source so powerful that it can heat 88 tons of hydrogen to 6500K while fitting into an engine that weighs 2kg. Maybe solar or beamed lasers or antimatter could do this. Fission or fusion would probably be heavier. I'm also assuming that hydrogen will be cheap since you will need to use more than 10,000 kilos of hydrogen for every kilo of payload. If we use more conservative assumptions of 28 kg tank weight, 20kg engine weight, 3000K chamber temperature, and thus 10km/s exhaust velocity we get a mass ratio of 881 and a total delta V of 68km/s. After accelerating to 34km/s it would take 51 days to go 1AU. Really more Conestoga wagon than interstate.

What about using a multi-stage rocket?

Staging does very little when your tanks are light to start with. My single stage vehicle already has a mass ratio of almost 10,000.

What about electric thrusters that have higher exhaust velocity?

The problem with electric thrusters is their low thrust to mass ratio which puts an upper bound on acceleration and a lower bound on trip time. You would get to your destination before reaching your max speed so increasing max speed wouldn't help. Electric thrusters would certainly be used on slow freighters, but not on fast passenger ships.

I don't trust NEP--I'm more an NTR man myself. You have to have a big booster to put enough good tankage up there. And since HLLV's are under attack from all sides--sadly I predict we will not be on Mars--in 300 years...
... at any speed.

Even if theoretically possible, I doubt it would happen by 2100. If Mars is visited regularly by 2100, then I suspect it will be visited by the cheapest method possible. A 6 month trip aboard a Mars Cycler would not seem to be a hard price to pay for immigrants, to say nothing of cargo.

On the other hand, if we are talking of once-in-awhile missions or Mars, or the very first crewed Mars expedition by 2100, yes maybe then NEP, NTR, Magsail, whatever...

If we don't have some sort of fusion drive by 2100, then there's something very badly wrong with our technological development. So no, it won't be "Mars in 19 Days".

It'll be 4.

hopefully nobody will go to Mars by 2100 because faster than light travel would have been invented and planets similar to Earth will be colonized.

I wish. Not likely though. That is why I hate movies like Star Wars. People see these impossible space-fighters and things, and get bored when you show them real rockets--and automatically call those 'primative' when that is about all we have.--Then they listen to every kook on Art Bell who has an 'over-unity' device--and aerospace suffers from lack of interest.

The recent Discovery Channel specials just showed old footage of Moller and Graham Hawkes contraptions--that are still mere curiosities.

Nothing ever changes.

Yeah, practical, large-scale superluminal travel is probably a good deal more than 100 years away.

We should focus on a reliable interplanetary infrastructure.

The only thing that can warp space enough for that kind of travel is a collapsar like a black hole or a neutron/quark star.

And I don't see us 'making' one.

Ever.

Sigh.

oh come on stop saying never


there's physics out there that can allow you to do anything other than the obviously imposssible things.

Impassable. Nothing's impossible. -Alice in Wonderland (well, the Disney version anyway).


The problem with breakthrough physics, is you can't plan around them or for them, for that matter.

This is something I've wondered about. Would a fusion drive be low thrust to weight like an ion thruster? If so it would take longer than four days to accelerate even if it could eventually reach a speed that would travel the distance in four days. Now, you could use fusion to heat a bunch of hydrogen to get high thrust which is kind of like my design. Would fusion allow you to do better than 20km/s exhaust velocity at a 500-1 thrust to weight ratio?

The idea is not to worry about top speed -- but to keep the whole thing on a constant 1g burn. If you were to gradually adjust down to 0.78g (Mars-normal), then it'd take a bit longer (say a little under a week). A fusion drive (at least theoretically) can do that.

Take hydrogen. Add heat. Squeeze. Eject using magnetic "nozzle". Observe speed increase rapidly....

...

Wow ... that is absolutely kick-ass! But why aren't we burning this stuff already?

Oh ... damn ... boron oxides. Nasty. But is it any nastier than what gets burned now?

DKH

For the same reason we aren't burning anything else in a fusion reaction. We don't know how. At least not in a way that produces more power than it takes to cause the reaction. Yet.