I am thinking of making a story set in the First Extraplanetary War era of the "To The Planets Beyond Extended Edition" set during a military confrontation between an UNAPA Task Force sent to the Galilean Moons to conduct a ground invasion of those moons and a smaller Ganymedean Task Force. However, in the story, the UNAPA Task Force is intercepted near the moon/captured asteroid of Carpo.

UNAPA (Main) Ships

  • Antimatter-catalysed nuclear pulse propulsion (first generation) engines (primary engines); Gas Core nuclear thermal engines (fourth generation; Open Cycle version) (secondary engines)
  • 132 km/s exhaust velocity (Primary Engines); 45 km/s (Secondary Engines)
  • 180 kN of thrust (primary engines); 4,500 kN (secondary engines) x3
  • 70 – 120 km/s of delta v
  • 707 - 1200 tonnes of wet mass
  • 331 - 706 tonnes of dry mass
  • 0.02 Earth gees - 0.03 Earth gees of acceleration (primary engines); 1.2 - 2 Earth gees of acceleration (secondary engines)

Ganymedean (Main) Ships

  • Helium 3-Deuterium ICF (second generation) engines (primary engines); Gas Core nuclear thermal engines (fourth generation; Open Cycle version) (secondary engines)
  • 110 km/s exhaust velocity; (Primary Engines); 45 km/s (Secondary Engines)
  • 195 kN of thrust (primary engines); 4,500 kN (secondary engines) x3
  • 50 – 100 km/s of delta v
  • 817 - 1400 tonnes of wet mass
  • 482 - 740 tonnes of dry mass
  • 0.014 - 0.024 Earth gees of acceleration (primary engines); 1 - 1.7 Earth gees of acceleration (secondary engines)

Drones (Both Sides)

  • Gas Core Nuclear Thermal (Third Generation; Closed Cycle; Mini-version) engines (Main Stage); UDMH/N204 Chemical engines (Terminal Attack Stages)
  • 23 km/s exhaust velocity (Main Stage); 3.3 km/s (Terminal Attack Stages)
  • 460 kN of thrust x5 (Main Stage); 1,830 kN of thrust (Terminal Attack Stages; 1 per stage) x8
  • 30 - 37 km/s of delta v
  • 60 - 80 tonnes of wet mass
  • 12 - 22 tonnes of dry mass
  • 3 - 4 Earth gees of acceleration (Main Stage); 2.3 - 3.11 Earth Gees of Acceleration (per Terminal Attack Stage); 19 - 25 Earth Gees of Acceleration (combined Terminal Attack Stages)

The spacecraft all have kinetic weaponry to attack each other, with 2 ships in each task force also equipped with drones. The kinetic weapons are all with depleted uranium rounds of ammunition, each slug being 41 kilograms, each coilgun being between 150 to 332 km of maximum range, with an exit velocity of 20 km/s. Effective range for these weapons is 45 to 115 km. And this is just the range that these craft can use to get a targeting solution or lock on hostile craft, since in a general sense, the range is technically infinite.

Each of the coilguns has 41 kilogram slugs fired at a rate of 3 rounds per second, with the burst duration being 5 seconds and an accuracy (on average) between 0.01 metres position error per kilometre of distance to 0.9 metres position error per kilometre of distance for both sides, depending on how advanced their targeting systems are. The slugs are assisted by a 30 kilogram (wet mass) liquid-fueled LH2/LOX guidance stage to assist in staying on the target. ECM effectiveness is basically around a 50% - 72% effectiveness in the sense that they can disrupt communication systems and disrupt the targeting solution of a hostile craft. So, in the former this means that if you want to contact the departure point, or another craft, communications are going to be affected. In the latter, you can miss and in a few millennia, the depleted uranium round will hit the surface of another object in who-knows-how-many years.

A clarification: this does not imply spacecraft in this setting use kinetics entirely. Some law enforcement spacecraft use directed energy weapons (or DEWs for short) spacecraft to target specific parts of a hostile spacecraft, but are impractical for large scale military use in an offensive sense. Military-grade DEWs are basically anti-drone defences, and since the two Task Forces have Drone Carriers in this situation, those are included and there are spacecraft designed specifically for this purpose. Anti-drone DEWs have a wavelength of 3600 nanometres, and require a series of large mirrors, which are vulnerable in combat, thus forcing spacecraft to conceal them away from combat and remove a fraction of the concealment in combat, depending on how much power is needed to dispose of incoming missiles or drone swarms (in this case, the latter is the target). On average, though, the power requirements are too large to be made into practical offensive DEWs in this setting (for now, that is).

