Space planes: optimal subsonic speed?

[ajburges]

Something I've wondered is what subsonic velocity offers the best fuel efficency (WRT distance) if you drop out of Mach without enough energy for a dead stick landing. Assume RAPIER and Whiplashes for jet Isp

Just looking for rule of thumb here. Is it better to fly near stall speed, hold a high fraction of Mach 1, or gun the engines?

[Gaarst]

Jet engines have constant Isp, so constant efficiency. The optimal speed is defined by your planes' aerodynamics. If your only constraint is fuel efficiency, then you want to find the speed at which your L/D ratio is the best.

[TheKorbinjer]

I gun the engines. Speed is my friend, though friction is not.

Honestly, I'd look around. 

Speed of sound ASL =  340.29 m/s

I would say that planes in the real world have a cruise speed of around 500 knots (260 m/s)

If you're looking for efficiency, i'd say that'd be perfect.

I honestly get around at any of my plane's top speeds. For instance, my 1:1 scale B52H replica can go 900 m/s (I know. it's insane.)

But i mostly say around mach 1

 

Try this:

Test different speeds at different altitudes say 100-330 m/s between 50-15,000m

See which speed at any of those altitudes offers the highest efficiency, and go with that.

(

Fly to 50 meters at 100 m/s, record your fuel consumption for 2 minutes, fly to next desired altitude and repeat

)

Should take only about 20-30 minutes.

 

Hope my suggestion help you!

Cheers!

-TheKorbinger

[bewing]

You get your best fuel efficiency flying high and fast. If you are way short of the runway, light your engines when you are at 23km to 25km, and fly at 1200 to 1400 m/s to your destination -- this minimizes flight time and drag. If you drop out of Mach, you will fall into the thick air, and you're screwed.

Edited February 17, 2017 by bewing

[A_name]

26 minutes ago, bewing said:

You get your best fuel efficiency flying high and fast. If you are way short of the runway, light your engines when you are at 23km to 25km, and fly at 1200 to 1400 m/s to your destination -- this minimizes flight time and drag. If you drop out of Mach, you will fall into the thick air, and you're screwed.

I second this.

[ajburges]

Obviously max aerobic velocity is the most efficient.

I'm more concerned about short distances; when you realize you are short of the runway, but breaking Mach 1 again may not be sensible. I expect that either TheKorbinger is correct and gunning the engine is always best or that the drag wall that starts around 300 m/s is significant enough that there is a noteable distance where you are better off governing your speed to minimize drag losses.

[GDJ]

It really depends on the spaceplane. Most of the planes I build they can land easily around Mach 0.2. Any faster and you risk overloading the landing gear due to shock loading. I would say practice doing short hops and turnarounds and find out how slow you can fly while being able to maintain altitude over the landing area without stalling.

As far as cruising to your landing area, Mach 1.5 should be fine, and start slowing down 20 km away. That will give you plenty of time to line up and set up your landing procedures.

Edited February 17, 2017 by GDJ

[bewing]

If you're just a little bit short of the runway, then the nasty part of the drag curve starts at 240 m/s. So pin it there.

[ajburges]

4 hours ago, bewing said:

If you're just a little bit short of the runway, then the nasty part of the drag curve starts at 240 m/s. So pin it there.

Excuse me as I bang my head into a wall...

Of course! Your fuel cost is directly proportionate to drag. Constant velocity implies thrust=drag and Isp (fuel per thrust for this aplication) is constant. Since distance is the integral of velocity WRT time, your peak fuel efficiency is the minima of drag over velocity. Said function is roughly constant until it ramps up for the Mach wall. Any sub critical Mach velocity will offer similar drag efficiency, but faster also offers better lift!

Edited February 18, 2017 by ajburges

[Wanderfound]

Alternately, to avoid the issue entirely...

 

Keep it high and fast until you reach the KSC mountains:

 

Then burn off the speed and altitude with some high-G S-turns:

 

Then settle in for a nice slow landing:

[AeroGav]

Best possible lift:drag ratio in the 1.4 mach up to orbital velocity (supersonic) range is about 4 to 1.   There is little change between mach 1.4 and mach 6, so you might as well go fast and high, because at mach 5 orbital freefall is supporting more than half your weight.   At a constant lift:drag ratio, that means lower drag which means less thrust needed to keep flying. In other words, if you're doing mach 5+,  you'll be able to fly really high despite keeping AoA low,  which makes for low drag and fuel consumption.

If you're in a panther powered low tech ssto,  you cannot go fast enough to benefit from this effect - these engines go from hero to zero between mach 2.5 and 3.    So , you might be better off switching to dry mode, which doubles your ISP,  and you should still be able to hold > mach 1.3.

Failing that, of if the fuel situation is really desperate, go subsonic, as slow as you can bear.  There is a really high drag region between mach 1.3 and mach 0.86, but once below that , the slower you go, the better your L/D.  Obviously, you still need to avoid high AoA, so you have to get low when you get slow.

In practice, this means not using the engines and just gliding down till just above ground level.  After that, trim the nose up for 2 degrees of AoA and add just enough throttle to maintain speed.      Not fun though.

[ForScience6686]

On 2/17/2017 at 10:01 PM, Wanderfound said:

Alternately, to avoid the issue entirely...

 

Keep it high and fast until you reach the KSC mountains:

 

Then burn off the speed and altitude with some high-G S-turns:

 

Then settle in for a nice slow landing:

Just to add to this, if too high, just roll over and pull back.  That will decrease speed as well as altitude.

[bewing]

4 hours ago, ForScience6686 said:

Just to add to this, if too high, just roll over and pull back.  That will decrease speed as well as altitude.

My Kerbals prefer outside turns. (ie. just push forward)

Wanderfound: but for those 15G S-turns -- you're lucky your wings didn't pop off.

Edited February 19, 2017 by bewing

[Wanderfound]

7 hours ago, bewing said:

My Kerbals prefer outside turns. (ie. just push forward)

Wanderfound: but for those 15G S-turns -- you're lucky your wings didn't pop off.

It was a slightly extreme example...

Normally I'll keep it at 20,000m/Mach 5 until I hit the west coast of the KSC continent. At that point, I'll throttle off and drop the nose enough that I'll glide in over the mountains at about 10,000m. After that I get lined up with the runway and drop it down to about 1,000m before another drop to treetop-height just before landing.

[Jarin]

On 2/19/2017 at 10:34 AM, bewing said:

Wanderfound: but for those 15G S-turns -- you're lucky your wings didn't pop off.

Autostrutting is an amazing thing. I'm still torn on whether it feels like cheating to me or not.