Flight Question Direct reentry into earth's atmosphere

[Wrangler]

Now this could be an FAQ, in which case please refer me to the relevent documentation.

I was wondering if it was possible to enter into the earth's atmosphere from an interplanetary return trip (like for instance, Mars) without first establishing an orbit around earth. I am planning to build a flight plan around the premise that an XR2 returning from Mars can land on an earth base, without making any course correction burns inside the Earth's SOI (maybe one 15-sec burn if it is unavoidable).

My own theory in this regard is that it might be possible on a very shallow reentry angle and a very large anticipation angle. Another possibility might be to perform an aero-capture maneuver to bleed off some energy before reentering.

Now being a Novice in interplanetary flight planning, I was hoping for some guidance. Which course would be the correct one and will IMFD and Basesync MFD will be sufficient to plan the entire trip? A bigger question. Is it even possible or am I just being my naive self again? :lol:

 

[Axertan]

With XR2 it should be possible to do a hyperbolic reentry. You will need to cover a lot of ground tho, it may be enven an inverted reentry. I might have done one, but the memory is fuzzy...

I believe TransX and MapMFD could work for you, in planning the time needed to reach Earth above (or on trajectory to) the base of your choice. In MapMFD you should choose your base, and in TransX enable viewing of bases. Your base should show up on TransX allowing you to plan your maneuver or mid-course correction to end up on track.

I'll be honest, I never did this myself. Good luck!

 

[Cairan]

Yes, you can, but first your reentry corridor would be very narrow... In other words, you won't have a big margin of error as to the velocity, angle of reentry and lift vector your ship will need to prevent burning up or skipping out. Also, you might get to a point where there is just no possible combination of angles and lift vectors to prevent skipping out. If you can't get low enough without burning up at your highest velocity at perigee to generate the required lift to keep you from skipping out, you won't make it.

Hence, you need to have a trajectory which minimizes your encounter velocity with Earth. That means, you won't be able to come direct from anywhere other than Venus or Mars. In other words, if you come from say Jupiter, then you'd need a gravity assist at Venus to shed some velocity before coming back to Earth.

From Mars, it is possible to aerobrake directly, but as I said, you need to point your ship "inverted" in the atmosphere, so that the lift provided by the wings of the XR2 initialy works to push you toward down the ground (nadir), instead of "up".

You won't be able to have a roll of 180 degrees on reentry and keep the proper aerobraking altitude, so you'll need to roll a little to the left, a little to the right (S-turns) so that the lift provided counteracts exactly the centripete force that wants to pull you away from Earth as long as your orbit is hyberbolic, and then while your apogee still is out of the atmosphere. When you reach that point, you can roll over "right side up" and continue like a normal reentry.

 

[Wrangler]

From Mars, it is possible to aerobrake directly, but as I said, you need to point your ship "inverted" in the atmosphere, so that the lift provided by the wings of the XR2 initialy works to push you toward down the ground (nadir), instead of "up".

You won't be able to have a roll of 180 degrees on reentry and keep the proper aerobraking altitude, so you'll need to roll a little to the left, a little to the right (S-turns) so that the lift provided counteracts exactly the centripete force that wants to pull you away from Earth as long as your orbit is hyberbolic, and then while your apogee still is out of the atmosphere. When you reach that point, you can roll over "right side up" and continue like a normal reentry.

Inverted reentry!... that would be so cool if it comes off! though I will have to engage the attitude autopilot after the ship is inverted. Also that Gravity-gravity assist idea is a good one too. Did not think of that!

Any ideas on the reentry corridor? My own estimate is about 65-70 degrees of anticipation. What I can't figure out is the Reentry angle itself. Coming in from that high up, I know that it will have to be a steeper than normal angle. But how steep? I read someplace that anything over 2 degrees is not recommended for manned spacecrafts. but then no manned spacecraft has ever had to do what I am imagining (yet :lol: ).

Still some great pointers. Thanks.

Wrangler

 

[statickid]

this is a pretty good thread:

http://www.orbiter-forum.com/showthread.php?t=29779&highlight=hyperbolic+reentry

I'm pretty sure someone did a successful aerobrake from Jupiter in an XR vessel, IIRC

It can be easier not to worry about the actual "angle" I always have the best success adjusting the Pe (closest approach TransX). For example, returning from the moon I usually set Pe around 60km. While trying to do speed runs the other day, I decelerated from a hyperbolic LEO transit by lowering Pe to about 64 and it was fine. I was going pretty fast, so I bet 64 is a good place to start. The only thing that is crucial is that you start with a high AOA (like 55-60 degrees for xr-2) and then lower it RIGHT after the hottest part and keep the hull burning hot. In the inverted reentry there is no proper reentry angle because you will be basically flying your spacecraft through the upper atmosphere, balancing lift/drag/hull temperature.

 

[Cairan]

Exactly, the hyperbolic aerobraking maneuver has 3 phases:

1) Pre-perigee inverted roll high-alpha drag maximization and perigee altitude targeting
2) Post-perigee inverted roll medium-alpha lift maximization and constant heat load management
3) Sub-orbital reentry landing site targeting

In phase 1, you come in fast, so you want to be sure that you'll hit your target aerobraking altitude precisely, there are no second chances. The high drag attitude will help to shed some speed, and in the real world, would help create a nice bow shock to increase the distance between the heatshield material and the shock-wave.

Primary concern: Hit your target altitude

In phase 2, you need to fight the tendancy of your ship to want to skip right back out. So you need to manage your lift to "push" you toward the Earth, using the atmosphere, while keeping an eye on the thermal load trend. If the temperature starts increasing, look at your vertical velocity. If it is increasing toward the ground, you need to decrease lift real quick or you'll burn up. If the temperature starts dropping, and your vertical velocity starts to increase away from the ground, you need to maximize lift fast or you'll skip out. Here the limiting factors are heat load and vertical velocity. Doing S-turns is quite hard here because during roll reversals, you can easily overshoot and loose your balance of lift, centripetal force and heat load.

Primary concern: Maintain vertical velocity near zero and heat load within tolerances.

Phase 3, is just a regular reentry, now you've rolled right side up, and you try to manage your trajectory to hit your landing target, as you would normally in any other circumstances.

Primary concern: Manage energy to reach the landing site.

 

[Tommy]

It's quite possible, using the inverted re-entry method outlined above. An Example of that can be found in the Home Direct tutorial (also included in IMFD Full Manual) available at OH. I should point out that the newer versions of the XR can use the AoA hold AP while inverted - so you don't need to use manual CoG adjustments and trim during the inverted portion. This makes things MUCH easier!

I usually use a low AoA (inverted) for the entry, and initial braking. This leaves a long re-entry, but you will have the range to travel well over a full orbit before landing so you don't need to worry about timing the arrival to target a particular base.

Should you decide to use a high AoA method, I suggest an Anticipation of about 90 degrees - more if coming in from far away (like Jupiter). Leave the Entry Interface at 120k, and set the ReA to about 4.5 degrees. When approaching, aim for a specific PeA (around 60k to 70k depending on velocity).

Hint: In the XR config you can change the "limits" on the AoA Hold AP to allow bank angles up to 90 degrees (IIRC default is 60 degrees). This lets the AP control the vessel through the "flip" from inverted to regular flight when your velocity drops below orbital.