Flight Question Checklist - DG to ISS

[Relkari]

Hi Guys,Gals & Visitors from other worlds.

I'm fairly new to Orbiter but have plenty of experience using MS FSX, i have just a few questions

1)I'm running thru the checklist scenarios and i'm having some issues with launching the delta glider.
I take off pitch up, turn to heading, pitch up to 70degrees, when the nose starts to drop at 30km i turn on rcs.
In the F1 help it says when you get thru the dense part of the atomsphere drop the nose to 20degrees, now from searching over the internet and reading thru 'go play in space' this seems to be at about 40km however without using the hover thrusters i always seem to loose height and end up coming back down into the atmosphere.
Although in the documentation it does not mention using the hover thrusters.

2) When i do make it into orbit, i can get close to ISS about 50km but then it starts moving away.

These are probley stupid newb mistakes but any help/advice would be greatly appreciated.

Thanks

 

[johan]

Hi! I'm fairly new to this myself, so have recently gone through this same learning process. I've found that the 20deg pitch that the F1 help advises needs to be taken with a pinch of salt. Make sure of these things:

1. Your throttle is all the way open when you pitch up to 70deg
2. When you get to 30km, pitch down to somewhere between 20 and 30deg (so your nose should be between 20 and 30deg above the horizon). Throttle should still be full open. Watch your forward velocity marker (the circle with the cross or plus in it), and make sure that it doesn't drop below the horizon (if it drops below 1 or 2deg upwards pitch, pitch up your DG's nose to lift it up).
3. When you've gained enough prograde velocity, you will see the forward velocity marker start to move upwards again. Now you can pitch down a little to gain prograde velocity more efficiently.
4. Now watch your ApA (turn on the Orbit mfd and hit the DST button so that ApR turns into ApA). Once your ApA is well out of the atmosphere (200km or so should be fine), you can turn off your engines, coast until you hit ApA and then turn prograde and fire your engines to lift your PeA out of the ocean (and the atmosphere )

If you still struggle, try pitching up to 60deg instead of 70deg after finding your heading. This should give you more prograde velocity by the time you hit 30km altitude, which will make that part of the process easier.

Good luck, and let me know how it goes!

 

[Tommy]

I use s slightly different method for the DG (and winged vessels like it such as the DGIV) that is a bit more efficient. It's closer to the ascent profile used by the XR series vessels, but without the scram engines, of course.

The profile given in the checklist is based on the theory that you want to reduce aerodynamic drag as much as possible during the entire ascent, but this means using thrust to overcome gravity quite a bit. You've noticed that the velocity indicator drops below 5 degrees (indicating a high AoA) most of the ascent.This means that more of your thrust vector is in a "upward" direction instead of a forward direction - and forward (tangential) velocity is what you need to attain orbit. Also, the higher AoA increases the drag coefficient quite a bit, so it can often be better to be a little lower. Even though you are in a denser part of the atmosphere, the lower drag coefficient results in LESS velocity lost to drag.

I basically break down the ascent into different stages.

1: Launch. At very low altitudes drag is very high, so get into the air and reduce throttle to maintain speed, about 200 m/s. and turn to the desired heading. Once the heading is attained, go full throttle and pitch up to about 75 - 80 degrees. Watch the ACC on SurfaceMFD, and use pitch to attain a slight ACC of a couple m/s. One you've done this leave the stick alone for the rest of the ascent - use elevator trim only (Insert and Delete keys, IIRC)

2: Transition. Once you are at least 8k to 10k alt (and should still be subsonic - 250m/s max) use downward elevator trim to begin lowering the nose. The stock DG's trim is a it "weak". so use full downward trim. A DGIV's is a bit stronger, about -.8 is better. The goal is to get near level flight around 25K alt. Watch the VS, target is about 200 m/s at this stage.

3: Acceleration: Use elevator trim to gradually reduce VS, about 150m/s at 30k, as sow as 100 m/s by 35k. If the nose drops even with full up trim just let it. As you gain speed it will come back up. By thi time you get to 70k you want a velocity of over 7km/s, and your ApA should be climbing rapidly.

4: MECO: Aoround 70k to 75k you should be near 7.5kms, and your ApA should have risen to around 200k. Cut the main engines and coast. Watch the ApA, if it's rising apply a bit of downward trim, if it's lowering trim up. Ussually a couple clicks of down trim will keep the ApA stable. Once over 125k you can engage prograde AP and some time accel.

5: Circularize. Once you get to your Apo, use main engines to raise the PeA and circularize the orbit.

Note that RCS wasn't used until the prograde Ap was engaged- in fact it's hand off the stick entirely after the intial pitch up manuever.

