XR2 Reentry from Moon

[caningo]

Hi everyone,

I have a quick question. I was making a trip back home from the moon using the XR2 and IMFD and got toasted during reentry. I guess I was coming in way too fast and my reentry angle was too steep I suppose. I used the default values of Alt 120km, ReA 6.20* and ReA 21*.

When I do these trips coming back home I usually do an Orbit Insert and then I do reentry. However this time I'm trying to be more fuel efficient and go for the reentry all together.

Does anyone have suggestions on what values would be appropriate to make a succesful reeentry without roasting the XR2 and the crew? :lol:

Thanks.

 

[astrosammy]

A direct reentry from moon is impossible with the XR-2.
You have to do Aerobraking:
First get a Periapsis of 70 km. Now aerobrake the ship into Earth orbit (Aerobrake MFD is a good help there). You should do that until you are in a LEO. Now just deorbit and land. I've tried it out with the XR-1, but it should be the same with XR-2.

 

[agentgonzo]

A direct reentry from moon is impossible with the XR-2.

No it's not, but it's not exactly easy. On a return trajectory from the moon, you will be travelling at about 10km/s when you hit the atmosphere. Aim for a PeA of about 65km. The higher you go, the more chance you will have of just bouncing off the atmosphere and the lower you go, the more chance you will have of being burned to a crisp.

When you re-enter, you need to do it inverted - that is, have your head towards the earth so you are flying upside-down. Start out at an angle-of-attack of about 50°. Watch your altitude on the HUD or in SurfaceMFD and as your vertical velocity reaches about zero, you then need to pitch down a bit so that you keep your vertical velocity about zero. As you do this, you use the aerodynamics of the vessel to pull yourself towards the Earth instead of just ploughing through the upper atmosphere and back off into space. It's easiest to use the CoG offset on the XR2 to help maintain the AOA necessary and prevent your nose from pitching down and you burning up (something the DGIV didn't have which makes this a lot easier in the XR2). As your velocity drops to about 7.5km/s, pitch-and-roll around so that you are no longer inverted and are flying head-up, then reenter as normal.

I've done this quite a few times (I did it a lot with the DGIV), and perform a direct reentry and dead-stick all the way into the GTF at Canaveral having dumped all the main fuel after the course correction burn half-way back from the moon.

 

[Messierhunter]

Thanks for the tutorial gonzo, I would have never thought to try a "heads-down" orientation. I'll have to give that a shot. Does it also work for direct re-entries from mars or is 10km/sec a hard limit?

 

[agentgonzo]

Thanks for the tutorial gonzo, I would have never thought to try a "heads-down" orientation. I'll have to give that a shot. Does it also work for direct re-entries from mars or is 10km/sec a hard limit?

No idea what the maximum you can do is. 10km/s isn't a limit, it's just the normal speed of approach from a lunar return. It's probable that you can do it at faster speeds. No idea what the reentry speed would be from a Hohmann transfer from Mars. Give it a try and post your results.

 

[Linguofreak]

The maximum will depend somewhat on how much weight you have on you. If you're going in with full tanks and cargo bay, you might not be able to make a successful reentry at all on a lunar return. (Although as long as your orbit isn't an Earth escape trajectory you should be able to make multiple passes, which will allow you to eventually get back down.)

With near-empty tanks and no cargo (The XR2 has a cargo bay, right?) you may well be able to make a successful return from Mars.

 

[caningo]

XR2 Aerobrake

Gonzo and all:

Thanks for the reply. I tried following your procedure and wasn't able to get your results. I think it's because I was coming in at 11 km/sec and my ship was full of fuel and had a large crew.

I was able to lower down my OS to 9400 km/sec on the first pass and then came back around and got it down to ~7300 km into a 160km x 60km orbit. After that I was good to go. I also found that exposing as much wing area provided me a more braking instead of doing the upside down thing. :huh:

Anyways please give me some feedback. If you can try the XR2 coming back form the moon or mars and post your results here.

Hope to hear from you soon! :lol:

 

[astrosammy]

I've tried out it from Mars, but got lost in space. I've attached my scenario.

