I started writing this as a "help guide" for my Design A Starship for the Sublight Universe tool. But it's grown and metastasized into a topic that needs its own discussion.

I am assuming that if you are reading this, you have had at least a high school education. And that probably included at least an introduction to basic physics. If it has been a while for you, or if the words Newton's Laws of Motion sound like a legal doctrine, take a minute out and review them.

While we are going to be building on Newton's laws, the math we are going to use is a little beyond Newton's equations. The problem with rocket science is that we are causing something to accelerate at the same time we are making it lighter. That old F=ma... not much help. To really make sense of the process we have to fast forward a few hundred years in Physics to the Tsiolkovsky rocket equation. Which looks a bit like this:

ΔV deltaV The difference in speed between your rocket before the burn and after
ve effective exhaust velocity A fudge factor that describes how fast your propulsion technology can excite the propellent
m0 wet mass The mass of the spacecraft and all of its propellent.
mf dry mass The mass of the spacecraft at the end of the burn.

How much DeltaV you need depends on how fast you want to get there. For the moment we are ignoring the difference in orbital speeds between things close to the sun and farther away because they are tiny compared to what it takes to get there. At least with a fusion powered spacecraft. Here as a handy little table to help you visualize the distances between places in the solar system and how much deltaV would be required to get there in 1 day, 7 days, 14 days, 28 days, and 60 days

Point A Point B Distance AU 1 Day DeltaV 7 Day DeltaV 14 Day DeltaV 28 Day DeltaV 60 Day DeltaV
Main Belt Asteroids Mars nearest 0.28 484808 69258 34629 17314 8080
Earth Mars nearest 0.4 692582 98940 49470 24735 11543
Main Belt Asteroids Earth nearest 0.68 1177390 168198 84099 42049 19623
Main Belt Asteroids Venus nearest 0.942 1631032 233004 116502 58251 27183
Main Belt Asteroids Mercury nearest 1.353 2342661 334665 167332 83666 39044
Earth Venus farthest 1.828 3165103 452157 226078 113039 52751
Main Belt Asteroids Jupiter nearest 3.29 5696494 813784 406892 203446 94941
Earth Jupiter nearest 3.97 6873884 981983 490991 245495 114564
Main Belt Asteroids Mercury farthest 5.416 9377572 1339653 669826 334913 156292
Main Belt Asteroids Earth farthest 6.05 10475315 1496473 748236 374118 174588
Main Belt Asteroids Mars farthest 6.61 11444931 1634990 817495 408747 190748
Earth Saturn nearest 8.07 13972859 1996122 998061 499030 232880
Main Belt Asteroids Jupiter farthest 10.41 18024469 2574924 1287462 643731 300407
Earth Saturn farthest 11.22 19426949 2775278 1387639 693819 323782
Main Belt Asteroids Saturn farthest 15.07 26093059 3727579 1863789 931894 434884
Main Belt Asteroids Uranus nearest 16.74 28984593 4140656 2070328 1035164 483076
Earth Uranus farthest 21.2 36706892 5243841 2621920 1310960 611781
Main Belt Asteroids Uranus farthest 25.05 43373002 6196143 3098071 1549035 722883
Main Belt Asteroids Pluto nearest 28.04 48550059 6935722 3467861 1733930 809167
Earth Pluto nearest 28.72 49727450 7103921 3551960 1775980 828790
Earth Neptune farthest 31.5 54540901 7791557 3895778 1947889 909015
Main Belt Asteroids Neptune farthest 35.35 61207011 8743858 4371929 2185964 1020116
Earth Pluto farthest 50.4 87265441 12466491 6233245 3116622 1454424
Main Belt Asteroids Pluto farthest 54.25 93931551 13418793 6709396 3354698 1565525

Once you know the DeltaV you can figure out just how much propellent you are going to need. Here is a handy table of DeltaV and the relationship between m0 and mf. To keep things easy, we are keeping mf constant at 1.0. So if you have an idea for a space ship, you can just multiply it's mass by the number in this table to see how much propellent would be required to get it up to that deltaV. (Assuming your space craft's mass and the propellent's mass are in the same units.) We are also using a ve for a fictitious fusion propulsion system with a value of 2000000.0 (m/s). Our propellent is basically shooting out at 0.006666666666666667 the speed of light.

