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Question for Veteran: Delta-V and rocket with wings


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Hi guys,

I know that getting into a 80km orbit around Kerbin takes 3400 delta-v. I built many rockets in the last few months with no issues but something happened yesterday night that left me puzzled.

I built a tiny plane with two small rocket engine that has 3600 delta-v and a TWR of around 0.7. The plane slowly climb up to around 60 000 meters, reaching a speed of 900 m/s  ...then run out of fuel. Before someone ask, the Delta V calculation for the plane is provided by Engineering Redux.

My question is ... what is the difference in the Delta-V budget between a standard rocket and a plane (using rocket engines)? How come a plane cannot do it even though it has more delta v than necessary? It's probably something related to air friction on the wings or something but cannot put my finger on it.I am missing something but I am pretty sure that someone here can explain that to me.

Thanks guys, this forum is awesome.

 

 

 

 

 

Edited by Frag2000
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Launch profile, the path your craft takes to space, matters a lot. If you could fly your plane up to 7000 m and then expend all of that 3600 m/s at once, you'd suddenly have a plane going ~3600 m/s---in the atmosphere. Drag would hit it like a brick wall, and it would rapidly slow down to just a few hundred m/s, if it didn't burn up first. All that delta v would be lost.

If you climb slowly, then the same thing happens in slow motion. You're expending fuel just to stay at or near terminal velocity over a longer period than is necessary for an equivalent gravity-turn launch. This is why a successful spaceplane design generally uses jet engines to get out of the lower atmosphere; they're a lot more efficient than rockets when there's air in the way.

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1 hour ago, Frag2000 said:

My question is ... what is the difference in the Delta-V budget between a standard rocket and a plane (using rocket engines)? How come a plane cannot do it even though it has more delta v than necessary? It's probably something related to air friction on the wings or something but cannot put my finger on it.I am missing something but I am pretty sure that someone here can explain that to me.

The 3400 number is for a well-optimised craft flown in a more or less optimal ascent profile. If you're really good you can get to orbit with a good deal less than that (3100 is doable), if you're not so good or your craft are not so optimal for whatever reason, you can easily burn 4000 without doing anything wrong wrong.

Thing with getting into Kerbin orbit, is you have to beat two forces: gravity and drag. If you have a TWR of 1 and are going straight up, you'll never get to orbit: all the fuel you burn will be lost to gravity. Similarly, if your drag is exactly equal to your thrust, you'll spend all your fuel fighting it without ever going any faster.

So, to answer your question, your problem is the low TWR and shallow climb: you're burning most of your fuel pushing against air rather than accelerating your craft.

In my experience, the a well-made rocket-powered spaceplane can get to orbit with about the same dV as a well-made rocket. You lose some more to air resistance because you're spending a bit more time in the lower atmosphere, but you lose a little less to gravity because your wings are producing lift and you're accelerating the craft, and you win back a bit more by getting away with a lower starting TWR -- 1.0 isn't enough for a rocket, but it is enough for a rocket-powered plane. You probably shouldn't go much below that though -- I have flown some that start out as low as 0.85 but that's really pushing it.

To fly a rocket plane to orbit, take off, point it up at 45 degrees, and when you hit 10 km and 500 m/s, follow prograde: from there on out you'll be on the same vector as a rocket on a gravity turn. If your rocket plane doesn't have enough power to fly up at 45 degrees immediately after take-off (it's okay if it doesn't accelerate in the beginning, it will a bit later), add more engines or remove some mass or improve its aerodynamic efficiency.

Edited by Guest
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