 

[agentgonzo]

I've always dumped all my fuel (Main and SCRAM) before reentry. No idea how many passengers/payload I had - I just used the standard scenario files (landed at BB or KSC for a flight to the moon and back) and config.

 

[caningo]

I've tried out it from Mars, but got lost in space. I've attached my scenario.

Astrosammy:

I tried your scenario last night and figured that you are coming in way too fast. If we can somehow reduce the ground speed of the ship to around 10 km/s and maintain the 65km periapsis then a proper reentry or at least orbit insertion can be made without any fuel.

Do you know of anyway of doing this? Reducing the eccentricity of the orbit without affecting other parameters?

I'll keep looking into this.

 

[astrosammy]

I tried to get the velocity as low as possible, I think making a slingshot around Venus or Mercury could lower it a bit, maybe a slingshot around Moon is also good.

 

[agentgonzo]

I tried to get the velocity as low as possible, I think making a slingshot around Venus or Mercury could lower it a bit, maybe a slingshot around Moon is also good.

I haven't looked at your scenario, but a slingshot around Venus or Mercury won't help if you are coming in on a Hohmann transfer from Mars as the deltaV of Venus and mars is comparable, and the delta-v to get to mercury is massive (delta-V to get there is the same as the capture delta-v coming back from there). A sling around the moon on the way in would reduce your velocity, but not by a huge amount.

 

[agentgonzo]

OK, I've just done a reentry from the moon in the XR2 with a full load of fuel (exited out just before reentry and filled up SCRAM, main, RCS and APU fuel in scenario file) and full load of passengers.

Max speed was 10.64km/s at about 80km altitude. Got down to 64km altitude and held it there at an inverted AOA of about 25° just over the north west coast of Africa. Gradually dropped down to about 60km over the next bit to keep hull temp high, but not critical (about 2000°). As I got to about 7.8km/s I flipped right-side up and reentered as normal, landing just on the east Coast of Afirca. Next up, see if I can do it from Mars!

Edit: Yep. Did a Hohmann transfer from Mars. Ended up coming into the Earth's atmosphere at a little over 12 km/s (this time without any fuel except RCS and APU). Held it at about 67km as I enterred over Indonesia, then dropped down to about 62km and held it there until sub orbital. Flipped it over and reenterred as normal. Ended up a couple of thousand km off the east coast of China.

Edit2: Yes, it's possible to do that with full tanks too. Same reentry, except this time had to hold it at about 65km for much longer. Took the best part of half an orbit to reenter, keeping the hull temperature about 2000° the whole time. Fairly simple, just takes a lot of patience otherwise you die.

 

[Messierhunter]

Thanks for all the details gonzo, just completed your technique for the first time on a simple reentry from the moon. I haven't yet started my mars mission, but needed to know if this would work from the start for mission planning. Did a round trip in the XR2 from the earth to the moon, landed, then launched straight back to earth. The whole process consumed every drop of fuel I had (didn't refuel at any point) so I ended up reentering with dry tanks. I began to run out of RCS fuel during the long 65km phase; I was finding it too touchy to use CoG adjustment, but my depleted fuel tanks forced me to roll back over at a little over 8000m/s and use CoG from there where I had more wiggle room. It's interesting that if you pitch up too much while inverted you expose the cockpit to too much heating, but you'll burn up anyway if you pitch too far down. You really have to hold a steady trajectory. In the end I landed it deadstick back at the Cape in one direct reentry from the moon, quite a thrill.

 

[Tommy]

Posted an annotated flight recording of a BB - Cape Canaveral direct re-entry. It was my first post to Orbit Hanger so there's a wait. Should be available tomorrow, it's called Home Direct.

To answer an earlier question, I find that setting the Ant to 90 in IMFD's Base Approach program works.

Tommy

 

[agentgonzo]

I was finding it too touchy to use CoG adjustment

After setting the CoG (roughly), use the trim (insert and delete keys) to make your AoA stable without having to use RCS fuel.