Some SSTO questions

[jsisidore]

I haven't played this math simulator in a very long time in fact I was sure I will never return to it as I hate math.

I did landed ssto on Duna but that was a long time ago and I forgot all about the nightmare that it was. Now I'm trying to build something practical to go to Minmus but I want to take processing plant with me so I will need mk2 cargo bay.

1. Does CoL has to be before CoM or after? How far?
2. I remember using wing incidence but what about the engines attached to the wings, should they be parallel with the fuselage? In the Skylon design they apparently do not.
3. Whats the best engine combo for ~30 tonnes? I got experience with 2 nukes and 2 rapiers at 33 tonnes but it seems I can no longer replicate it.
What speed do I need before 30km? If I reach 1500m/s at 10 degrees the prograde falls to 0 and there's nothing I can do to raise it without losing control.
4. How important were wings? Skylon is almost wingless, not that I seen it actually fly.

Edited July 10, 2018 by jsisidore

[bewing]

2 hours ago, jsisidore said:

1. Does CoL has to be before CoM or after? How far?

Behind, and only a little. The two balls touching each other is good.

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2. I remember using wing incidence but what about the engines attached to the wings, should they be parallel with the fuselage? In the Skylon design they apparently do not.

Generally, your engines should be attached to your fuselage. If you want them to look like they are attached to the wings, then you use the Move tool to adjust them.

But no, in general you want your engines aligned with your prograde vector, which you want to have aligned with your fuselage.

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3. Whats the best engine combo for ~30 tonnes? I got experience with 2 nukes and 2 rapiers at 33 tonnes but it seems I can no longer replicate it.

There really isn't one. It kinda depends on your flight profile. A shallow flight profile with lots of wing allows you use minimal numbers of engines.

Two Rapiers are easily enough all by themselves, for 33 tonnes -- with a shallow ascent.

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What speed do I need before 30km? If I reach 1500m/s at 10 degrees the prograde falls to 0 and there's nothing I can do to raise it without losing control.

You don't want to raise it. A prograde at zero means you are thrusting horizontally, which is pretty much what you want. So lock your nose prograde and burn like hell until your Ap is at 71.5 km. But if you are losing control, then you should be looking at adding more stability to your design. However, you should probably be hitting 1500 m/s at something more like 24km. Between 26 and 29km you want to switch the Rapiers into closed cycle mode -- and by 30km, you should be at something more like 1700 m/s.

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4. How important were wings? Skylon is almost wingless, not that I seen it actually fly.

Now you are getting into spaceplane aesthetics and religious questions.

If you don't have any wings, why the heck aren't you launching a rocket in the first place? The wings are there to provide lift for your plane while it flies horizontally and picks up speed. They create drag, but provide lift and fuel storage, so they pay for themselves. But if you use wings, it takes longer to get to orbit. Many players seem to like to rush it. So they put on little tiny things that they call wings, and put their nose 30 degrees above horizontal, and blast their way to orbit on massively overpowered engines. The smaller the wings, the more engine you need. The two methods get you to orbit with similar deltaV and cargo capacities.

 

Edited July 10, 2018 by bewing

[jsisidore]

45 minutes ago, bewing said:

1. Two Rapiers are easily enough all by themselves, for 33 tonnes -- with a shallow ascent.

2. You don't want to raise it. A prograde at zero means you are thrusting horizontally, which is pretty much what you want. So lock your nose prograde and burn like hell until your Ap is at 71.5 km. But if you are losing control, then you should be looking at adding more stability to your design. However, you should probably be hitting 1500 m/s at something more like 24km. Between 26 and 29km you want to switch the Rapiers into closed cycle mode -- and by 30km, you should be at something more like 1700 m/s.

3. If you don't have any wings, why the heck aren't you launching a rocket in the first place? The wings are there to provide lift for your plane while it flies horizontally and picks up speed. They create drag, but provide lift and fuel storage, so they pay for themselves. But if you use wings, it takes longer to get to orbit. Many players seem to like to rush it. So they put on little tiny things that they call wings, and put their nose 30 degrees above horizontal, and blast their way to orbit on massively overpowered engines. The smaller the wings, the more engine you need. The two methods get you to orbit with similar deltaV and cargo capacities.

 

Thanks bewing

1. Can you go to Minmus with mk2 and 2 rapiers? What twr I will need? I reckon I will need more wings... I did tried just rapiers but was out of fuel half way to minmus.

2. Although I gain speed after 30km I lose altitude, even if I make it to orbit I lose all my fuel. I may be too heavy even at 1 twr?

3. Single Stage that's why lol. I remember there was a wing formula, I'll have a look maybe on wiki

[AeroGav]

6 hours ago, jsisidore said:

I haven't played this math simulator in a very long time in fact I was sure I will never return to it as I hate math.

I did landed ssto on Duna but that was a long time ago and I forgot all about the nightmare that it was. Now I'm trying to build something practical to go to Minmus but I want to take processing plant with me so I will need mk2 cargo bay.

1. Does CoL has to be before CoM or after? How far?
2. I remember using wing incidence but what about the engines attached to the wings, should they be parallel with the fuselage? In the Skylon design they apparently do not.
3. Whats the best engine combo for ~30 tonnes? I got experience with 2 nukes and 2 rapiers at 33 tonnes but it seems I can no longer replicate it.
What speed do I need before 30km? If I reach 1500m/s at 10 degrees the prograde falls to 0 and there's nothing I can do to raise it without losing control.
4. How important were wings? Skylon is almost wingless, not that I seen it actually fly.