I list 3 different figures in the table below:

DeltaV M0 flat out M0 one way M0 round trip
1000 1.0005 1.001 1.002
3000 1.0015 1.003 1.006
5000 1.0025 1.005 1.0101
10000 1.005 1.0101 1.0202
25000 1.0126 1.0253 1.0513
50000 1.0253 1.0513 1.1052
100000 1.0513 1.1052 1.2214
500000 1.284 1.6487 2.7183
1000000 1.6487 2.7183 7.3891
1565525 2.1875 4.7852 22.898
2000000 2.7183 7.3891 54.5982
4000000 7.3891 54.5982 2980.958
10000000 148.4132 22026.4658 485165195.4098
93931551 2.4944634104437785e+20 6.222347706042806e+40 3.871761097489618e+81

For low DeltaV we don't need much propellent at all. 3000 m/s is the DeltaV to boost from low Earth Orbit to the Moon. 10000 m/s is the DeltaV to boost from the surface of the Earth into Low earth orbit. You can read more on those figures here. Keep in mind, the Wikipedia pages is using km/s. An there are 1000 m/s in one km/s.

Example Missions

All of these examples are for a 7 day trip across 2 astronomical units of space, at a cruising speed of 247351 m/s (0.143 au/day). Even though the vessels are all travelling at the same speed, you can see that depending on the the mission or the cargo, the wet mass can vary considerably.

Ferry Missions

Ferry missions fly from one place in the Solar System to another. In this case we are flying from one space port to another that is 2 AU away. This is a pleasure cruise taking its passengers on a 7 day getaway. The ship will be resupplied at the next spaceport, so it does carry any propellent or provisions for a return trip.

For math purposes, it is easiest to start with our destination and work backwards.

Stage Delta V Wet Mass Dry Mass
Decelerate to Destination 247351 1.132 1.0
Cruise to Destination 0 1.132 1.132
Accellerate to Destination 247351 1.281 1.132

So for our simplest mission, we need to bring along 0.281 kg of propellent per kg of ship.


Let us assume we are dispatching a spy craft to travel to a remote part of the Solar system and clandestinely monitor something. The ship will not be maneuvering at all on station. (At least with the main engines.) So 0% of our DeltaV reserved for the mission. Our cruising speed to get to our patrol station would be 0.25 of whatever we decide our deltav should be. We can then work out how long we want our Spacers out on patrol. Let's say 60 days. Next we need to work out how far away they will be patrolling. Let us assume we are in the Asteroid belt, and the areas we need to patrol is 2 AU away. We want to take 7 days to arrive, so our cruising speed is 0.143 au/day.

We are going to work backwards from our return trip. In the last stage of our journey:

Stage Delta V Wet Mass Dry Mass
Decelerate to Home 247351 1.132 1.0
Cruise to Home 0 1.132 1.132
Accellerate to Home 247351 1.281 1.132
Mission 0.0 1.281 1.281
Decelerate to Station 247351 1.449 1.281
Cruise to Station 0 1.449 1.449
Accelerate to Station 247351 1.64 1.449

The human mind would intuitively think "ok, I'm traveling twice the distance at the same speed, twice the fuel". But then the math says "no." We need 0.64 kg of propellent per kg of ship for this mission, not 0.562

We need to think of our return flight, with propellent, as the cargo for the first flight. In fact, you can see the entire home-bound leg of the trip is exactly the same as for a one-way flight.

