questions about the ∆v science


I got some excellent questions on the last few articles and the answers deserve some space, so here’s that space.

I’m about to reveal how clueless I am about these topics, but these are making me reflect on recent sifi fiction. Could weapon recoil provide ∆v significant enough to be strategic (assuming weapons use power that doesn’t steal from thrust capacity)?

This will become clearer in a later answer but the short version is: probably not. Ships are going to be very hard to move due to their mass. Additionally, you probably don’t want a weapon that costs you ∆v unless you point it in exactly the right direction: the odds of that being both the direction of your target and opposite your desired vector change is mighty small.

Could you use weapons as thrust in an emergency? You could probably use some weapons to rotate the vessel rather than apply ∆v. Rotating your ship is comparatively cheap! In fact any weapon with recoil probably has a compensating jet to avoid this. I believe this is the case with the Rocinante in The Expanse! From the entry on PDCs in the fandom wiki:

They also utilize thrusters on their rear to counteract the recoil of the firing cannon, that would otherwise knock the ship off course.

To be clear, though, it probably wouldn’t affect the ship’s course, but it would rotate the ship. And I suppose if you’re burning the drive while firing that would indeed knock you off course.

Or nearby detonations (does space conduct shockwaves)? It’s intriguing to weigh the ∆v cost of any sort of space confrontation or skirmish.

Since there’s no atmosphere in space (by definition) there’s nothing intrinsic to transmit a shock wave. There will be some shock from the expanding plasma of the explosion of course, but we normally call that “damage”! And maybe a little photon push as well. Nothing that you would want to use as thrust.

However! If the explosion is energetic and close enough (and ideally shaped for the task), you can indeed propel your space ship with nuclear bombs. This would be a very poor ad hoc solution to a problem, but not an infeasible design. Obviously there are significant drawbacks to the design.

How does starship mass impact ∆v strategy? I’m probably wrong but I’m assuming greater mass requires greater thrust to achieve the same vector, and more thrust requires more fuel or efficiency, which all rolls into a single measure of total ∆v capacity.

I’ve only put the thumbnail here because it’s massively detailed and you should go to Winchell’s site to read about it and download the whole thing.

You are absolutely correct! For any given drive capability you can calculate the ∆v of the whole system by estimating the proportion of reaction mass (the mass you store only so you can shoot it energetically out the back) to the payload mass (the mass you have to keep). This is because of the “rocket equation” which I’m not going to go into, but Winchell Chung has an awesome chart showing ∆v for any given hypothetical drive type and any given mass ratio! It’s the basis of the game design that’s emerging here.

So yes, one of the joys of using ∆v as the core resource is that it encapsulates all kinds of information about the ship.

But would reducing your mass increase ∆v capacity through fuel efficiency?

The Marie Therese before ejecting the spin-grav luxury cabins and swimming pool.

Essentially yes! Let’s say you were the players in my last Diaspora campaign and were escaping in a luxury liner with a huge rotating spin-gravity living space. And you’re being chased. Ejecting that useless mass (which was huge) would change your r-mass:p-mass ration substantially, and give you a ton of spare ∆v.

Mass also impacts gravitational vectors, right?

Nope. The force you experience is dependent on your mass, but the acceleration you experience is not — it’s pretty much 9.8 m/s² for everything on Earth (but an elephant gets a lot more harm falling from a height than a mouse does — that’s the force). But not everywhere, and certainly not at different altitudes. But that’s way more detail than we need.

Does anything in space provide opportunity for aerobraking other than atmospheres?

Atmosphere is all I can thing of. Most interstellar gas clouds are way too tenuous to be interesting at this scale. Doesn’t mean you can’t invent one though!

Renewable sources of ∆v reserves look increasingly important (e.g. rechargeable solar vs consumable fuel?).

Well, basically your ∆v is going to be based on how much and how fast you can throw something out the back. So renewable is pretty tough unless you have a way to convert energy into mass. At least for rockets anyway. Remember for a rocket we’re not talking about power (although you might need some power to run the rocket, and that power supply will have its own energy needs which might include solar) but rather reaction mass.

Many thanks to Adam Minnie for the questions!

6 thoughts on “questions about the ∆v science

  1. >>Renewable sources of ∆v reserves look increasingly important (e.g. rechargeable solar vs consumable fuel?).

    >Well, basically your ∆v is going to be based on how much and how fast you can throw something out the back. So renewable is pretty tough unless you have a way to convert energy into mass.

    Solar sails get you dV in one direction, maybe more if you can use sligshots to realign those. Set up a series of those in a steady stream all over the solar system (time investment and air traffic control problem, not a industrial one) and you can pacman one up as it passes to absorb it’s dV. It’s like an icehockey player gaining speed by being pleted by pucks with little sails on.

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      1. I thought of another source of dV – catch.

        To follow ananlogy above, it’s two icehockey players facing each other and hitting a puck back and forth. every time they catch the puck, it accelerates them backward. Every time they hit the puck back to the partner, they accelerate backwards.

        Now swap the puck out for a big mass, and the stick for a railgun and the catching mitt for *something involving a pun on MagNets that I can’t do the maths for*.

        so the simple case two simiarly equipped ships can accelerate away from each other. As they move apart, the time between momentum boosts will increase (as the slug has further to travel), so I think the method gives a relatively fixed top speed.

        In a busy solar system though, we’re talking billions of ships active on millions of vectors and headings and paths (nearly all of which would be the in solar plane, which further condenses things into a 2d problem). So could you have a dV trading economy? We can get all the vector we need of Ship a, but it’d get us closer and if we pickup a slug from the wagon train heading widdershins, and a slight bend on that moon, it might just work?

        The trading economy would need a range of slug ‘coin’ sizes for different size vectors (and giving change), and ships might be equppied to only be able to take up to a certain size slug. Taking a hit from something heavier might be a worthwhile risk, like a small business taking a huge loan to expand.It might even be profitable to have nomad ships wandering the system, selling slugs high and buying them low.

        Liked by 1 person

      2. The “catch” method has a couple of issues that might not be issues. One is that they still need to move that mass — it’s really a very coarse-grained rocket. That’s not really a problem, but rockets rely on very high exhaust velocities, so catching this miss might be a bit of a trick. The other is that you can only accelerate in a straight line, but as the ships’ orbits expand they will no longer be in a position where this helps them — it starts to get inefficient fast. Still, there might be a place for it!


  2. Well now I want to do a monte carlo simulation of a inhabited solar system, With number of ships on transport paths between habitats a function of the pop of Hab A * Hab B, and see at any one time, which ships could credibly improve their path by trading a 0.1% of their velocities….
    with moving planets.

    Liked by 1 person

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