High TWR low ISP vs low TWR high ISP (VAC)

[Tyoi]

So, got a question for my fellow kerbalnauts here: Let's say I make a satellite for, uhm, -insert planet name here-. I want it to orbit on a 90 degrees inclination because I want to scan it with some kind of scanning addon. (or whatever)

The lifting stages did their job and I'm now in stable orbit around kerbin, ready for a transfer to the other planet. Now, did I put a engine on the satellite with a high(er) TWR but low(er) ISP (VAC) or vice versa? Isn't TWR less important in VAC? (All engines produce enough delta-V to reach my goal)

Or: (let's make this question 2...) When I know I'm gonna need 1000m/s delta-V, do I just take the engine that gets me there? I.e.: the highest TWR engine that fits the delta-V requirement, ignoring the ISP?

Let's say for argument sake I do not alter the final stage except for the engine (so the fuel amount stays the same) due to weight limitations the lifter stages have.

I do know ISP is your 'bang for buck' or fuel-economy gauge (to a certain extend, not really really) so I was thinking the ISP is more important in VAC over TWR, since I recon a higher TWR only shortens the time you need to (de)accelerate to make the transfer.

(For the newest kerbalnauts here: ISP: Specific Impulse. TWR: Trust to Weight (ratio). VAC: Vacuum. Delta-V: Erm... Google it.)

Edited December 27, 2013 by Tyoi

[Kimberly]

Your thrust-to-weight ratio is not very relevant in vacuum. Maneuvering with a very low TWR will test your patience, but that's it. However, ISP can safely be ignored, too--delta-V is partially derived from your engines' ISP, so if e.g. Kerbal Engineer says you have enough delta-v, you don't need to worry about ISP. Delta-V is what really counts.

You've pretty much got it right. Assuming you have enough delta-V, you want the highest TWR you can, simply because this makes maneuvering easier. But with enough delta-V, you ought to get there either way.

[Champ]

So, got a question for my fellow kerbalnauts here: Let's say I make a satellite for, uhm, -insert planet name here-. I want it to orbit on a 90 degrees inclination because I want to scan it with some kind of scanning addon. (or whatever)

The lifting stages did their job and I'm now in stable orbit around kerbin, ready for a transfer to the other planet. Now, did I put a engine on the satellite with a high(er) TWR but low(er) ISP (VAC) or vice versa? Isn't TWR less important in VAC? (All engines produce enough delta-V to reach my goal)

Or: (let's make this question 2...) When I know I'm gonna need 1000m/s delta-V, do I just take the engine that gets me there? I.e.: the highest TWR engine that fits the delta-V requirement, ignoring the ISP?

Let's say for argument sake I do not alter the final stage except for the engine (so the fuel amount stays the same) due to weight limitations the lifter stages have.

I do know ISP is your 'bang for buck' or fuel-economy gauge (to a certain extend, not really really) so I was thinking the ISP is more important in VAC over TWR, since I recon a higher TWR only shortens the time you need to (de)accelerate to make the transfer.

(For the newest kerbalnauts here: ISP: Specific Impulse. TWR: Trust to Weight (ratio). VAC: Vacuum. Delta-V: Erm... Google it.)

TWR is only relevant for takeoff and landing. Here, you will only travel from one body's orbit to another body's orbit. You want high ISP, and don't care for TWR. The only interest in a high TWR here is it makes your burns shorter (saving your real-life gaming time - the most precious ressource IMHO), that's why you don't want to use ridiculously-low thrust engines (like ion) for interplanetary travel.

TL; DR : Use nuclear engine. Take a lot of spare DV for plane-change if you are going to a massive planet/moon.

(All engines produce enough delta-V to reach my goal)

Changing the engine (and then, the ISP) will only make you use more fuel to go to the same destination. You might want that spare fuel for plane change, or to achieve a secondary objective after the first is ok, or anything else...

[Champ]

(...) so if e.g. Kerbal Engineer says you have enough delta-v, you don't need to worry about ISP. Delta-V is what really counts.

I partially disagree with it. Yes, if you have enough delta-v, you are sure to get to your destination. That's true. But you might need to change your inclination (Tyoi says he wants a polar orbit for scanning), and no dv-map can take this in account. So, he wants to have a lot of spare dv. Then, he cares about ISP.

By the way, spare dv is always usefull.

[enki]

Tyoi already knows he wants a polar orbit. That would be a trivial increase in dV requirements as long as the polar orbit was achieved before or right after the SoI change.

