Raise periapsis with very low twr

[Irvin]

Hi,

I'm launching from Kerbin an ion ship using no rocket fuel at all, it works but I have the feeling I could be more efficient.

What I'm doing is zipping through the atmosphere with a set of rapiers and get as much kinetic energy as I can, it ends up with an apoapsis around 100/110km and an horizontal speed around 1300 m/s. At 30km or so the rapiers stop working and I detach the ion craft, at this point I'm 2 to 3 minutes before the apoapsis and I do not want to raise it any further. What I want to do is to raise my periapsis in the most efficient way, but as the ion craft has a ridiculously low TWR I have to burn as soon as I detach, I can't wait to be any closer from the apoapsis.

If I burn prograde my apoapsis is raised and I end up with a terrible elliptical orbit, so I've tried to burn well below the prograde marker so that the apoapsis stays almost constant and the periapsis is raised, but is this the best thing to do ?

Thanks.

Edited August 20, 2016 by Irvin

[GoSlash27]

Irvin,

Yes it is. Burn radial-in and accept the steering losses.

Best,
-Slashy

[Irvin]

Ok, thanks !

[Laie]

As a general rule of thumb, for low-TWR final stages, it's easier (and uses less dV) if you launch into a higher orbit. Thats because you need to be on a steep trajectory to begin with, just to have the time for your put-put upper stage to make orbit before it falls back into the atmosphere. Forcing a low orbit just outside the atmosphere requires extra work. To that end it may well be most efficient if you launch into a lopsided orbit, then circularise at PE.

Not necessary but helpful: get MechJeb and use it's SMART A.S.S. tool for controlling the vessel. This lets you set your direction to (eg) +20 degrees relative to prograde and will hold it without hassle. Very convenient for that kind of maneuver, also makes it easy to do and compare different approaches.

 

42 minutes ago, Irvin said:

If I burn prograde my apoapsis is raised and I end up with a terrible elliptical orbit, so I've tried to burn well below the prograde marker so that the apoapsis stays almost constant and the periapsis is raised, but is this the best thing to do ?

Depends on what's "best". In your situation, you can only control the shape of your final orbit by a) pitch and b) throttle. I take it that your TWR is so low that you have to go full throttle for most of the time, so controlling by pitch is the right thing to do. What pitch is "best" depends on your priorities.

The first thing I'd try, if I was given a vessel like yours, would be to burn prograde, maybe even pointing a bit higher than prograde; this will raise AP, but it will also shift it outward, buying more time to orbit. If this leads to a 200km AP, that's just how it is. From there, I'd work my way towards determining how much lower I can point and still make space.

If you insist on not raising your AP any further, you have indeed no choice but to point downwards in order to keep the AP static. Then again, doing that also brings your apoapsis closer, in the sense that you reach it even sooner. Leaving you with less time to raise PE. If you can point downwards and still make orbit, that means you have a higher TWR than strictly necessary. Your craft and ascent could be more efficient if you bring less thrust, or start your burn later.

I guess the "best" ascent profile, in terms of dV consumed, will have you point prograde-ish or maybe even slightly lower at first, then gradually pitching ever more upwards in order to stay aloft. It may even include overshooting the AP (that is you start falling down again) and catching your fall before you hit the atmosphere.

[Irvin]

That's a very interesting answer, I'll try what you suggested and compare what I end up with in terms of dV.

My goal is to go to the Mun and Minmus with only air breathing rapiers and ions, I can reach the Mum but I'm still a bit short of dV to reach Minmus and come back to Kerbin.

OTOH I can try to skip circularisation and burn straight to the Mun...

Edited August 20, 2016 by Irvin

[Superfluous J]

Remember that while it's true that burning off prograde "wastes" dV, efficiency is a far more complex idea. If you waste a few hundred m/s of dV but gain thousands from using that ship over another, then to me it sounds like the ion ship is the way to go.

[Irvin]

Actually the ion ship is just a challenge I set to myself :-)

[CSE]

23 hours ago, Laie said:

I guess the "best" ascent profile, in terms of dV consumed, will have you point prograde-ish or maybe even slightly lower at first, then gradually pitching ever more upwards in order to stay aloft. It may even include overshooting the AP (that is you start falling down again) and catching your fall before you hit the atmosphere.

This is a common launch profile for larger planets (64k, RSS, real life). If you have Kerbal Engineer available, then once you have left the atmosphere the KER time-to-atmosphere item will show you the time until you re-enter, which I find quite useful for any low-TWR upper stage that will complete circularizing after apoapsis. Use it the same way you'd use the time-to-apoapsis indicator for a higher-TWR launch.

