Spaceplanes: when to go supersonic?

[GoSlash27]

foamyesque,

 I know your question was directed at @Brikoleur, but I'd like to weigh in on this too. I'm kind of an oddball in that I never use spaceplanes to transfer cargo, only crew and supplies. I find the cargo bays restrictive, and don't like to design my missions to fit inside them. Thus, for me "payload" is strictly the mass of supplies that get transferred in orbit. For crewed missions, I consider the "mass" to be the mass of the passenger compartment(s) divided by the number of Kerbals contained.
 My passenger ferry spaceplanes are always small; 4 crew members (not counting flight crew) each way per flight. I rarely find a need to move more crew than that in a single go.
 My supply tankers, OTOH, tend to be as large as I can make them while keeping them reliable and easy to fly.

Best,
-Slashy

[foamyesque]

@GoSlash27: You can use planes to transfer cargo without using cargo bays, you know. I posted an example upthread. Of course, that means you need to work out an alternate means to streamline things, but that's by no means impossible.

 

 

[GoSlash27]

40 minutes ago, foamyesque said:

@GoSlash27: You can use planes to transfer cargo without using cargo bays, you know. I posted an example upthread. Of course, that means you need to work out an alternate means to streamline things, but that's by no means impossible.

 

 

foamyesque,

 Haha yeah, I'm aware. I don't remember who it is, but one of the members likes to place the cargo in a nosecone in the CoM. That seems to work pretty well also. Still... I prefer not to do it that way. Better to leave the airplanes with a uniform mass and shape IMO.

Best,
-Slashy

[Guest]

9 hours ago, foamyesque said:

@Brikoleur:I'd be interested in seeing those heavy-duty spaceplane designs. Also, a question: When you say 'payload', is that counting all the miscellanceous hardware in the launcher (wing, engines, etc) or is it mission gear -- passenger modules, deployable craft (satellites, spaceport modules, etc), left-over-fuel if you're doing refuel runs...?

Only mission gear.

Here's my current heavy lifter, the Rukh, in action, lofting Jool Station on the first stage of its journey.

IIRC the payload was about 150 tons for this flight, but I've tested it with a 210-ton one. Once I get Experimental Aerodynamics it'll improve a bit more, with some selected shuttle parts instead of the jumbo jet parts that melt kind of easily.

(This is also rather amusingly a passively safe design on re-entry, although that was an unintended benefit and I have no idea why it works like that: first it settles into a very steep 60-degree pitch, and when it hits thick-enough air it transitions to normal flight. By itself. Regardless of what I do with the controls. The only difficulty being that at that point it'll be going at about 200 m/s and 15 km, which means "normal flight" means a steep dive -- but one it's easy to pull out of, although I can avoid that by using the Vernors on the tail to force it level a little bit before that point. And in normal flight it's a lamb, stable, easy to handle, easy to land, although pretty draggy due to those big engine pods. So basically to re-enter, I burn Pe down to 50k, point it at the sky, go have a coffee, and take the controls when it's flying aerodynamically again. "The Rukh -- so simple an engineer can fly it.") 

Edit: more about this mission here: 

 

Edited January 18, 2018 by Guest

[foamyesque]

Interesting idea, underslinging it like that. I would have expected ground clearance to be an issue, but evidently not. I'm a bit surprised those pods don't warp your wings, though; it's always been a problem for my high-thrust planes.

[Guest]

59 minutes ago, foamyesque said:

Interesting idea, underslinging it like that. I would have expected ground clearance to be an issue, but evidently not. I'm a bit surprised those pods don't warp your wings, though; it's always been a problem for my high-thrust planes.

Autostrut. Couldn't build anything this size without it. 

Ground clearance isn't a major issue, my main gear is on the pods. I did build a "blister" on the nose to accommodate a steerable nose gear. 

Thrust torque once out of the atmosphere is a bit of an issue, but it's manageable. I have Vernors on the tail which help maintain attitude, and I've set up a custom action to shut down two engines in each pod asymmetrically to reduce it.  

[foamyesque]

Hm. I usually wind up building my pods as close to centerline as possible, and running a strut from nose in to the body, as well as one or two struts to interlink the wing sections so's they don't flop around. The drag penalty's atrocious, but I got my building habits in the days before autostrut; I always wind up spacetaping things, even when it's not the best choice. To tell the truth, I don't even know how to turn auto-strut on

 

Totally feel you on the thrust torque though. Like you, I have an action group, but in my case I use it to terminate all the RAPIERs; orbital manouvering's done either on RCS vernors (which also maintain attitude control as the aerodynamics fail & the CoM shift increases in the upper atmo, as with you) or on a single Thud, whose huge vector range and extreme-rear placement allow it to work regardless. Nothing quite like doing your circularization burn with a <0.1 TWR

[Laie]

On 1/16/2018 at 8:10 AM, Spricigo said:

So, there is the method you use  and there is the method you more often see people using. And you want to know which one is "better".

So you just nees to try that method you see so often and see how it goes. 

Too many variables for this to be decided by simple experiments, but by and large it seems that one shouldn't climb all that high at low speeds, even if the vessel allows.

I guess it's the good old drag vs. gravity losses all over again. A slow climb needs a lot of energy no matter what.

 

[Spricigo]

45 minutes ago, Laie said:

Too many variables for this to be decided by simple experiments, but by and large it seems that one shouldn't climb all that high at low speeds, even if the vessel allows.

I guess it's the good old drag vs. gravity losses all over again. A slow climb needs a lot of energy no matter what.

 

By all means, it can be determined by experiments. Just:

1.take [spaceplane]

2.follow [flight profile ], see results

3.repeat (1-2) , maintaining [spaceplane] and changing [flight profile]

4.compare results.

Profile A get to orbit with 1km/s remaining while Profile B had 1,2km/s? => Profile A consumes more fuel. That is already an useful fact for the spaceplane tested, further refining our experiment can only improve the knowledge about the matter.

Granted, it will take a lot of iteration to reach conclusions that are applicable to a wide range of designs, target orbits and flight profiles. In any case, the process is nothing more than a series of simple experiments.

 

[GoSlash27]

8 hours ago, Laie said:

Too many variables for this to be decided by simple experiments, but by and large it seems that one shouldn't climb all that high at low speeds, even if the vessel allows.

I guess it's the good old drag vs. gravity losses all over again. A slow climb needs a lot of energy no matter what.

Laie,
 It's really more a matter of finding the lowest altitude at which you can punch through Mach 1 without incurring excessive heating and drag afterwards. A low drag/ low thrust plane behaves a lot differently than a high drag/ high thrust plane, and the behavior of any plane changes radically once you clear Mach 1.3 or so.

 The key to the whole thing is getting supersonic as efficiently as you can. Once you clear that point, the plane goes all "super saiyan" and it's easy to get to orbit. Some can do it right on the deck, others need more altitude.

Best,
-Slashy