Just how much better is a gravity turn than direct ascent?

[GoSlash27]

18 minutes ago, Jarin said:

Does this mean I should be running my SSTO spaceplanes lower, hour and faster? Assuming they can take the heat, that is. Flying flat at 30km or so seems like drag would still be an issue.

The thing about space planes is that their air breathing Isp is so crazy- high that atmospheric drag is a relative non- issue. I tend to dwell for a while around 20km before engaging the rockets... so long as my airframe can handle the heat.

 Again, this is an example of DV not being a good mark of the efficiency of a vehicle. Space planes expend a whole lot more DV getting their payloads to orbit than comparable rockets. They just do it at higher payload fractions and much lower operating cost.

Best,
-Slashy

Edited September 7, 2016 by GoSlash27

[Jarin]

27 minutes ago, GoSlash27 said:

The thing about space planes is that their air breathing Isp is so crazy- high that atmospheric drag is a relative non- issue. I tend to dwell for a while around 20km before engaging the rockets... so long as my airframe can handle the heat.

Oh yeah, that's a given. I'm talking about in the 25km+ range where I'm running on rockets but still within the atmosphere. Did a trial run with my recently-built crew transport, since I had good efficiency numbers there across several tests.

It... uh... got a bit toasty.

I ended up sacrificing an RCS port to Pele in the name of science. Numbers came out a touch over my previous flights, but within normal margins. This was honestly a bit surprising, since I had to keep goosing the throttle while coasting to keep my AP up from drag. With how many struts and fuel lines this beast has on it, I'd probably see a lot better gains on some of my more streamlined craft. Something I'll have to remember.

Edit: And thank you slashy, for my 200th rep point!

Edited September 7, 2016 by Jarin

[GoSlash27]

1 minute ago, Jarin said:

Oh yeah, that's a given. I'm talking about in the 25km+ range where I'm running on rockets but still within the atmosphere. Did a trial run with my recently-built crew transport, since I had good efficiency numbers there across several tests.

It... uh... got a bit toasty.

I ended up sacrificing an RCS port to Pele in the name of science. Numbers came out a touch over my previous flights, but within normal margins. This was honestly a bit surprising, since I had to keep goosing the throttle while coasting to keep my AP up from drag. With how many struts and fuel lines this beast has on it, I'd probably see a lot better gains on some of my more streamlined craft. Something I'll have to remember.

FWIW, I've found good luck circularizing at 43 km in level flight, whether using space planes or rockets. This seems to get me to orbit with the least expenditure of propellant.

Best,
-Slashy

[foamyesque]

My experience has been that it's generally best to engage a pitch-up manouver either once you're on rockets, or just before, to get some vertical altitude and out of the high-drag region you're in ASAP. It also helps deal with the heat problem, of course. I don't generally go level-flight again until I've got an apoapsis projection of 45-50km.

[Jarin]

On 9/6/2016 at 7:31 PM, GoSlash27 said:

FWIW, I've found good luck circularizing at 43 km in level flight, whether using space planes or rockets. This seems to get me to orbit with the least expenditure of propellant.

Best,
-Slashy

Just a follow-up here. Further testing shows very nice efficiency gains if I climb to around 30km, then lock autopilot to orbital prograde (which is generally just a touch above horizontal at that point). There's little enough drag in that range that burning on rocket-mode doesn't cause too much heat trouble. All told, it moved me from about 4-500dV left in orbit up to about 750dV, so very respectable improvement.