2 Questions about Delta-Vee & Launching Small Launch Vehicles

[Turk_WLF]

As I understand it when Squad changed the Atmosphere in V 1.0 they changed the Delta-Vee from 4500 down to I believe 3200 to get into Low Kerbin Orbit, is this so?

Also with the changes to the Atmosphere, launching Small Launch Vehicles (the Stock Z-Map Satellite) is harder because they tend to flop over in middle of the flight. Are there any tips for launching smaller Launch Vehicles?

I’m just wondering is that are any methods that I don’t know about? By the way I’m using KSP 1.0.4 & Mechjeb.

Thanks,

[obsidian_x]

Put some fins down low as you can get them on the rocket. 3 or 4 will do it.

- - - Updated - - -

Sorry didn't answer the other question. I belive it's around 3400 dv to LKO now. That's what I plan for and depending on how good the launch, I can have some left, or I need about 100m/s of the next stage.

[Fearless Son]

The changes to the atmospheric model have necessitated a few "best practices" changes on the part of players.

Previously you wanted to do a straight ascent, then a sharp gravity turn. These days, you want your gravity turn to be long and gentle. Start turning just a little off the launch pad, and turn a tiny bit at a time. If your rocket is well balanced, you should keep the nose pointed at the prograde vector as it very slowly slips sideways. You want to make one continuous turn between 500 and 20,000 meters, starting pointing vertical and ending pointing about 45 degrees from parallel with the surface, though your mileage may very depending on your design.

Secondly, center of mass versus center of drag is now a much bigger concern, and this is what is responsible for rockets flipping (the gentle turn mentioned above mitigates this somewhat.) You can avoid the dreaded flip by keeping the rocket's center of mass low, and keeping its center of drag even lower. This primarily means two things. First, shorter, fatter rockets are more stable than longer, thinner ones. Try to build them with the minimum number of stacked fuel tanks. A single large tank will be less in danger of drastically shifting the center of mass than two smaller tanks with the same total fuel would. In fact, having radial fuel tanks that feed into the main booster fuel tank (and jettison when they are empty) is in my experience an effective solution. The other is that you want to minimize drag on the top of the rocket, while actually deliberately adding drag to the bottom. Avoid excessively wide fairings, as while careful use of fairings can reduce overall drag, they can also shift that center of drag upward. Obviously place aerodynamic nose cones where you can, particularly if they are angled to deflect air away from the main body. Adding fins to the bottom of rockets is now almost mandatory, if only because they create drag down low and help keep the rocket stable. Put some with control surfaces on there and you can even increase your control of the rocket at lower altitude (where they can also help your SAS.) Airbrakes with freedom of control can also help with this, but they create a lot of total drag so they are only worth it in this capacity if your rocket is really big or has a lot of unavoidable drag at the top.

[Harry Rhodan]

You can avoid the dreaded flip by keeping the rocket's center of mass low, and keeping its center of drag even lower.

Or by using heavy upper stages that keep your CoM up.

[Wolfox]

Or by using heavy upper stages that keep your CoM up.

True. Center of Mass in the middle together with unbalanced Center of Drag is a recipe for a disaster.

Think of a CoM as of a hinge on which the whole rocket is screwed. Both very up and down are stabile in some way, but center is prone to rotate around its axis.

Edited August 6, 2015 by Wolfox

[brettuzzi]

Using asparagus-style staging (fuel lines) will do wonders for keeping your rocket more stable -- your main stack rocket won't drain (keeping the majority of your vessels mass balanced front-to-back) until the very last atmospheric stage, and the radial tanks won't all drain from the top at the same time.

Your center of mass will stay way closer to the front of the rocket for the entirety of the ascent, which will do wonders in preventing flipping AND give you better delta V to boot.

~3200 is the lowest amount of delta V that I've heard of people reaching orbit, but I usually plan on somewhere between 3400 and 3700 depending on how awkward the payload is.

[Warzouz]

Yes, 3200 is a good target dV. To get this dV you have to get a very good trajectory and a smooth flight. Further more, you payload has to be very streamlined.

I usually get to LKO at 3200 with almost all payloads (except very non streamlined ones - up to 3500). This is vaccum dV. The real dV (including atmo ISP loss is a bit lower). You don't have to take that into account. Just don't build a launch stage with Poodle, Terrier or Nerva.

Just stick 3200m/s to your launch stage or 3400 to be safe. I also recommend to add a bit more to small rockets as they are a bit harder to fly than heavier ones.

[Harry Rhodan]

Using asparagus-style staging (fuel lines) will do wonders for keeping your rocket more stable -- your main stack rocket won't drain (keeping the majority of your vessels mass balanced front-to-back) until the very last atmospheric stage, and the radial tanks won't all drain from the top at the same time.

Your center of mass will stay way closer to the front of the rocket for the entirety of the ascent,

By piling up all the fuel at the back of the rocket you keep your CoM closer to the front?

[Red Iron Crown]

By piling up all the fuel at the back of the rocket you keep your CoM closer to the front?

Not so much that it keeps the CoM forward, but that it keeps it from moving rearward. A single stack rocket drains top to bottom, so it is very possible (if not likely) that the CoM will move rearward as the upper tanks empty. Aparagus (or other radial boosters) also help keep the center of drag rearward as well.

[brettuzzi]

By piling up all the fuel at the back of the rocket you keep your CoM closer to the front?

edit; see next page

Edited August 6, 2015 by brettuzzi

[Red Iron Crown]

Er, contents don't shift around in single tanks, the CoM of the tank itself remains in the same position whether full or empty or whatever the G forces.

[brettuzzi]

My bad.. Duh, contents under pressure!

I think my original point is still (but only) relevant if you have multiple tanks in your main stack then.

[lugge]

Just stick 3200m/s to your launch stage or 3400 to be safe. I also recommend to add a bit more to small rockets as they are a bit harder to fly than heavier ones.

Sorry for interruption, but that's something I always wonder:

When a Dv Map (or you) tell me an amount of needed Dv for orbit at a body with atmosphere, is this Vacuum Dv?

Or, in other words, do I have to set KER to Vacuum or ground altitude when building my lifting stage for 3400m/s?

[Sharpy]

3400 is the vacuum dV as calculated by KER. The actual, "true" dV to equatorial prograde LKO is something of order of 2000m/s (a bit over 2200 of LKO speed, minus initial speed of Kerbin rotation). The remaining 1400 is the extra that accounts for unavoidable losses to atmospheric drag, gravity drag and poor ISp in low atmosphere.

It's the same 2000m/s that were there before 1.0.3, but since the air got much thinner, the proportions of losses changed: you lose less to air drag, so you may start gravity turn earlier, to gain horizontal speed faster, thus reducing losses to gravity drag as well. But since you're moving slower vertically, your ISp losses increase somewhat - not so much as to outweigh the gravity and air drag benefits though.

In the past you had to keep your speed below 200m/s in low atmosphere, increasing time of ascent and so losses to gravity drag, the air drag was generally higher, and since you started the turn much later the gravity drag swallowed even more. That's why the overhead above the obligatory 2000m/s was much higher.