In addition, the sensors of the spacecraft used in the manoeuvre (and throughout both wars in general) will be addressed in Part II. :/

UNAPA Task Force Composition

  • 12 Space Dominance Vehicles (SDVs); 2 being Drone Carriers (100 - 120 drones each) and 8 Space Offence Vehicles (SOVs), 4 Space Defence Vehicles (SDeVs)
  • 3 Space Control Vehicles (SCVs) with 3 battalions each SDV with a complement of Transatmospheric Vehicles (TAVs) and/or Transatmospheric Pods (TAPs), the former for regular armed forces and latter for rapid deployment Espatiers and SpecOps units.

Ganymedean Attacking Task Force Composition

  • 6 Space Dominance Vehicles (SDVs); 2 being Drone Carriers (100 - 120 drones each) and 2 Space Offence Vehicles (SOVs), 2 Space Defence Vehicles (SDeVs)

Defences on UNAPA Task Force Target

  • 1 Orbital Defence Platform (ODPs; anti-spacecraft/missile satellite network server(s))
  • 8 - 12 SDVs ( 4 - 6 SOVs, 2 - 4 SDeVs, 2 Drone Carriers)
  • 32 - 45 Orbital Guard Vehicles (OGVs)*

UNAPA Objective: Launch a ground assault on the moons of Europa, Ganymede, Callisto and Io.

Ganymedite Objective: Prevent the UNAPA ground assault by either destroying them or forcing them to turn back (by this, I refer to return to the point of origin via gravitational slingshot).

Soft Detection Time: 1.2 Days Post-Departure

Hard Detection Time: 3.1 (Brachistochone Transfer)- 87 Days (Hohmann Transfer) Post-Departure

Identification Time: 5.8 (Brachistochone Transfer) 97 Days (Hohmann Transfer) Post-Departure

Target Lock Time: 6.9 - 7.9 (Brachistochone Transfer) ~100 Days(Hohmann Transfer; Depends on positions of both objects at the time of departure) Post-Departure (Arrival)

Oh and for those of you unaware of the characteristics of Ganymede or Carpo, 2 links, coming up! Ganymede link there and Carpo link there.

In addition: the x[insert number of engines here] is an engine cluster, and only applies to secondary engines of main craft and drone engines.

Asterisk #1 = Law Enforcement Only; To be used to minimise civilian casualties, if any.

With this in mind, can such an intercept manoeuvre be pulled off in a realistic fashion?

NOTE: The following provides a Reddit link to the scenario itself: UNAPA vs Outer Solar System Perspectives Part I.

  • 3
    $\begingroup$ Future Historian, generally speaking this board considers [hard science] to be a more stringent tag than either [reality-check] or [science-based], so you don't need to include those tags in your question. I'm going to make some tag changes to your post, feel free to change them back if you don't like them. $\endgroup$
    – Jim2B
    Commented Apr 19, 2016 at 15:14
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – HDE 226868
    Commented Apr 20, 2016 at 0:21
  • 1
    $\begingroup$ The Atomic Rockets website has lots of information to do these calculations, and probably discover other things you are missing or haven't considered yet: projectrho.com/public_html/rocket/index.php. Also look up the Space War threads on Rocketpunk Manifesto: rocketpunk-manifesto.com. You will find many ideas about how a war in space "should" be fought. $\endgroup$
    – Thucydides
    Commented Apr 20, 2016 at 4:58
  • 1
    $\begingroup$ Would be space warlords should look up this URL: rocketpunk-manifesto.com/search?q=space+war Enjoy. $\endgroup$
    – Thucydides
    Commented Apr 20, 2016 at 5:15
  • $\begingroup$ It strikes me as there are two problems here. The first one is easy "how do I get from point A to point B?" I think there's a hard science answer to that one, if we better understand the positions of both moons at the time the UNAPA strikes. The latter question is harder: how do you repel an attack. That requires much more information, ranging from the lethal range of your (unguided) coilgun rounds, to sensor array questions defining how much early warning they get, to strategic questions about how UNAPA and Ganymite forces choose to engage in combat. That may be harder to answer $\endgroup$
    – Cort Ammon
    Commented Apr 20, 2016 at 18:52

4 Answers 4



The answer to this question is very long and complicated. I'm not sure whether I'll commit to answering it to the standard of hard-science. However, I'll start with a general outline and work towards that goal. That way if I give up part way through, at least you will have something.


I'm basing my answer upon primarily these references:

  1. Atomic Rockets: Space War - Strategy and Tactics
  2. Rocketpunk Manifesto (at least 15 entries regarding space war)

Although it will take you a long time to do so, anyone wanting to write semi-realistically about space warfare should do so. It will change your perspective a great deal.


Space war is not similar to other types of war.

