Switching propulsion on SSTO planes

[Evanitis]

How do you do it these days (other than the Rapier)?

I socialized in the assymetric flameout times when it was kinda required to shut down the jets and intakes when firing the rockets. Since it changed, I always went the fire-and-forget way - not a single action group, just staging the LFO engines once I'm at the desired height and velocity. It works nicely, the jets keep burning until they get bored... but I started to wonder if what I'm doing is efficient. Is it? What's your experience?

Edited January 21, 2016 by Evanitis

[Vegetal]

I don't see why it wouldn't be efficient. You are squeezing the most you can from that high Isp thrust. I think this is the most efficient actually...

[Evanitis]

Dang, I was hoping to find my missing few dozen m/s dV there. 

Thanks!

[suicidejunkie]

What about closing your air intakes to reduce drag?

That doesn't mean shutting down the air breathers immediately; you can let them run on the remaining air in your hull after closing the intakes.

But at some point as they spool down in the thin air, the drag from the intakes is going to exceed the thrust from your engines.

[Val]

11 hours ago, Evanitis said:

Dang, I was hoping to find my missing few dozen m/s dV there. 

Make sure your speed is well above 1200 m/s (using Whiplashes) before engaging LFO engines, and that you're not ascending more than 15° above horizon, unless you have very high TWR (<15 t per Whiplash and TWR >2 on LFO)

And once you're above 30 km, follow prograde, or slightly above with lower TWR.

[Evanitis]

13 minutes ago, Val said:

Make sure your speed is well above 1200 m/s (using Whiplashes) before engaging LFO engines, and that you're not ascending more than 15° above horizon, unless you have very high TWR (<15 t per Whiplash and TWR >2 on LFO)

Thanks!

Minimal fuel-composition adjustments and slower pitch-up after firing the rockets helped. I was kinda' flying blind during the seaplane challenge as it disallowed MechJeb and I was lazy to install KER (or to slap the MJ module on and off), so I didn't realize my TWR is lower than what I'm used to.

9 hours ago, suicidejunkie said:

What about closing your air intakes to reduce drag?

I read it somewhere that closing the intakes doesn't change the drag theye days. I kinda' believed that without checking... and since I'm not a good enough pilot to keep the very same ascent profile each time, I'm not sure how could I test that.

[Nefrums]

Several resent test have confirmed that drag does not change when closing intakes.

The optimal accent profile is slightly different depending on how much drag you have.  For draggy ships I try to get to level flight at 10km before accelerating past mach 1.  For low drag ships I accelerate to mach 3 at see level and then climb at 4-6 degrees to get to 1600m/s before 20km. This however require you to have the very draggy shielded docking port or heat shield at the front of your ship, or else it will overheat. So it is not really valid for small (mk1 parts) ships.