Spaceplane gliding advice (FAR)

[dlrk]

I'm looking for some advice on how best to glide a spaceplane without running out of airspeed before the runway or arriving over the threshold with two much airspeed. I'm using Trajectories which does a great job getting in the area, but it doesn't handle planning the glide on final approach (nor would I expect it to). Does anyone have any tips for energy management during the approach to landing?

Edited March 2, 2020 by dlrk

[Guest]

I usually target Pe at 40k above KSC. Then I control my trajectory with my attitude during re-entry. If it looks like I'm about to fall short, I drop into a shallower attitude, if it looks like I'm overshooting, I pull into a steeper one. If I'm at about 15 km going at < 1000 m/s when over the mountain range just west of KSC, I'm good, from there it's just a dive and a brake to landing on the runway.

If I claimed I can do this every time with every spaceplane I make I would be lying, but I can usually get it in the ballpark on my first try with a new design, and then refine it from there on out; if I overshot by a lot I lower the target Pe for my next attempt, if I fell short, I raise it.

[dlrk]

Thanks, but it's not getting in the general a problem with, it's specifically managing airspeed and altitude on final. I'm thinking it's probably the best idea just to keep a bunch of extra airspeed, then airbrake right before the runway.

[Cavscout74]

A lot will depend on your actual spaceplane design, but if you are purely gliding its almost always going to be better to have a little too much airspeed until you're at the runway.  At that point, airbrakes and/or drogues can shed the excess. 

[Guest]

1 hour ago, dlrk said:

Thanks, but it's not getting in the general a problem with, it's specifically managing airspeed and altitude on final. I'm thinking it's probably the best idea just to keep a bunch of extra airspeed, then airbrake right before the runway.

Ah, sorry, I misunderstood. There too it depends on the design. I generally keep airspeed around 100 m/s until at about 150 m radar altitude, then I level off to start bleeding it. My planes glide easily down to 40 m/s or so, and I touch down around 50, which leaves some room for error. 

The glideslope varies for each craft but I give them enough wing that it is never all that steep. The closer you can get them to aerodynamically neutral while still keeping them passively stable the easier the final approach and landing is. Nose heavy designs are tricky to glide down easily as they tend to want to faceplant when they slow down.

Sadly there is no one size fits all answer to this, every plane is different.

[Zeiss Ikon]

The Space Shuttle had a specific system for this, Terminal Energy Management (TEM).  They used the  only available strategy, with  very limited drag brakes (split rudder only), gear that couldn't be retracted in flight, and a drag chute, also of limited effect: they reentered on a path to be a little high and hot, then used what amounted to S turns in the last minute or two of descent to dispose of excess energy.  And then they landed on long, long runways...

[sturmhauke]

I build my spaceplanes (mostly shuttles these days) to glide at an angle of about 20 degrees; minimum airspeed tends to be around 100 m/s. Lower airspeed/shallower glide angle requires more wing surface, but that's at something of a premium, especially on a shuttle. I also build custom airbrakes out of multiple elevons (the stock airbrakes respond too slowly and have poor thermal resistance), and use multiple chutes (standard chutes, not drogues) on touchdown. I also flare the nose at the last minute, just before touchdown, to kill the last bit of speed.

[RizzoTheRat]

On 3/2/2020 at 8:11 PM, dlrk said:

Thanks, but it's not getting in the general a problem with, it's specifically managing airspeed and altitude on final. I'm thinking it's probably the best idea just to keep a bunch of extra airspeed, then airbrake right before the runway.

The technique for real word gliders is to use pitch to control the airspeed, and the airbrakes to control the sink rate, which is the other way around to what you'd think.  With Trajectories giving you an impact point this means you should be able to use the pitch to maintain a speed comfortably above your stall speed, and then use the airbrakes to bring the impact point on to the runway, once you get low enough pitch up gently (flare) to trade speed for reduced sink rate, but not so much that you start to climb again (known as ballooning).  Ballooning can result in a rather hard landing and I may have managed to crack the wheel faring on a Grob Viking doing that