I've been playing Lord of the Rings online most of this year but came back to KSP because compared to learning a zillion boss fight dance moves, repeatable quest timers, and locations of all the vendor npcs,   building airplanes in KSP is really accessible and something you can just drop in and out of at will.   Also it has a pause button,   KSP lets you pee.    This is important.

3.   For 30 tons, a lot of things work.     One Whiplash two Nervs ?  2 Rapier 1 Nuke ?  1 Rapier 2 Nukes?  All can do the job.    My last endgame build was a 40 ton crew transport,  2 nukes, and one panther and one rapier clipped into each other to avoid asymmetry probs.   That combo covers all speed regimes pretty well.

 

Flight profile :  I don't like thinking in terms of this pitch angle , that pitch angle.       Pitch depends on your payload and TWR, the optimum changes throughout the flight.     But the optimum AoA does not.    For wings that is 5 degrees.   For every other part it is zero (prograde).

 

During the speedrun, you want to get as much velocity out of air breathing mode as possible.   This means getting into level flight at about 21km, and holding that until acceleration starts to die away .    You should be able to do 1400 - 1500 m/s,  but engine thrust falls away rapidly thereafter.   You are probably holding the nose down during the speedrun,  to stop the plane climbing.          Up to 21km thrust declines more slowly than air density (on the RAPIER) ,  which is why it's a good altitude for the speedrun, but after that it falls away more quickly than air density (drag).

Once you've lit the nukes,  it's time to forget about  pandering to your jet engine's intake needs, the only thing that matters is lift to drag ratio - you  maintain the AoA that gives best exchange rate possible.     If  the plane climbs, it climbs, if it then starts descending for a bit before heading back up again - let it.   Just nail that AoA sweet spot to minimise drag loss.

   

You can sort of see from this one,  i'm holding the nose down a bit to maintain level air breathing flight at 21km till we reach 1500, then i start the nukes.    At this point,  i let the nose rise back to prograde again.    The wings are angled up at 5 degrees so they are at 5 degrees making lift while the rest of the airplane is on prograde for low drag. I'm tweaking the deployed flap surface to keep the nose close to prograde.         Now, when i stop pushing the nose down we end up with an excess of lift and immediately start climbing.  

In that video,  I'm doing 1800 when i pass 30km, and am climbing at a three degree angle.      

 

[jsisidore]

1 hour ago, AeroGav said:

I've been playing Lord of the Rings online most of this year but came back to KSP because compared to learning a zillion boss fight dance moves, repeatable quest timers, and locations of all the vendor npcs,   building airplanes in KSP is really accessible and something you can just drop in and out of at will.   Also it has a pause button,   KSP lets you pee.    This is important.

3.   For 30 tons, a lot of things work.     One Whiplash two Nervs ?  2 Rapier 1 Nuke ?  1 Rapier 2 Nukes?  All can do the job.    My last endgame build was a 40 ton crew transport,  2 nukes, and one panther and one rapier clipped into each other to avoid asymmetry probs.   That combo covers all speed regimes pretty well.

 

Flight profile :  I don't like thinking in terms of this pitch angle , that pitch angle.       Pitch depends on your payload and TWR, the optimum changes throughout the flight.     But the optimum AoA does not.    For wings that is 5 degrees.   For every other part it is zero (prograde).

 

During the speedrun, you want to get as much velocity out of air breathing mode as possible.   This means getting into level flight at about 21km, and holding that until acceleration starts to die away .    You should be able to do 1400 - 1500 m/s,  but engine thrust falls away rapidly thereafter.   You are probably holding the nose down during the speedrun,  to stop the plane climbing.          Up to 21km thrust declines more slowly than air density (on the RAPIER) ,  which is why it's a good altitude for the speedrun, but after that it falls away more quickly than air density (drag).

Once you've lit the nukes,  it's time to forget about  pandering to your jet engine's intake needs, the only thing that matters is lift to drag ratio - you  maintain the AoA that gives best exchange rate possible.     If  the plane climbs, it climbs, if it then starts descending for a bit before heading back up again - let it.   Just nail that AoA sweet spot to minimise drag loss.

   

You can sort of see from this one,  i'm holding the nose down a bit to maintain level air breathing flight at 21km till we reach 1500, then i start the nukes.    At this point,  i let the nose rise back to prograde again.    The wings are angled up at 5 degrees so they are at 5 degrees making lift while the rest of the airplane is on prograde for low drag. I'm tweaking the deployed flap surface to keep the nose close to prograde.         Now, when i stop pushing the nose down we end up with an excess of lift and immediately start climbing.  

In that video,  I'm doing 1800 when i pass 30km, and am climbing at a three degree angle.      

 

Hello Gav you're still alive. Sorry that is my informal greeting. I think I need 2 nukes to save fuel going to Minmus or even Mun. It is hard to balance oxidizer and liquid fuel using rapiers.

[juanml82]

For 33 tons, I'd try 2 rapiers, one nerv.

The nerv will have awful TWR, but it should still do its job