Getting around the solar system quicker

[Marclev]

Getting to other planets typically involves waiting for a transfer window, and then either increasing or decreasing one's orbit until it intercepts the target, I.e. A Hohmann Transfer.

One thing that's always slightly bothered me about this, it seems that no matter how big a behemoth of a rocket you build it will take the same amount of time to travel to the target (give or take a few minutes of initial burn time), as it will only work with a certain orbital velocity.

So my question is, given some ludicrous amount of fuel and power (say enough to get you near light speed), how could one use it to vastly reduce the time of transit to a distant planet (e.g from 4 down to 1 years)?

[Supernovy]

One way to do this is to constantly accelerate all the way to the target, e.g. taking a torchship on a Brachistochrone trajectory.

Another way is to simply do a non-hohmann transfer - burn toward the target planet until you get to some ludicrous speed, and turn around to brake once you get there. I'm fond of calling these "hyperbolic transfers" as they almost certainly involve being over escape velocity during transit.

You can safely ignore orbital mechanics if you're travelling at a high enough speed!

[Taki117]

The problem is the rocket equation. The more fuel you have the more fuel you need and so on and so forth. However, it is possible to reduce this time if you point straight at your target and burn until you get an intercept.

[Eric S]

Actually, hohmann transfers can take various amounts of time depending on the amount of delta-v allocated to getting into/out of the transfer orbit. The most efficient hohmann transfers involve one planet at the periapse, one at the apoapsis of the transfer orbit, but that's not required. By injecting the craft into an orbit where the periapse is below the lower planet, the apoapsis is above the higher planet, but both planets intersect the transfer orbit at the correct time, you can shorten transfer times at the cost of delta-v at both ends.

If you go to http://alexmoon.github.io/ksp/ you'll find a calculator that tells you the correct phase angle/ejection angle/ejection delta-v/etc to get from one planet to another at almost any time. By the time you're talking about burning 15k-102k delta-v (depending on the phase angle) to go from Kerbin to Duna, you're arriving in about 30 days, as opposed to the 264+ days of the delta-v-optimal transfer.

Beyond this, there are transfers that are constant thrust maneuvers, but these tend to dramatically increase the fuel required unless you're talking low-thrust craft, so you'd probably only see them in KSP if you're using far-future engines or the infinite fuel setting.

[rgriffith26]

Basically, just point and shoot. lead your target and fire away.

You have to remember, though, that the amount of fuel and power it takes you to get up to speed will be about the same as what's required to slow you down (firing retrograde). It really becomes a matter of escape velocity around the body in which you're orbiting. If you speed up too much, you'll simply be going too fast for the body to maintain it's "pull" on your ship. Again, if you have enough fuel and power, you don't have to worry about that. You can just power your way to wherever you want.

Hope that helps.

-Ryan

[problemecium]

If you want to go REALLY fast, and realism isn't important to you, I suggest looking into Interstellar or my Jump Drive mod.

[Mr Shifty]

Basically, just point and shoot. lead your target and fire away.

You have to remember, though, that the amount of fuel and power it takes you to get up to speed will be about the same as what's required to slow you down (firing retrograde). It really becomes a matter of escape velocity around the body in which you're orbiting. If you speed up too much, you'll simply be going too fast for the body to maintain it's "pull" on your ship. Again, if you have enough fuel and power, you don't have to worry about that. You can just power your way to wherever you want.

This is almost entirely incorrect. 1) You can't just lead your target. 2) Ejection burns are not generally the same as injection burns. 3) When you intercept another planet from outside its SOI you'll always be going faster than escape velocity. 4) While it's technically true that enough fuel and power will get you anywhere, you can't achieve the speeds you'd need without hugely overpowered (though fun!) mods like the nuclear pulse rockets.

As others have mentioned above, alexmun's intercept planner lets you plan intercepts for any launch window and decide about tradeoffs between travel time and delta-V requirements. (It also works for eccentric and inclined orbits, which Hohmann transfer calculations can't handle.)

[cantab]

The Hohmann Transfer's the most fuel-efficient direct route. It's certainly possible to expend more delta-V for a faster transfer. If you're using Alexmoon's transfer calculator, you can cap the transfer time in the advanced options, or just click the spot you want on the chart.

Even in stock, you can get loads of delta-V out of the nuclear engines if you're prepared to put up with some long burns. An LV-N and a Jumbo-64, propelling a 1.75 ton payload (I bet you can do a Duna lander in that), would get over 12 km/s of delta-V. During a launch window you could get from Duna to Kerbin in 10 (24-hour) days with that, and that's without aerobraking.

[rgriffith26]

I know that realistically, there is no way other than to do orbital transfers, intercepts, etc. However, the original post asked:

So my question is, given some ludicrous amount of fuel and power (say enough to get you near light speed), how could one use it to vastly reduce the time of transit to a distant planet (e.g from 4 down to 1 years)?

I was addressing the "unlimited power" idea. I agree with you on all the rest.