Set up

  • Both sides have decent sensor nets, stealth is not possible.
  • Drive flares reveal acceleration & velocity vectors, position, engine type, rough performance capabilities of the ship, and masses of ships.
  • Computers can easily calculate possible destinations for any fleet.
  • Any decent defensive planner will understand that the launch of a UNAPA fleet from Earth could very well signal the start of offensive operations and begin defensive preparations.
  • The UNAPA force gets to choose the engagement parameters (time of launch, location of intercept, etc.).
  • The offensive force always knows whether they plan to attack or just scare the defenders.
  • Offensive forces have more than twice $\Delta v$ to perform this mission within 1 year - if it's a one-way trip. $\Delta v = 120 km/s$ is enough for a round trip to Jupiter with a one-way trip time of 6 months
  • UNAPA forces will reserve $\Delta v$ so they can maneuver during the fight.
  • Plan on UNAPA forces to be en route for 6-12 months (the defenders have plenty of time to prepare a reception).
  • The UNAPA will either try to keep the defenders guessing about their true objective (i.e. which body in the Jovian system they plan to assault) or they'll choose to arrive at the worst time for the defense (probably when the bulk of defender's forces are on the opposite of Jupiter from their objective.
  • Given the masses of the ships involved and a reasonable density, these are tiny ships (smaller than US Naval frigates). Considering 2/3+ of their volume will be filled with fuel, expect the crews to be numbered in tens of people.
  • Distances in the Jovian system are about 0.1% of the distances between the planets inside Jupiter's orbit ($\frac{r_{Callisto}}{r_{Jupiter}} = \frac{1.8 \cdot 10^6 km}{7.8 \cdot 10^{8} km}$)
  • Since the spacecraft involved possess roughly similar performance characteristics, the defenders can easily throw every resource at attackers.
  • Given that the defenders can see attackers coming, that's what they'll do.

Defenders can always intercept attackers

Using the above setup:

  1. 6 month advanced warning
  2. Similar craft performance

That should hold true even if you require the more difficult "0-0" ("0" distance and "0" relative velocity) interception.

The 6 month warning provides the defenders with plenty of time to prepare.

  • $\begingroup$ Makes sense. So, since the Ganymedeans are doing the intercept, how much Delta V is required to make it in a reasonable amount of time (in a Hohmann Transfer from Ganymede to Carpo, that is)? $\endgroup$ Commented Apr 22, 2016 at 13:21
  • $\begingroup$ @FutureHistorian That's a completely different question, and one better suited for Space Exploration because it has nothing to do with building a fictional world. Specify the starting and final orbit and the time allowed, and someone can tell you how to calculate (and probably do the calculations for that particular case) the $\Delta v$ budget required as well as how it breaks down into the specific maneuvers. $\endgroup$
    – user
    Commented Apr 22, 2016 at 17:30
  • $\begingroup$ Bear in mind all ships have plenty of $\Delta v$ meaning they'll choose a faster transfer than a Hohmann transfer. They do not have enough for a constant acceleration though. Calculating a "0-0" intercept is tough. You basically have to pick an intercept location and velocity and then work backwards. Finding "the best" intercept requires many iterations of solving the problem. If I get time to do it, I might run it through a spreadsheet later but it won't be today. $\endgroup$
    – Jim2B
    Commented Apr 22, 2016 at 17:42
  • $\begingroup$ @Jim2B Thank you. Feel free to notify me when you finish. :P $\endgroup$ Commented Apr 22, 2016 at 17:44

Given the rudimentary state of development of the opposing forces, I see no reason that the defenders can succeed.

First, though, the ships are far too small. The troop carriers are sized for 3 battalions, or something in the vicinity of 3000 to 3600 troops for US Marine battalions Assuming similar ship sizes with wet navy ships, a Tarawa-class amphibious assault carrier will run about 45,000 tons to carry 1700 troops. Figure the landing aircraft for the Tarawa class are equivalent to landing shuttles as a first approximation. So something on the order of 100,000 tons is needed for your 3 battalions per troopship.

Your drone carriers are hauling 100 to 120 drones at 60 to 80 tons per drone, so a loaded carrier will mass in excess of 10,000 tons (and that's just payload). Figure at least 10,000 tons for the vehicle itself, and probably more in the vicinity of the 100,000 tons of the troopships. Presumably the carriers have to house maintenance crews, stores and parts.

It's important to realize that the defenders MUST intercept the troopships before they drop their troops. Once the invaders are down and near the settlements, bombardment from above is not an option, unless the GATF wants to take the Ben Tre option. This means that the defending forces are forced to loiter close to the target moons. It also means that they cannot spread out to cover all 3 moons, which would invite defeat in detail. As a result, they are vulnerable to detection by recon drones from the UNAPA task force.