Long Range Patrol

Let us assume we are dispatching a patrol craft to perform pirate interdiction. We want to leave plenty of propellent in the tank to chase down the pirates. So let us say we want 50% of our DeltaV reserved for the mission.

Stage Delta V Wet Mass Dry Mass
Decelerate to Home 247351 1.132 1.0
Cruise to Home 0 1.132 1.132
Accellerate to Home 247351 1.281 1.132
Mission 989404 2.1 1.281
Decelerate to Station 247351 2.377 2.1
Cruise to Station 0 2.377 2.377
Accelerate to Station 247351 2.69 2.377

Our patrol craft needs to launch with 1.69 kg of propellent for every 1.0 kg of ship (and crew, warheads, etc.). We do spend 0.819 of it flying around and doing our Errol Flynn impression. And our return flight still only uses .281. But the cost of carrying all of that mission fuel meant our outbound flight cost extra. 0.59 vs 0.359 for our recon flight.


Let us assume we are dispatching a supply craft to travel to a remote part of the Solar system. Like our recon mission, they are flying out and back again. But the twist for this mission is that they are dropping off a load of supplies, and flying back with empty cargo holds. The cargo is 90% of the dry mass of the ship on launch.

Stage Delta V Wet Mass Dry Mass
Decelerate to Home 247351 0.113 0.1
Cruise to Home 0 0.113 0.113
Accellerate to Home 247351 0.128 0.113
Mission 0 1.028 0.113
Decelerate to Station 247351 1.163 1.028
Cruise to Station 0 1.163 1.163
Accelerate to Station 247351 1.317 1.163

We need 0.317 kg of propellent per kg of ship. Less than the recon mission. Still more than the one-way trip. The savings comes from only having to lob 10% of the original mass back again.

Resource Collection

Let us assume we are dispatching a mining craft to travel to a remote part of the Solar system. This is the opposite of our cargo mission. The vessel is going to pick up mass during the mission phase. To make the math easy, we'll say its the same 90% ratio we used for the cargo example. I.E. the vessel itself only has a mass of 0.1, but it will arrive home with a mass of 1.0. The 0.9 extra is whatever resource it is collecting from the mission area.

Stage Delta V Wet Mass Dry Mass
Decelerate to Home 247351 1.132 1.0
Cruise to Home 0 1.132 1.132
Accellerate to Home 247351 1.281 1.132
Mission 0 0.381 1.281
Decelerate to Station 247351 0.431 0.381
Cruise to Station 0 0.431 0.431
Accelerate to Station 247351 0.487 0.431

We need 0.387 kg of propellent per kg of ship. This is a little more than dropping off cargo, but less than carrying that mass for the entire trip.


Rocket Science is ... weird. But we now know if you are going to run a service, you will spend a lot less in propellent if you drop something off, instead of picking something up. Both are cheaper than flying somewhere and neither picking something up, or dropping off. Only a military can afford to fly somewhere, fly some more for grins, and then fly home again. But the cheapest way to travel, by far, is between spaceports.

You can also see the power of the fusion engine to make traveling around the solar system relatively low cost. Let's look at our ferry missionm, with a propellent:vehicle ratio of 0.281. By comparison, a Boeing 747-8 has a max takeoff weight of 442250 kg. Of that, 191420 kg is fuel. That's gives us a propellent:vehicle ratio of 0.43. Oh yes, and a 747 burns Kerosene, our fusion ship "burns" water. Or lunar dust. Or whatever you can ionize into a plasma.

If you are curious, the ratio for the Space Shuttle at launch has a mass of 2,040,000 kg, and lands with a mass of 104,328 kg. That's a propellent:vehicle ratio of 19.55. The key difference is that the ve for the shuttle's propulsion system is only 4400 m/s (when operating in a vacuum). That's tiny compared to our fusion engines, and thus to develop the incredible speeds it needs to reach orbit it needs to use a lot of mass to get there. And that sucker ran on rubberized solid fuel and cryogenic oxygen and hydrogen.