Flight Question Landing at a base on earth

[Jason210]

I haven't yet got the hang of landing where I want (DGIV)...usually end up 500km - 1000km off target then usually run out of fuel trying to cover this distance!

Most of the time I end up in the sea.

Any tips tutorials for this? Cheers!

 

[Urwumpe]

What does 500km - 1000 km mean exactly? Are you landing behind the target, are you coming too short, do you land to the side of it?

Generally speaking, your goal is not to do the deorbit burn at the same place, but pass a window in space called "Entry Interface" at the same distance to the base, in the same altitude, like 120 km, at the same angle as specified for your vehicle (usually -1.25°).

Then, you need to fly your spacecraft to the base. Not just deorbit and let Sir Isaac Newton do the steering for you. You keep AOA constant, and use bank to control how fast you dive into the atmosphere. The faster you drop, the faster you slow down, but you also got more heat.

There is an old add-on called reentry MFD, that calculates how fast you should slow down for arriving precisely at a base.

 

[agentgonzo]

There are some good tutorials for AerobrakeMFD around. Using that is the only way that I can get within 100km of a base reentering for Earth. There are a couple of others that help you, but that's by far the most popular (and best IMO).

 

[Cras]

With Aerobrake MFD, it becomes very easy to get close to the base, but if you are flying the DGIV, keep in mind that Aerobrake gets confused when the retro doors are open.

 

[dbeachy1]

I like using BaseSyncMFD to calculate my deorbit burns; you can check it out on the bottom of this page.

 

[Cras]

Forgot about to mention BsaeSync MFD. Yes, that is extremly useful, both to make sure your orbit passes close to the base you are looking to land at, and also for timing the deorbit burn.

 

[Jason210]

What does 500km - 1000 km mean exactly? Are you landing behind the target, are you coming too short, do you land to the side of it?

Generally speaking, your goal is not to do the deorbit burn at the same place, but pass a window in space called "Entry Interface" at the same distance to the base, in the same altitude, like 120 km, at the same angle as specified for your vehicle (usually -1.25°).

Then, you need to fly your spacecraft to the base. Not just deorbit and let Sir Isaac Newton do the steering for you. You keep AOA constant, and use bank to control how fast you dive into the atmosphere. The faster you drop, the faster you slow down, but you also got more heat.

There is an old add-on called reentry MFD, that calculates how fast you should slow down for arriving precisely at a base.

Usually I'm before or after the base. I'm quite good at flying towards / past it since all you need to do there is align your orbital plane so that it's passing over the base...

How did the NASA shuttle guys manage it?

 

[Urwumpe]

How did the NASA shuttle guys manage it?

1. No exact alignment of the orbit plane is needed. There is a concept called cross-range, which is the maximum distance you can divert laterally from your orbit plane. Most winged spacecraft have a cross-range of about 1000 km. NASA just waits until the landing site is safely inside the cross-range of the spacecraft.
2. All NASA spacecraft with controlled reentry used constant AOA and control the reentry by the bank angle.
3. If you need to slow down faster and your heat is still within limits, you bank more to the side.
4. If you need to slow down slower or you are getting hot, you bank less to the side.
5. Bank means you fly a huge curve, so your ground track will bend away from the orbit ground track. First you bank towards the base and turn towards it. Then your ground track will pass over the base and turn away from it. When you have reached the limits of your spacecraft there (usually about 10.5° difference in your direction of travel compared to the direction to the base) you invert your bank angle: From 90°L you switch to 90°R for example. This is called Roll-Reversal.
6. All NASA spacecraft with controlled reentry (Gemini, Apollo, Shuttle) did roll reversals. This results in your spacecraft flying to characteristic S-turns of the space shuttle reentry.
7. The final trick is called TAEM or Terminal Area Energy Management. During this phase shortly before lining up for landing, NASA makes sure the Space Shuttle arrives in the right altitude and the right speed for a good landing. The S-Turns become smaller, and the speed brake is deployed for maneuvering the energy (speed and altitude) of the spacecraft into the "box" for landing.

It isn't more. Some small misalignment during deorbit is compensated by the cross range, the reentry is controlled by flying S-Turns and the landing is targeted by using the speedbrake on the final 150 km.

 

[Jason210]

Some small misalignment during deorbit is compensated by the cross range, the reentry is controlled by flying S-Turns and the landing is targeted by using the speedbrake on the final 150 km.

Thanks. I don't understand the purpose of the S-turns, unless it's just to reduce temperature?

 

[C3PO]

Thanks. I don't understand the purpose of the S-turns, unless it's just to reduce temperature?

You control altitude (= breaking) by banking. For altitude control it doesn't matter if you turn left or right. To avoid turning away from target base, you reverse the bank to get back on track.

 

[Urwumpe]

Thanks. I don't understand the purpose of the S-turns, unless it's just to reduce temperature?

They are the result of flying at bank. If you keep AOA constant, your vehicle produces the same lift to drag ratio all the time. By changing the bank angle,you change the angle between lift vector and gravity vector. at 90° bank angle, all lift points to the side and makes you turn, at 0° you have full positive lift, etc. This way of controlling flight has the advantage, that the angle between heat shield and airflow never changes - the best protected sides always point forward.

You don't just want to reduce temperature, you also want to slow down exactly so fast, that you arrive about 90 km away from the base at Mach 3 in about 25 km altitude. This means you sometimes need to get more deceleration as by flying only for minimal temperatures.

Also, you need to remember, that if you slow down too much in the higher atmosphere,you will drop finally at a steeper angle and get more heat and more drag force. You want to stay on a optimal glide angle that gets you low at the right speed.

So, the goal is to really control every aspect of your descent... not just glide down, fly down.

Look here, this image shows the S-curve pretty well:

 

[C3PO]

This is a vertical profile of a typical reentry. The exact values will vary for different craft.

The main part of deceleration happens between 65-25 km altitude.

 

[Jason210]

Thanks guys!