Comment by jhgb | Hacker News Reader

Comment by jhgb | Hacker News Reader

Parent Full thread

> I don’t know that voluminous tanks and heavy engines are necessarily a problem for something that’s designed to permanently live in space - the tanks can essentially just be onion-layered gasbags, and could be km3 in volume if you wanted.

It's the opportunity cost. At low and moderate speeds (we're talking delta-Vs of 10 km/s and less), the same tankage simply gives you higher performance with chemical propulsion, so for no size of tankage may it actually be advantageous to use an NTR instead of a chemical engine. Only at extreme delta V levels do NTRs actually get better performance, but that's not a mission-to-Mars territory. LANTRs could possibly lower the crossover point, especially with variable Isp, but properly estimating how much requires calculus of variations, as I already said elsewhere.

> As to fuel - don’t get it from heavy bodies. Mine asteroids, minor moons, whatever.

Same issue. Your supply may be limited and/or require effort to extract. NTRs throw oxygen away; hydrolox and methalox engines use it for propulsion. For every tonne of water extracted, you'll go MUCH further if you go chemical, or at least with LANTR instead of NTR.

>Only at extreme delta V levels do NTRs actually get better performance, but that's not a mission-to-Mars territory

Why is that not mission-to-Mars territory? You can shave months off the transit time with >15km/s delta-v.

1) The travel time benefits are degressive owing to increasingly eccentric heliocentric trajectories - the changes in trajectory length get smaller and smaller as your velocity vector stops being colinear with the planet's orbit upon intercept, so you don't really save a lot of additional time. (But you get the most benefits with even small increases above Hohmann transfer speed.)

2) Intercept velocities, on the other hand, are progressive -- pretty much for the same reason, combined with Pythagoras' theorem. At one point you stop being able to aerocapture, even with exerting downward lift in Martian atmosphere to prolong the braking phase.

Owing to these two things, I'm not quite sure that propelling yourself from LEO to Mars at 15 km/s would be a good idea, unless you intend to crash into the planet.

I am assuming that you can spend the delta-v at both launch and intercept, it sounds like you are assuming it is all used at launch?

This starts becoming disadvantageous even faster than just trying to speed up. At that point a much better improvement for you is the development of a magnetoshell decelerator. Mass-wise, there's no situation where propulsive intercept is better since a magnetoshell decelerator will be much more lightweight than even an NTR stage.

Why? Propellant is relatively light in an NTR system as far as I'm aware. I am also assuming that making a nuclear ship aerodynamic would be unfeasible (it will likely be long and only structurally reinforced longitudinally).

That's not quite right.

Using a Hohmann transfer orbit [1], you get from Earth to Mars in about 9 months.

Using an Aldrin Mars cycler [2], you can get in as little as 75 days. Of course, the Aldrin Mars cycler requires more delta-v, but that's the point, if you have more delta-v you get there sooner.

[1] https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

[2] https://en.wikipedia.org/wiki/Mars_cycler

Reply on news.ycombinator.com