IMFD Back to Cape Canaveral

[Ululuru]

Hey,

I played around with Orbiter for a few weeks. Now i decided that going to the moon or the ISS isn't enough. So i read markl316's IMFD Documentation and watched some Tutorials.

Now i try to fly the Venus weather mission from the DG4. I can execute the whole missione without any real problems: I get to the iss, refuel, get into an equatorial parking orbit with the correct altitude. But when i eject from Venus back to Earth, I have some Problems:
In the Target Intercept Program, i enter the Correct Lat and Lon coordinates for the Cape. But when i use the Base Approach a few hours/days before i arrive back at earth, i have some Problems: The required Dv is about 6m/s, but when i hit AB or Burn manually (doesn't matter) the Dv does not change. AB will just sit there and Burn forever.

Also, how do i survive a reentry in the DG4 with such high approach speed (12 km/s)
Should i just get the PeA to something like 70km and only Aerobrake, then get into an Earth orbit and Reenter later?

 

[Mojave]

You should set the PeA to 150km so you can even out your orbit allowing you to set up a better reentry approach to the Cape.

 

[Ululuru]

i dont want to use so much fuel to get into an earth orbit...

that would be a dv > 4000 m/s

 

[Mojave]

Sorry, I was thinking you would have enough fuel to do the burn. I have no other advice to offer. Sorry

 

[RisingFury]

Aerobreaking into a parking orbit is a good idea, but it's a tough thing to do. You'll need to go in inverted, so that your DGIV's top is facing the Earth and if you can handle it, at an AOA of 35° or less. DGIV practically stops producing lift beyond that....

An altitude of 70 km should be survivable, but save the scenario before the entry just in case, so you still have time to repeat.

 

[Grover]

i prefer to use the XR2, but i supose its not too different:

i fly with little AoA, at around 70Km, max lift is between 10 and 12 degrees AoA, so i keep the pitch at about 5-7, and pitch up if i have an emergency and start to climb, so i dont accidentally skip off the atmosphere and get lost in orbit around the sun

just keep at 70km, you can get down to 65 on a return from the moon, so you might want to aim for that, but 70 should do, then just watch vertical speed and acceleration to keep in the safe zone where you can prevent yourself flying off

personally, i find it hard, because it takes too long, and i get bored halfway through and do a nosedive

good luck!

 

[Ululuru]

Ok thanks.
What Reentry Angle do you suggest?

 

[Grover]

donw use reentry angle, just set PeA to 70Km, then enter inverted, then control your altitude like a plane, only you're inverted and at escape velocity... and if you go too low you get fried.... easy!

 

[Ululuru]

can't do it...

i watched again, my approach speed is about 13.5km/s, thats just too much.
I now captured myself with the engines into an highly elliptical orbit, and did 1 aerobrake into a normal circular leo. From there i just did a normal reentry.

 

[hrothlant]

can't do it...

i watched again, my approach speed is about 13.5km/s, thats just too much.
I now captured myself with the engines into an highly elliptical orbit, and did 1 aerobrake into a normal circular leo. From there i just did a normal reentry.

Plan ahead to set up a proper return / approach to Earth. The lower your approach speed, the less you have to blow off inside the atmosphere.

You can fly this mission without refuelling at the ISS, you just don't need it - if you use an aeorbrake maneuvre encountering Venus and also landing back on the Cape.

My "flight plan" in short terms:
I first planned my return journey from Venus to Earth using IMFD. I set up a hypothetical Hohmann transfer (using Target Intercept - Tangential Transfer) from Venus to Earth. This would ensure an Earth encounter with the lowest velocity.
The next step was to plan the journey to Venus, arriving at a date let's say two weeks before the return.
This plan made me arrive at venus with a quite higher speed, but i found out it's easier to aerobrake in the Venusian atmosphere with high velocity than in Earth's Atmosphere.
After Aerocapture at Venus, you circularize your orbit, deploy the satellite, await your launch window back to earth, eject to the journey home, set up the proper base approach some distance ahead, perform a direct reentry and glide back to the cape, after a journey of about one and a half years

 

[Tommy]

The probable reason you had such a high dV when using Base Approach was in the timing of your arrival. If you are approaching for "Re-Entry" rather than "Insert", WHEN you arrive at Earth is crucial.

To use the "Re-Entry" mode, you need to be arriving at the right time of day AND on the right trajectory - otherwise Base Approach will attempt to put you on that vector by brute force, and that often just isn't possible. It tries anyway, and fails, and that's why the dV doesn't change.

You can actually use the inverted re-entry technique at velocities over 14km/s in a DGIV - but you have to VERY precise. A bit too high and you won't get enough lift to hold you in the atmosphere. A bit too low and you burn. It can be done - but it's extremely difficult.

 

[Matrix Aran]

I've been playing with inverted re-entries from the Moon over the past few days in an XR5 and I've managed to come to much the same conclusions as others have in this thread: a PeA of about 70 or so is the best way to go and they're dastardly hard. The benefit to the technique though is that you can adjust for targeting errors during the maneuver.

The one thing that I can add is that from my experimentation I've found that it helps immensely to fly the inverted phase manually, at least for the first time as it will let you correct for errors much faster than the craft's autopilot can (at least in the case of the XR series).

I've found the simplest approach is to simply keep the craft inverted and level until your vertical speed begins to slow and right as it starts to grow again you add some up elevator or trim. Keep an eye on your temperatures and try to keep the vertical speed to +- 20m/s. The goal is to drop down as far as your ship can handle in the atmosphere and stay there. As you slow down you'll be constantly adjusting this as your craft will want to start sinking further and further, building up heat far too quickly. When you get used to which angles to use for which speeds you can later on leave things to an autopilot if you wish.

Aerobrake MFD is very helpful for targeting your landing at this point, also keep an eye on your orbit eccentricity, As it nears zero it'll be time to roll upright and carry out a relatively straightforward re-entry, with Aerobrake MFD helping to provide the correct AoA for the rest of the decent.

Alternatively, if you're a bit unlucky or if you're like me and on a flight you've taken just on a whim your landing point is directly under your periapsis, you can roll level a little early when your apoapsis is at a comfortable altitude for a small de-orbit burn.

I'm fairly sure this isn't the best way to go about things, but perhaps it might help you perform a slightly more elegant re-entry.

 

[Tommy]

Aerobrake won't be of much help during the inverted portion of the re-entry because your projected flightpath usually won't reach the base. If you set the Landing Target on IMFD's main config page, you can use IMFD's Map to help you stay on course. Just hit the MOD button until you see the landing target displayed, and keep the Ang as small as possible.

 

[Matrix Aran]

You know that actually works a lot better. I'd only had Aerobrake open really to just eyeball the trajectory on the map view during the phases of positive lift, but IMFD gives a much more accurate indication.

 

[Tommy]

I use that feature a lot - I don't even bother with BaseApproach anymore. I use Delta - V and Map for MCC's and approach - it's much more accurate.