[klappertjes]

As said before, with the notion that it can have basically infinite amounts of fuel and thrust (enough to go 0.99999999999c within at least the timeframe of a hohmann transfer) then this is a non-issue. Go as the photon flies and head pretty much straight for your target. In my layman's understanding, if we did this on Earth going to Mars, the trip would be 15-45 minutes. Mars won't have moved enough for us not to hit its "SOI". Maybe lead it jsut a tiny bit to be sure. There you go, fastest you can get there.

Since the one main driving force behind KSP's game mechanics are arguably fuel and thrust management, and we've taken those away, what are we discussing that's a problem exactly?

[magnemoe]

Actually, hohmann transfers can take various amounts of time depending on the amount of delta-v allocated to getting into/out of the transfer orbit. The most efficient hohmann transfers involve one planet at the periapse, one at the apoapsis of the transfer orbit, but that's not required. By injecting the craft into an orbit where the periapse is below the lower planet, the apoapsis is above the higher planet, but both planets intersect the transfer orbit at the correct time, you can shorten transfer times at the cost of delta-v at both ends.

If you go to http://alexmoon.github.io/ksp/ you'll find a calculator that tells you the correct phase angle/ejection angle/ejection delta-v/etc to get from one planet to another at almost any time. By the time you're talking about burning 15k-102k delta-v (depending on the phase angle) to go from Kerbin to Duna, you're arriving in about 30 days, as opposed to the 264+ days of the delta-v-optimal transfer.

Beyond this, there are transfers that are constant thrust maneuvers, but these tend to dramatically increase the fuel required unless you're talking low-thrust craft, so you'd probably only see them in KSP if you're using far-future engines or the infinite fuel setting.

Thank for this, has bookmarked it.

This is especially useful going to Duna or Eve as the transfer windows are rare, 1500 m/s or a 40% increase gives you far more options.

Also for return trips, here you have a fixed but larger dV budget.

If you use a ship with very high dV budget you can pretty much burn a bit ahead of target. Still smart to use nodes as its more accurate. Remember using the orion nuclear rocket and did a 30 days trip to Jool this way.

[cantab]

The more "realistic" case would be a continuous 1 g acceleration, spending half the trip speeding up and the other half slowing down. Given unlimited delta-V, this is likely to be as fast as humans will want to travel over long distances, though for short trips or in the military higher accelerations might be accepted.

If you want to try this in KSP, then of course the simplest approach is to use the infinite fuel cheat. Be prepared for some long burns though, since you'll be flying the whole trip in at most 4x real speed.

[Kasuha]

If you click inside the graph generated by Alexmoon's calculator, it will display you a transfer corresponding to that selected point - transfer duration and launch date. That way you can get even very fast, non-Hohmann transfers. At a corresponding price of spending way more dv.

Edited June 6, 2014 by Kasuha

[Mr Shifty]

Actually, hohmann transfers can take various amounts of time depending on the amount of delta-v allocated to getting into/out of the transfer orbit. The most efficient hohmann transfers involve one planet at the periapse, one at the apoapsis of the transfer orbit, but that's not required. By injecting the craft into an orbit where the periapse is below the lower planet, the apoapsis is above the higher planet, but both planets intersect the transfer orbit at the correct time, you can shorten transfer times at the cost of delta-v at both ends.

Just a point of semantics: Hohmann transfers are two-tangent transfers. Their transit time is fixed. If you're not intercepting on a tangent (i.e. at one of the apses of your transfer orbit), you're not doing a Hohmann transfer.

[Northstar1989]

The Hohmann Transfer's the most fuel-efficient direct route. It's certainly possible to expend more delta-V for a faster transfer. If you're using Alexmoon's transfer calculator, you can cap the transfer time in the advanced options, or just click the spot you want on the chart.

Even in stock, you can get loads of delta-V out of the nuclear engines if you're prepared to put up with some long burns. An LV-N and a Jumbo-64, propelling a 1.75 ton payload (I bet you can do a Duna lander in that), would get over 12 km/s of delta-V. During a launch window you could get from Duna to Kerbin in 10 (24-hour) days with that, and that's without aerobraking.

The ideal launch window changes the more Delta-V you spend on the transfer.

For transfers costing many times the Delta-V of a minimal-energy transfer like the Hohmann, you want to launch closer to Closest Approach. This is because gravity will curve your trajectory less over the course of a shorter, higher-speed transfer, and thus you want to be performing a transfer that can take best advantage of the relatively straight line...

Orion Nuclear Pulse rockets aren't unrealistic- they're a real life technology we've had the capability to construct for decades. The only thing that's been missing is political willpower.

You can also get some frankly insane Delta-V value with the (perfectly realistic) technologies of thermal rockets and Microwave Beamed Power in KSP-Interstellar (the last breakthroughs needed for this in real life were discoveries made in gyrotrons in 2005), with asparagus-staged drop tanks in orbit (asparagus staging may not make sense in real life in-atmosphere, but it's definitely the best strategy for high Delta-V interplanetary transfers...) and relying purely on LH2 for maximum-ISP fuel... (with enough beamed power, you can still get respectable thrust with this- in fact with enough nuclear reactors at the KSC, you can even launch small dozen-GW plasma-thruster powered spaceplanes from the KSC runway, as spaceplanes can get by with TWR less than 1...)

Regards,

Northstar

Edited June 7, 2014 by Northstar1989