The UNAPA, on the other hand, has the luxury of considerable excess $\Delta$v, which means they can delay their arrival and choose their attack vector.

Attack drones will presumably be armed with large fusion warheads, since their terminal maneuvering capabilities are far too limited to allow kinetic intercepts. Assuming a 30 km/sec terminal velocity, a 25 g acceleration, and a terminal guidance range on the order of the minimum KW targeting systems, terminal guidance can only provide about 500 meters of correction. Also, of course, a near-miss by a nuke will presumably have a useful EMP effect on the target.

Conspicuously absent from ship capabilities is point defense. Ships are sitting ducks. Armor will, in fact, work quite well against nuclear blasts, but the KW rounds are simply unstoppable. A 41 kg slug travelling at 20 km/sec will have an impact energy of 8.2 GJ (about 2 tons of TNT), and will core through any armor and any mere ship. Lengthwise. And trying to stop an incoming bogey by shooting it is pretty laughable if it's on a collision course. Even if you turn it to scrap, that scrap is still coming at you at 20 or 30 km/sec.

Note that the points below assume that at least the manned ships, and probably the drones, can be stealthed against passive detection, particularly in the IR.

Given the lethality of the weapons and the vulnerability of the ships, the victory will go to the one who shoots first, which will place a premium on drone deployment. Defensive picket drones need to be constantly radiating in RF or optical in order to spot intruders, and this makes them visible from long distances and avoidable. In the event of a meeting engagement (both sides in stealth mode until they spot each other visually), neither side can hit the other quickly. Drones accelerating at 25 g's will take 3 minutes to reach full attack velocity, so engagements at ranges of 4,000 km or so will provide lots of warning to the side being attacked to launch their own, unstoppable salvos of missiles. KW launches are much faster, with flight times of about 4 seconds for shots taken at maximum (115 km) range. However, the launch rails have to be precisely aimed, and this will involve rotating the entire ship which houses the guns. Turrets are not an option. Launching a 41 kg slug at 20 km/sec using a 10 meter rail will require launch forces of 20 MN for 1 msec. That's a peak g load on the projectile of about 2 million g's and a peak recoil force of 2000 tons on the mount.

So, the UNAPA will approach from some selected course to one of the moons, preceded by a swarm of recon drones which are on full active search mode, succeeded by a larger swarm of attack drones to service any targets located. These will be moving fairly fast. A second swarm of drones will arrive later, at lower velocity, preceding the main fleet, with the troopships braking hard in order to drop their troops.

If the GATF is in the first system, the attack drones will probably kill it. If not, the successive moons get harder for the UNAPA to take without getting killed, since their location is given away by the first attack.


1) Change the mass of your ship/missile to increase/decrease the chance to intercept.

2) Use gravity from nearby objects to aid your acceleration and hinder your enemies. a)This may require being able to predict the enemy's most likely path/course of action.

3) Defensive positioning. Place your forces in such a way that even with the lower thrust there is no incoming vector that could not be intercepted before planet fall.

4) Use stealth to hide your forces and only launch intercept forces when the target is within the threat envelope.

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    $\begingroup$ Hard science, please? :P $\endgroup$ Commented Apr 19, 2016 at 14:57


Your best bet, by far, for intercepting the opposing force will be artillery. Pepper the incoming targets with lots and lots of small, fast projectiles while they're en route instead of sending ships to meet them.

Cold, dark projectiles, below a certain size, will be virtually undetectable. At orbital speeds, the relative velocities between the incoming ships and your projectiles will be huge, resulting in intense damage from collisions. Furthermore, you'll be able to afford lots and lots of them. You don't need to worry about relatively low accuracy if you can launch a cloud of a million projectiles at someone.

Even an egg-sized projectile will be far larger than the sorts of micrometeorites that most spacecraft are equipped to deal with. Investing in huge orbital coilguns that can rapid-fire such projectiles with good accuracy will be farm more economical than investing your delta-v in lifting lots of air, crew, and life support equipment.

Furthermore, almost all of the expense in setting up good artillery will be upfront, and you can continue to fire them the whole time your opponents are approaching. If your opponent wants to survive the barrage, they'll need to invest so much in extra fuel and armor that their fleet won't be strong enough to face even a small orbital fleet once they get close to Europa.

Intercept in orbit

For any ships which can survive the long-range artillery bombardment, the best option for the defenders will be to wait as long as possible before intercepting. The best time to intercept will be in orbit of the target moon. This is to conserve fuel and allow the defenders to field the largest possible defensive fleet.

Moving anywhere in space requires a lot of fuel, relative to the mass of the ships being moved, as well as a lot of additional structure in terms of fuel tanks and engine stages. Waiting until the last possible minute will allow the defenders to field as much defensive materiel for as little cost as possible.


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