Low TWR SSTO ascent profile

[katateochi]

I've been messing around with a couple SSTO designs that use only liquid fuel; J-X4 and LV-N engines.  I have 1 which has a pair of LV-Ns and another with just 1, and both have a very low TWR of about 0.42.  Both can reach orbit, but I'm wondering what is the most efficient way to fly such low TWR SSTOs.

What seems to work ok is to start flying very low (10 deg pitch after take off) and get up to around 1000m/s. At this point I can either pull up hard to (around 30 degs pitch) and use the last bit of time on the jet engines to gain as much vertical speed as possible before flame out, so when I switch to the nukes I'm higher up. Or increase pitch just a bit and push on in the lower atmo. The latter seems to be slightly better.

Either way I can't maintain the climb and end up dropping a few km from around 40km up.  But I'm still gaining horizontal speed and not overheating (well, everything's glowing but not at point of explosion) and eventually the craft starts climbing again. Also the Pe has been raised a lot by that point (usually to around 10-15km by the time the Ap reaches 100km).  

So I guess the Q is; with a low TWR SSTO, is it better to try and punch up into the higher atmo before the jets flame out, or keep pushing on in the lower/mid atmo and focus just on horizontal speed (climbing only enough to avoid dangerous overheating).
 

[fireblade274]

I'd say both options are more or less the same in their results, barring extremes on either end. You probably already know this, but the way I see it and have noticed in my SSTO's is going up quicker at a higher AoA will cause less fuel to be burned during air breathing engine cycle and make the craft heavier for the rocket engines to deal with at a higher altitude, then a lower AoA assent profile. However since your ascending faster in the former then then latter, you reach orbit and more fuel efficient altitudes quicker, thus, generally the 2 factors, in my experience, cancel each other out with a negligible difference.

[katateochi]

Thanks @fireblade274, that makes sense and does seem to match up to what I'm finding.  I guess the decider between the two approaches is how much drag the craft has, and those with more drag would favour the higher AoA approach.  I'm finding that for my current craft the lower AoA approach is slightly better, but we're only talking about a matter of ~50 dV difference. 

[HMIC]

@katateochi I'd go with #2, pushing the limits of the jet engines as much as you can until you get as close as you can to the +70km Ap.  Nukes engines are heavy, so it's a challenge to get a decent TWR and ascent profile using only liquid fuel.  So as long as you make it and have enough to compleat your objective, you can throw efficiency out the window.  

[Wanderfound]

When are you lighting your nukes? They should be going on well before the jets flame out; I normally kick mine in at 20,000m or so.

[katateochi]

4 hours ago, HMIC said:

@katateochi I'd go with #2, pushing the limits of the jet engines as much as you can until you get as close as you can to the +70km Ap.  Nukes engines are heavy, so it's a challenge to get a decent TWR and ascent profile using only liquid fuel.  So as long as you make it and have enough to compleat your objective, you can throw efficiency out the window.  

On just the jets I can't get near to 70km Ap, If I take a steep AoA then Ap goes up to around 40-50, but what I'm finding is that if I take a steep AoA, and have a vertical velocity of around 400m/s before flame out, I end up loosing all that vertical velocity and start going back down again. The lower AoA doesn't give as much vertical velocity, but I don't drop a much either.
One of the craft (with twin jets and twin nukes) can reach orbit with about 1600m/s dV spare, enough to reach Mun orbit and return. The other (single jet and nuke, very strange design) can reach orbit with around 2600-2700m/s dV, enough to land on Mun and return. Reason I'm asking is I just want to squeeze every last drop of dV out of them as possible! Unfortunately the one that can land on Mun and return is very unstable on re-entry, so needs more work. 

3 hours ago, Wanderfound said:

When are you lighting your nukes? They should be going on well before the jets flame out; I normally kick mine in at 20,000m or so.

yeah 20km is about when they go on, before flame out. I fire them up as soon as surface velocity stops increasing.

[fireblade274]

On 4/28/2016 at 10:06 PM, katateochi said:

 I guess the decider between the two approaches is how much drag the craft has, and those with more drag would favor the higher AoA approach.  I'm finding that for my current craft the lower AoA approach is slightly better, but we're only talking about a matter of ~50 dV difference. 

Exactly

[numerobis]

When you're getting to 40km, what's your horizontal orbit speed?

At 1900 m/s orbit horizontal speed at 40km, apparent gravity is about a third of surface gravity. You can beat that with your 0.45g of thrust, and with the help of your wings I bet you'll only need around a 30 degree pitch.

In fact, the ideal is to pitch up (but not as much) long before you reach Ap and maintain the pitch, so that when you actually reach it you're at orbit speed and apparent gravity is nil.

(Apparent gravity is mu/r^2 - v^2/r where r is distance from the core of the planet [add 600000 to the altitude] and v is your orbit horizontal speed.)