As for your second question, 50k is close enough for an initial rendezvous if your relative velocity is low ( under 300m/s). You should be using DockMFD and have the COM/NAV 1 set to the ISS's transponder (not an individual port freq), and ensure the docking hud is active and set by pressing the <HUD> key once DockMFD shows it's getting a signal. Pay attention to the RVel when you stop getting closer - if it's over a couple hundred m/s, it likely means your planes have gotten out of alignment - fix that then try again. The DG has tons of thrust, so if your RVel is under 500 m/s you can usually still succeed. One you stop closing on the ISS, turn toward the negative RVel indicator (the plus sign, will be 180 degrees from the "plus sign in a ball". Use main engines to reduce the RVel as much as possible, 1 or 2 m/s max. Then point at the ISS and give a bit of thrust to get about 20 m/s RVel. Use lienear RCS to keep the plus sign near the RCS as you close. Use bursts of time accel - don't increase RVel. Then you can pick a port and dock.

IMFD has a program (under the Orbital Ops menu) to null relative velocity, this can be handy as it's very difficult to do this manually - once RVel gets below 5 m/s the vector will start moving quite a bit.

BTW, welcome to the forum, and congratulations on getting that close without help! The main secrets to rendezvous and docking are patience and precision. Keep the planes aligned within a couple hundreths of a degree, and keep the RVel low.

 

[Relkari]

Thanks Johan & Tommy
Tried 2 launches last night & I was able to make it into oribit without using the hover thrusters, the velocity marker did dip slightley bellow the horizon but I was able to recover and i'm sure with some more practice it will get better.

Thanks again

 

[johan]

Well done! :thumbup:

Once you're comfortable getting into orbit, I'm sure you can use Tommy's instructions to get real close to the ISS and dock. If I were you, I would try to get comfortable with docking to the ISS before I tried to return to the Earth's surface from orbit. I found it more challenging than the docking maneuver... I lost count of the number of times I alternated between burning up and overshooting my target base by >1000 km! Overshooting by that much inevitably ends badly, by running out of fuel, or air, or overheating the internal systems (if you're flying an XR vessel):facepalm:

 

[Tommy]

If the velocity indicator dips below the horizon (even with full up trim) it means you are too high for your velocity. Your experience with MSFS is as much a liability as an advantage. MSFS doesn' usually happen at the same hypersonic speeds as an ascent to orbit, and "level flight" (the holy grail of normal flight) is rarely desirable in Orbiter. Sure, for a "perfect" ascent you will always be gaining altitude, but it's not uncommon for "dips", especially in the 30k to 35k range. For any given altitude, there is a minimum speed needed to maintain altitude using elevator trim alone (this also varies with the vessel). Using the stick to keep the nose up (and keep climbing) is wasteful - the increase in drag coefficient outweighs the reduction in drag from the lower density (and losses from a misaligned thrust vector).

Too achieve a stable orbit, two conditions must be met. You must have sufficient altitude to clear obstacles (including atmosphere - about 200k for Earth) and a sufficient tangential velocity that centrifugal force balances the gravitational pull. The sooner you build tangential velocity, the sooner centrifugal force helps overcome gravity, so it's best to concentrate on tangential (forward) velocity - only have as much vertical velocity as required to to lower the drag (measured in m/s) until t is less than the gravitational pull (also measured in m/s) that you are using thrust to overcome. Put simply, speed is more important than altitude, because speed will become altitude - the reverse is not true.

In fact, it's possible to achieve orbit (in a stock DG simply by putting the throttles to full, and waiting until the ApA is around 200k, then coasting to Ap and circularizing. A touch off back stick to get off the ground doesn't hurt, but once you are airborne and the gear is up, you can use neutral stick and trim and the DG will climb all on it's own. It's not very efficient, but it will get you into orbit.

The whole "20 degree" thing is part of a technique known as a "pitch table", where you pitch the vessel according to yur altitude. it's a simplified way to help get above the thickest part of the atmosphere and then gain speed. It's very useful for a more ballistic ascent profile used by non-lifting vessels such as a Saturn V or Space Shuttle, and also works for gliders It's not as efficient for gliders as an aerodynamic ascent profile like I outlined above. A DG has wings for a reason - let them do the work of gaining altitude so the engines can concentrate on gaining speed.

All that being said, efficiency is something that comes in time, with practice. Even getting to a stable, circular orbit is an achievement to be proud of. Once you can get into orbit reliably you can start to worry about fuel efficiency. Then you can practice with the Lua Challenge #1 (in the 2010 version). Martins's high score isn't bad, but it's beatable by an experienced orbinaut (he spends his time developing Orbiter, not flying it!) My first try at that was 5656.57 kg used, compared to 5986.67 kg used, but I've spent a LOT of time refining my ascent profile. I'm sure it can be done much better.

