How to land at the launch pad.

[Frostiken]

That's true of the Shuttle's computers, which never advanced beyond early-80s tech, and of the computers used in space probes (*still* 486es!), partly for reliability, and partly because with time comes skill in radiation-hardening a given chip design.

And probably partly because... really how up-to-date do your computers need to be? It's spaceflight - there's nothing to it more than math equations. And not particularly hard ones at that.

[DoctorEvo]

And probably partly because... really how up-to-date do your computers need to be? It's spaceflight - there's nothing to it more than math equations. And not particularly hard ones at that.

Bu-But then the astronauts can't play Angry Birds when they get bored!

[rdfox]

That's why they take up the ThinkPads that you see them using in the ISS cabin.

[The_Stargazer]

If I recall correctly, the computers were so outdated because they were stable, because in space you can't have somebody go up and reboot Windows, or reset the modem.

No... Though

The computers were outdated because of 'design freezes'.

When you're developing a rocket or space vehicle (or any engineering vehicle really) you have to basically put a freeze on implementing new technologies or the vehicle would never get done.

For example, when the Shuttle was first developed in the 70s (didn't fly till the 80s, but was designed in the 70s), they made the choice of computers. Then the whole computer revolution began.

But if in 1980 we decided 'hey look, we have revolutionary cool computers that we should use on the Shuttle', then they could spend another 5 years implementing that into the design and.... wait... the 1980 computers are already out of date in 1985... So then they decide to put in new computers in 1985, takes 5 years... and 1990 comes around...

You get the idea. As soon as we decide to design a vehicle, the technology is out of date.

While yes, they want stable computer systems, and the SIMPLER the computer the more stable it is, computer tech that flies in space is usually out of date due to design freezes.

[rdfox]

Plus, of course, radiation hardening (486es are *GREAT* for that!), outgassing test issues, power requirements, the whole thing about whether you actually *need* more computer for the job they're assigned to, and human-rating requirements. As Wikipedia's page on the Shuttle's GPCs says, the AP-101S computer 'remains in service (formerly on the space shuttle) because it works and is flight-certified, whereas a new certification would be too expensive.'

[The_Stargazer]

Plus, of course, radiation hardening (486es are *GREAT* for that!), outgassing test issues, power requirements, the whole thing about whether you actually *need* more computer for the job they're assigned to, and human-rating requirements. As Wikipedia's page on the Shuttle's GPCs says, the AP-101S computer 'remains in service (formerly on the space shuttle) because it works and is flight-certified, whereas a new certification would be too expensive.'

Though there is a push to get more COTS (commercial off the shelf) computers flight certified. Heck, even Android phones are being flown (in a stripped down version) as the CPUs of some small satellites.

[Enzo03]

I absolutely love how much this thread derailed.

I have never done a successful orbit (last one ended in landing on the dark side of the planet, which is apparently made of antimatter or reverse-explodium) but I once did one of the ridiculous straight-up flights and then managed to land on the launch tower. It was pure luck though.

[Sabactus]

I have a theory that Im going to test. Im going to launch and maneuver into a circular orbit, say 50km. When I am on the opposite side of the planet from the space center I'll do an Hohmann transfer orbit burn from 50km to 34km. Im hoping that by doing that Ill place the perikee over the space center and hit atmo right over it.

I'll let you know my results.

[Rocket Surgeon]

I have a theory that Im going to test. Im going to launch and maneuver into a circular orbit, say 50km. When I am on the opposite side of the planet from the space center I'll do an Hohmann transfer orbit burn from 50km to 34km. Im hoping that by doing that Ill place the perikee over the space center and hit atmo right over it.

I'll let you know my results.

Works great, but your timing has to be brilliant. When passing over your landing point: MET + Orbital period - Hohmann transfer time. When your MET clock says that number, deorbit, wait and pray.

[Sabactus]

Works great, but your timing has to be brilliant. When passing over your landing point: MET + Orbital period - Hohmann transfer time. When your MET clock says that number, deorbit, wait and pray.

How do you know the transfer time?

[DoctorEvo]

How do you know the transfer time?

Should be half the orbital period, which can be determined from the semimajor axis, which can be determined by averaging your apsis (from the center of Kearth, not the surface) and Periapsis.

Here's the formula I came up with:

T_transfer = 3.14sqrt(a³/u),

Where 'u' (because Greek letters like 'mu' aren't recognized) is Kearth's gravitational parameter (5.29x10²² km³/s²) and 'a' is the semimajor axis.

[Sabactus]

Flight log

-Way overshot insertion into orbit at 50km, ended up with a highly eccentric orbit.

-Apogee was approx 165km. deltaV 2148 m/s

-Hohmann transfer to 50km deltaV 2058 m/s

-Orbital injection at 50km deltaV 2423 m/s

-Minor burns to stabilize orbit

-Ended up in a fairly stable orbit at approx 64500m +- 1km

(If my theory is correct performing the burn on the opposite side of the planet should have me hit atmo over the space center)

-At T+ 01:04:19 second pass over the space center

-At T+ 01:19:16 altitude is 64190m estimate opposite of KSC

-Calculate change in deltaV to 2278m/s

-Firing

-[hold breath for next 15 mins]

-T+ 01:26:00 altitude 52000m, suspect I may have misjudged the burn.

-T+ 01:30:00 altitude 38500m speed 2370m/s and certain I fouled the deorbit burn.

-T+ 01:32:00 ATMO

-T+ 01:35:31 Touchdown

Failed to land near the Space Center but take solace in that I ended up on the proper continent and on line with the target area.