[Clockwork_werewolf]

ISP is almost all you care about in space but there is one exception. If you burn times becomes too greater a percentage of your orbit time you end up wasting fuel. A perfect burn would be a one second burn at exactly you PE, any time longer than this is technically inefficient but if you can use a nuke with TWICE the ISP of the next best engine (I ignore Ions) then it is probably better in most cases but not all. I once had a ship that Kept its second to last skipper stage even after circularising its orbit (it was over designed). It had Nukes that were far enough out to also fire. I sent a few of these ships to the Mun, first one without firing the skipper (I would have had to drop about 3000 fuel if I dropped the engine). The second I fired suing the skipper. There were 4 Nuke engines and one skipper but in the end the single burn with all engines was actually better than just firing the nukes.

I know the most efficient thing to do would be to fire for 30second to a minute each time I get to the PE until the AP intersects with the Mun but I'm not that patient

For a polar orbit if you have not already placed a ship into an intersect that would let it be in a polar orbit then Multiple Short Nuke burns at the ascending node (I think its that one not descending but just make the node and work it that way).

That or just strap on a mainsail and bring 34K of fuel, it not like we have a budget

[Champ]

Tyoi already knows he wants a polar orbit. That would be a trivial increase in dV requirements as long as the polar orbit was achieved before or right after the SoI change.

You are right. Please ignore my last comment.

I was all about the "think about the dv requirement for plane change" because last weekend one of my missions was a partial failure because of this, but it was particular (Wanted to launch multiple probes with different inclination - I had a lot of spare DV, but not enough and last probe never attained polar orbit)

Still, spare dv is always usefull. You can set a secondary objective after fulfilling the primary objective (I personally do that a lot, cause i always overengineer my ships, yet the secondary objective is rarely attained).

[Skorpychan]

Generally, I go for low TWR/high ISP when I go interplanetary, because I like the nuclear engines, and not throwing away loads of fuel with a single error.

For Kerbin SOI stuff, I prefer higher TWR because distances and dV amounts are so small it makes little difference. The exception is when I haul large stations and large amounts of fuel around. That ends up using my interplanetary systems.

For in-system small probes, nuclear engines are too weighty, and ion thrusters end up unable to move a significant science payload, so it's the little orange inline motors, or their radial cousins for slightly heavier craft. Decent ISP and TWR on the motor itself, but a very small amount of total thrust makes it inadequate for larger things.

[Tyoi]

Awesome, good to know I'm on the right track then.

Ofc, I'm not traveling without some extra dV, but for the sake of the argument I made some 'fixed' notations so it doesn't get too complicated. Main point I wanted to know was the TWR/ISP thing.

[Murph]

My take on it, if you're trying to be efficient.

If you're fighting gravity (ascent/descent from/to surface), TWR is what matters more. Once you have about 2.0 TWR (for ascent, sometimes less is ok for descent), relative to local gravity, then you can look at ISP to try to be efficient.

If you're doing large dV burns in orbit, ISP is what matters, TWR is almost irrelevant as long as your longest burns are completed in a reasonable time (i.e. less than 5 minutes, although longer is ok sometimes). A TWR higher than 0.5 is likely inefficient, and you might even be ok with a TWR as low as 0.1 or 0.2. Once it's above a sane minimum for the mission, the higher the TWR, the lower the efficiency, pretty much.

[wilt57]

TWR is not irrelevant in space and Isp is not totally trumped by TWR for launch. Clustering higher Isp engines to give your ship the needed thrust can result in less reaction mass needed to achieve a certain amount of delta-v, like the 4,500 needed for Kerbin orbit. Likewise, adding extra engines to an interplanetary ship for additional TWR can result in less wasted delta-v during the burn. Just wanted to get that out to start.

In the case you presented, you mention all engine choices give you the delta-v required to achieve your goal. In this case it would make sense to use the engine with the highest TWR to give you the shortest burns, but you've over looked something. By using the engine with the highest Isp (assuming it gives you high enough TWR) you more than likely have extra delta-v. This means two things:

you have more burn time to make any additional adjustments you may want/need to make or

you can reduce the amount of reaction mass your ship has and still have enough for your planned maneuvers.

For the second case, by reducing reaction mass, you have naturally increased the TWR of the ship. As the mass of your ship is less, a smaller lifter is needed to lift it to the same parking orbit above Kerbin. From an efficiency standpoint the second option makes more sense. Less is more in space travel.

[SRV Ron]

For the long 5 minute burn when orbiting, you want to chase the prograde marker, not the maneuver marker. (Unless you have to burn radical.) That is most efficient use of fuel. Expect that prograde marker to change as your speed increases or decreases.