 

[johan]

Ha! This is the clearest explanation I've seen so far, thanks Tommy! It makes more sense now than it used to :tiphat:

 

[cinder1992]

<snip>Overshooting by that much inevitably ends badly, by running out of fuel, or air, or overheating the internal systems (if you're flying an XR vessel):facepalm:

if you're overheating, deploy the radiator! that's what it's there for.

 

[johan]

if you're overheating, deploy the radiator! that's what it's there for.

Not if you're flying at Mach 3, at 25km altitude because you have 2000km to go to reach your base, and you're trying to not run out of fuel before you get there! At least not if you want to get there with the radiator intact.

What I did once, was simply put 'er down in the middle of nowhere. This let me deploy the radiator and open the hatch (for some reason I was low on LOX as well). Good thing that fuel truck showed up so I could refuel and get back to base... I didn't bother asking the driver what he was doing in Mexico! :lol:

 

[Relkari]

Yay, just docked at ISS, taken me a week of trying, but was well worth it, quick trip over to MIR and then onto re-entry & landing :uhh:.

 

[Tommy]

AerobrakeMFD is pretty much essential for a completely deadstick landing on target. Search the forum for "AerobrakeMFD" and "BasesyncMFD" for tips on unpowered re-entry/landing. Even with the MFD's, it will take some practice.

 

[thesnorklemonkey]

If you'd like a piece of a tutorial I did that involved launching the Delta Glider, I have a link below. I don't know if it's the same as these other descriptions, but it's narrated and whatnot. I hope it helps.

 

[Tommy]

Thesnorklemonkey, while I appreciate the desire to create tutorials and help others learn, I've checked out your DG to ISS tutorial. It's so full of errors and horrible methods that I'm not sure where to start. The last segment (segment 8, final approach to ISS) is decent, but the rest is seriously flawed.

You don't point out that you've enabled "unlimited fuel" in the launch pad, and your methods probably wouldn't get you to the ISS without that. You maintain a steep pitch until you are at 60k (and have only built up about 375m/s velocity), that's why you need the hover thrusters to maintain altitude. This is extremely inefficient, and also causes the high eccentricity you end up with. Also, correct launch headings for the ISS are 42 degrees or 138 degrees, not 90. This is why you have 38 degrees of RInc (yes that does stand for Relative Inclination) upon reaching orbit.

You seem to think that the burn time when aligniung nodes is 1/2 the TThA (or TThD) when it is the actual burntime. The burn should start when TN = 1/2 TThA, not TN = 10 seconds. This is why it took you three nodes to align planes.

It's not my intention to sound rude (although I'm sure I do) but this kind of tutorial will confuse a newbie far more than it will educate them. It is very incomplete and full of errors, and will give new orbinauts many bad habits that will hinder their progression in the future. It's just as easy to learn how to do things correctly from the start than it is to learn an "easy cheesy" way that will let them down as soon as they try to apply those bad practices in another mission. This will lead to much frustration down the road, and ends up making the learning curve much steeper than it needs to be.

I also checked out the Apollo 11 TLI tutorial, and a free return trajectory does NOT have a lunar periapsis that is almost 125k BELOW the surface of the Moon. The 8M offset radius value is only a rough "starting value" and will need to be adjusted to get an appropriate PeA. Nor is your method for setting the TEj correct, and I don't recall you mention setting the TIn for a TOF of about 75 hours.

As I said, I appreciate the desire to help others learn, but you can't effectively teach something you aren't very competent in to begin with. I've been working on some segments for the upcoming IMFD Full Manual V2, and I've invested over 200 hours (so far) in testing and creating the techniques shown in them. This is on top of the 5 years of experience with Orbiter and IMFD I already had.

The ascent profile I outlined above is the result of dozens of tests to determine drag at various altitudes and speeds, and hundreds of ascents where I used scientific analysis methods to refine the profile for efficiency and repeatability. While it's not perfect, it's better than the majority of orbinauts can do. I don't make tutorials because I can simply do something, only when I can do it better than most, AND explain it clearly and completely so that even new pilots can understand it. If I can't, I don't feel I have anything worthwhile to teach.

I'm not claiming to be the best pilot here, several others are better at many tasks than me. I have earned a reputation for knowing what I'm talking about - mostly because I don't talk about things I don't know well (such as TansX, which I have yet to get the hang of). I do this because I experienced a lot of frustration early on because many of the turoials available were oversimplified, incomplete or just plain wrong, and I don't want others to have to go through that as well.