Apart from that, you have two ways to learn to live with a highly-eliptical initial orbit. If your final destination is LKO, you can aerobrake to bring the apoapsis down to something more desirable: effectively, this is a bi-elliptic transfer launch. Or if you want a transfer to Mün or Minmus, launch into an elliptical orbit and wait until your periapsis aligns correctly for an efficient transfer. The only case it doesn't work well for is a transfer to other planets, where you'd need a lot of calculation to get the ellipse aligned with the desired ejection angle when the transfer window arrives.

14 hours ago, 5thHorseman said:

Remember that while it's true that burning off prograde "wastes" dV, efficiency is a far more complex idea. If you waste a few hundred m/s of dV but gain thousands from using that ship over another, then to me it sounds like the ion ship is the way to go.

I think this is the way to go for more range (Minmus, Duna, ...). If the rapiers are quitting from altitude, then it sounds like your lifter might be able to get a bigger upper stage up to 1300 m/s at 30km. So bring more fuel, or a multi-stage upper, or ion boosters on the sides if your aerodynamics allows, launch into a high apoapsis and just correct it with ion burns once your up there.

[Irvin]

Hi,

So I've managed to reach the Mun and come back with ~1100/1400 dV left depending on how I launch, land, etc so I think getting to Minmus too won't be a problem for this craft. Thanks to all of you for your advices.

Here are a few pics if you're interested in seeing my craft, I'll make a video later this week.

I saw the trick of using a heatshield in front of a fairing somewhere on reddit, I can't remember the guy's name but he was building a lot of SSTOs  as far as I can remember. At the expanse of quite some fuel to overcome the drag you can zip through the atmosphere at any speed without risking a RUD.

The launch profile is a little bit unusual, I build up speed asl up to 800/1000 m/s then I pitch up 40°-ish and let the fuel burn, depending on the exact profile I end up somewhere between 110/130 km apoapsis and 1250/1400 m/s of horizontal speed. I can go up to 1500 m/s asl but then there is no way to pitch up, I used this trick for a single-stage-to-the-island speed run and I was below 40s (never posted it on the reddit weekly challenge though).

Then when the atmosphere is thin enough I detach the ion craft and its five motors and two gigantors are more than enough to reach the Mun. 

The full upper stage can land back on Kerbin using the chutes and can be reused, I plan to do something for the booster stage too even if it will fly like a frying pan without the upper stage attached.

Edited August 21, 2016 by Irvin

[Geschosskopf]

On 8/20/2016 at 4:43 AM, Irvin said:

What I'm doing is zipping through the atmosphere with a set of rapiers and get as much kinetic energy as I can, it ends up with an apoapsis around 100/110km and an horizontal speed around 1300 m/s. At 30km or so the rapiers stop working and I detach the ion craft, at this point I'm 2 to 3 minutes before the apoapsis and I do not want to raise it any further. What I want to do is to raise my periapsis in the most efficient way, but as the ion craft has a ridiculously low TWR I have to burn as soon as I detach, I can't wait to be any closer from the apoapsis.

Well, I think your main problem is that this mission profile of wanting to reach a 100km parking orbit isn't suited to an ion-powered ship.  As you know, ion-powered ships have minuscule TWRs and more dV than they'll ever use.  This has several important implications that means you should do things differently with than than with other engines.

First off, there's no benefit from ejecting from a low parking orbit.  The Oberth effect depends not only on the velocity of the ship, but also on the mass flow rate of the burned fuel.  Ion engines don't put out much in the way of exhaust mass so Oberth has no noticeable effect on them.

Second, you should never burn more than about 5 minutes at a time in LKO.  Otherwise, you end up losing a lot of efficiency due to cosine loss.  But a transfer burn of only 5 minutes with an ion engine is unlikely unless the probe is very tiny.  Therefore, if you start low, you'll usually end up having to break the total burn up into several sections of no more than 5 minutes each spread over several orbits, and this throws your timing off on doing the transfer burn, so you need a bigger correction down the line.  So it's usually better to start from a higher parking orbit, where the cosine loss for a longer burn is still acceptable, and you can do the whole burn in 1 go.

So here's what I'd do....  I'd burn the ion engine on the way up and accept the higher Ap you get.  And I'd circularize at whatever that ended up being, to be in a nicely high orbit from which you can do the whole transfer burn in 1 go.  This would be better in the long run than trying to keep the thing at 100km.

[Irvin]

Indeed the 100km parking orbit was a terrible idea, it's not suited at all for an ion craft.

[Sharpy]

It won't be a pure radial in burn - there will be a prograde component. How much?

To get the right burn angle (you won't have the time to do this during the actual launch, but you can "waste" one launch on planning, and perform the maneuver in the next launch, following the parameters from memory) - as your apoapsis is right, set up a maneuver node at the apoapsis, that gives you the circular orbit you need. Of course by the time you have the maneuver node made, you'll be too late to actually execute it, but remember the angle of the maneuver node on the navball, and perform the burn in that direction on the next launch.