[DoctorEvo]

Yeah, with such limited instrumentation, a brick-wall atmosphere, and effectively no option of a controlled lifting reentry, I prefer to use much shorter, steeper deorbit profiles. If I'm trying to hit the launchpad and I have enough delta-V left, I won't deorbit until I can actually SEE it. My FURTHEST landing using this method was on that little peninsula just to the North - Barely out of sight from the space center.

Ideally, a full Hohmann transfer to the upper atmosphere is the most EFFICIENT deorbit, but it's not the most accurate and you shouldn't be caring about 'wasting' propellant at the end of your mission anyways unless that's literally all you have left.

[Sabactus]

Yeah, with such limited instrumentation, a brick-wall atmosphere, and effectively no option of a controlled lifting reentry, I prefer to use much shorter, steeper deorbit profiles. If I'm trying to hit the launchpad and I have enough delta-V left, I won't deorbit until I can actually SEE it. My FURTHEST landing using this method was on that little peninsula to the northeast that's just barely visible in your picture.

Ideally, a full Hohmann transfer to the upper atmosphere is the most EFFICIENT deorbit, but it's not the most accurate and you shouldn't be caring about 'wasting' propellant at the end of your mission anyways unless that's literally all you have left.

Yeah you can't see but I was using the Sphaera mod. I had a tank and a half of deltaV left. >.<

[Rocket Surgeon]

Flight log

Well done.

First (and only) time I tried it I had to fudge some estimates but ended up splashing down in the water off the pad.. Just out of view range at sea level. I was satisfied.

[Sabactus]

Well done.

First (and only) time I tried it I had to fudge some estimates but ended up splashing down in the water off the pad.. Just out of view range at sea level. I was satisfied.

Im going to give Hohmann another shot today. Im wondering about your method of deorbiting within sight of the space center. How do you do that accurately?

[MaxSchram]

-Apogee was approx 165km. deltaV 2148 m/s

-Hohmann transfer to 50km deltaV 2058 m/s

-Orbital injection at 50km deltaV 2423 m/s

I recognized it earlier in some other posts of various people. You guys keep mixing up velocity and delta-V. The first one is the actual speed you are going, while the latter is a velocity change. These are two different things, but let me clarify it with an example:

If you are in a stable, circular orbit at 100km you have an orbit velocity of ~2245 m/s. To do a Hohmann Transfer to, let's say 200km, you need to apply delta-V of ~73m/s. So your final velocity after that change is ~2319m/s.

I know i'm arguing about trifles, but my mathematicians heart hurts so much while reading this. ;D

[Sabactus]

Ok those velocities aren't the velocities at the time those are the velocities that I had to change to. So at 165km I applied deltaV (velocity change) to 2148m/s.

[szerszak]

Man, this topic is offtoped hard.

I've just succefully realised my idea of landing near space center.

Plan was:

1. Achieve orbit

2. Make Hohmann transfer to 36km orbit

3. Wait for visual contact with space center area and deorbit to hit atmosphere

4. Make corections of your course while descending

5. Land

But I belive I wont be able to land near landing center in repeatable manner when doing other tasks.

I made my mission as simple as possible - heading straight east all the time I stayed on course to launch pad. More complicated missions might get you off the course and frankly I don't have idea how to get back on the right one (to fly over space center). Especialy when I have no idea how to use that pink indicator - it points direction but I feel I lack more info to make this thing usefull.

The other thing is that I need at least one full fuel tank to aim at launch pad (unless I got lucky and deorbit in the right moment) and that's quite a lot.

[Sabactus]

See im trying to do it completely unpowered after the deorbit burn

[Rocket Surgeon]

I recognized it earlier in some other posts of various people. You guys keep mixing up velocity and delta-V. The first one is the actual speed you are going, while the latter is a velocity change. These are two different things, but let me clarify it with an example:

If you are in a stable, circular orbit at 100km you have an orbit velocity of ~2245 m/s. To do a Hohmann Transfer to, let's say 200km, you need to apply delta-V of ~73m/s. So your final velocity after that change is ~2319m/s.

I know i'm arguing about trifles, but my mathematicians heart hurts so much while reading this. ;D

This isn't just trifles. If everyone speaks a different thing then no-one knows what's being said.

[kerbalkicker]

I managed to get within about 1 km of the launch center by timing my de-orbit burn properly and free-falling in. I was orbiting at about 48 km, burned pretty much an entire tank and let it drop in. About halfway through the burn I thought I was going to massively overshoot my target, but I didn't.

[Moach]

i find i can get myself pretty close if i deorbit 90 degrees uprange from circular at 65k towards a -2.5~3.5k below-ground perigee...

haven't gotten the exact values right yet... but i keep landing within recovery ships reach (assuming those sail off from the base) by doing it this way...

you get a steeper reentry at 90 degrees than you do from 180, but it's easier to get it right like this... plus, the pod with heatshield can very well take it :

a good way to determing the deorbit location is to mark your attitude upon passing over the 'other side' - which you can easily see from the purple marker... then leave the controls untouched and once the ship has pitched 90, burn retrograde to get yourself at the needed speed for a negative -2.5~3.5k perigee

then hope you did everything right and coast your way down

[defiler86]

Closest I got to the center is about +-1000 m. Landing it off shore, and the center was in view distance. It was dumb luck that it happen since it was the first mission I was practicing Hofmann Transfer orbits. XD

[TwoHedWlf]

My first try at landing back at the space center I started de orbiting from about 50,000m as soon as I could see the center in front of me. Ended up overshooting but had enough fuel to turn around and keep on a steep descent towards KSC. Ejected the last stage( fuel tank and LFE) so I could land using the chute. They hit the side of the assembly building. ;D